LIFE IN THE TREES: THE PRIMATES EVOLVE

EARLY IN THE MORNING OF 30 DECEMBER; ABOUT 996,000 METERS UP THE LINE

Now, the end of the year is in sight. The map of the world is gradually becoming one that humans would recognize, even though many changes are still to come. With the extinction of the non-avian dinosaurs, the mammals of the Cenozoic Era will begin to diversify remarkably in form, size, and habitat. They will reshape life on every continent except Antarctica, and will even make their presence felt in the world ocean and in the skies. As we saw in the last chapter, the earliest known eutherian mammal was adapted for climbing as long ago as 160 million ybp. Now, a hundred million years later, there are mammalian populations living among the tree branches of certain forest regions. We will probably never know when arboreal life became the norm for mammals such as these. And in regard to one particular variety of tree-dwellers—the primates—we need to look at the extraordinary physical traits they acquired over the course of their tens of millions of years of existence.

Primate Characteristics

In every evolutionary transition we have examined, the form of a new animal emerges before there is an animal that we can say is definitively of a new type. For example, the earliest tetrapods were not yet fully amphibian, even though they may have had certain amphibian characteristics. There were reptile-like amphibians before there were reptiles. There were non-mammalian synapsids that had a number of mammalian traits. So it was with the primates. In examining the lineage that led to primates, we have examples of animals that showed primate-like traits but which were not necessarily true primates. So when have we arrived at the point where we can say an animal is a true primate? How is a primate defined? Scientists from Carolus Linnaeus, who in the 18^th century devised the first truly scientific way of classifying organisms, to modern primatologists, have tackled that question. It is not simply a matter of listing particular features; it is also considering the possession of certain features in combination. There seems to be a consensus on the following points:

• In regard to the skeletal system: the possession of clavicles, the possession of specialized bone structures around the eye (either a postorbital bar or a more-encompassing structure called a postorbital closure, which is typical of monkeys, apes, and humans), a braincase larger in proportion than those of other mammals, incisor, molar, and canine teeth (at some stage of life), with four incisors in the upper jaw. [It should also be noted here that the possession of a petrosal bulla, part of the skeletal structure of the middle ear, is a trait absolutely unique to primates.]
• In regard to the appendages: the possession of nails or claws (tending toward the possession of nails in more “advanced” primates), a distinct big toe (hallux) on the feet, a big toe that is widely separated from the other toes except in the case of humans, hands and feet adapted for grasping (prehension), a tendency toward opposability [the ability to bring a toe or finger in contact with the other toes or fingers for the purposes of  grasping an object] in at least one digit on the hands and/or feet. [Opposable thumbs are the most common example in primates.]
• In regard to other general features of the anatomy: a brain size larger in relation to the body than those of other mammals, a shorter snout than most mammals, mammary glands centered in the chest, male genitalia that are pendulous, a well-developed caecum (a section of the large intestine).
• In regard to the senses: a greatly enhanced visual sense, with a tendency toward stereoscopic vision [using both eyes to focus on a single point, which creates the experience of depth-perception] and chromatic [color-perceiving] vision. There is a relatively poor sense of smell (olfaction). 
• Other traits: long gestation periods compared to maternal size, relatively slow growth of offspring in relation to maternal size, relatively late sexual maturation,  relatively long life-spans.¹

Again, it is the possession of these traits in combination that distinguishes primates. Further, the earliest primates did not necessarily possess the most fully elaborated forms of these traits. Primatologists often refer to these as the suite of primate characteristics. How were they acquired?

Hypotheses Regarding the Evolution of Primate Physical Traits

For many years, the dominant hypothesis in primate studies, one that was particularly stressed in anthropology, was the arboreal hypothesis. This set of ideas dates back to the early 20^th century, although it has been elaborated on since that time. Basically, this was the argument that such traits as prehensile appendages, stereoscopic vision, and chromatic vision were adaptations to tree-dwelling. This hypothesis also emphasizes a shift in the structure and physiology of the primate brain toward vision at the expense of olfactory capacities. There are seemingly persuasive arguments in favor of this position. Depth perception in the forest maze can indeed be crucial. An adaptation called brachiation, or traveling by swinging from branch to branch, evolved among larger, more “advanced” primates. Depth perception is absolutely essential to this adaptation. Prehensile hands are also well adapted for maintaining one’s grip not just on branches but on life and limb themselves.² And seeing in color, which is simply the product of the ability to perceive more diverse frequencies of light, is certainly a tremendous advantage in forest life, especially for spotting predators. But are these ideas the only possible explanations for the primate anatomical suite, or are there other, more basic explanations that need to be explored?

An alternative view to the arboreal argument is known as the visual predation hypothesis. First put forth by primatologist Matt Cartmill in the early 1970s, and since modified, its adherents maintain that the source of primate characteristics was the need to see and grab insects for food, insects that were found chiefly on small branches in the lower parts of trees. Indeed, there are many predators in nature with forward-facing eyes, and it is true that the original primates were very likely to have been insectivores. But since there are many predators which lack stereoscopic vision, a variation of the visual predation hypothesis has been offered. Its advocates now contend that nocturnal hunters require forward-facing eyes. Studies of prosimians (the most “primitive” variety of living primates) and comparisons between the eyes of prosimians and those of the earliest primates of the modern type, the euprimates (see below) appear to give credence to this contention. This would seem to argue that the basal primates were a group of nocturnal insectivores moving among slender branches in search of food.³

It should also be noted that, generally, advocates of the visual predation view argue that since there are many mammals that are adapted to arboreal living (such as squirrels) which do not exhibit primate-like features, and have very keen senses of smell, then therefore primate-like traits cannot be attributed to life in the trees per se. Elwyn Simons, a critic of this perspective has asked, rather pointedly, how the adaptations of tree rodents are relevant to the fact that modern primates exhibit the visual and appendicular qualities that they do. [I would ask at this juncture: why does natural selection have to choose only one way of living in a particular environment? All that seems to be necessary is for a way of life to be reproductively advantageous.] He also points out that the anthropoid [ape-like] primate specimens uncovered in the Fayum Depression of Egypt were from animals that were both arboreal and diurnal [daylight-living].⁴ This would seem to argue that modern anthropoid traits are of great antiquity, and that nocturnal predation wasn’t necessarily the only force that drove primate evolution. There may be other factors at work as well, which suggest that the visual predation hypothesis may not provide a complete explanation.

Still another view is known as the angiosperm radiation hypothesis. Proposed by Robert Sussman, he contends that the radiation of flowering plants (specifically angiosperms in developing rain forests) was the principal driving force in the shaping of primate traits. Sussman maintains that the flowering plants, primates, bats, and those birds that feed off of plants, formed a co-evolutionary relationship with each other, beginning around the boundary of the Paleocene [65.5 million ybp to about 55.8 million ybp] and the Eocene [55.8 million to 33.9 million ybp] Epochs. The angiosperms provided readily available sources of food, and these sources facilitated the rise of all those taxa that fed off of them. Those taxa in turn disseminated seeds, helping angiosperms spread more widely. In this hypothesis, primate traits were shaped by the desire of the earliest primates to reach food that grew on the ends of branches—flowers, fruits, and the insects feeding off of the vegetation. According to Sussman, primates operating in low light conditions needed enhanced visual and manipulation skills.⁵  

There is a synthesis of the visual predation/angiosperm radiation hypotheses emerging. A recent study has suggested that primates developed skills necessary to exploit angiosperms and while doing so grabbed insects in an opportunistic fashion. This foraging pattern was made possible by the ability to move around on small branches while using the appendages to capture prey. The advantage of being able to do these things would have strongly encouraged the evolution of prehensility, and in fact the developments of locomotor skills and prehensile hands seem to have reinforced one another. It is also worth noting that Eocene primates already possessed a high degree of prehensility in their hands.⁶

It can therefore be argued that the search for insects, fruit, and leaves attracted primate-like animals into the trees and contributed to the evolution of their attributes. Certainly the evolution of precision grip, the ability to grasp and hold very small objects between the thumb and finger, must have been influenced by predation on small insects. But it can also be argued, in my view, that adaptations for predation and fruit-gathering may have been just the beginning of the process of primate development, not its culmination. As primates were attracted to higher elevations in trees, it would seem to me that these habitats must have confronted them with particularly sharp selection pressures. Prehensility would have been of even more crucial importance in such a setting. Stereoscopic and chromatic vision must have been powerfully reinforced by tree-top living, in light of the ubiquity of these features in the primate order. The evolution of forward-facing eyes may have entailed a reduction in peripheral vision, but evidently it was not a fatal one. And we must account for the fact that primates may have started out as nocturnal animals, but the vast majority of them are now awake in the daylight hours. Primate characteristics are in all likelihood traceable to a number of factors operating in a synergistic fashion. In one sense, therefore, it may be said that the various hypotheses concerning the selection pressures that shaped primates are complementary to each other rather than contradictory. 

Sister Taxa and Clades

Evolution is a bush, not a ladder. Population A can give rise to an offshoot, Population A1, that can evolve into a separate, distinct species. But Population A doesn’t necessarily become extinct. Often it continues onward. The ancestral form and the descendant form can therefore coexist.⁷ Only if A1 has gained a strong reproductive advantage over A is A likely to eventually disappear. Further, when two or more kinds of organisms have all branched off from a common (usually extinct) ancestral population, they are known as sister taxa (singular sister taxon) to each other. For example, humans and chimpanzees, having genetically split off from a common ancestor, are sister taxa, and we would say chimpanzees are our sister taxon. At each point going back through time there are junctures of common ancestry giving rise to descendant forms which are sister taxa to each other. Groups of organisms that all have a common ancestor are known as clades. (The study of these branching relationships is called cladistics.) Entire groups of animals, such as the whole of Class Mammalia, can be considered clades. When all these phylogenetic relationships are rendered in graphic form, depicting lines of descent through time and charting evolutionary connections between and among organisms, it gives the appearance of a bushy, treelike, branching object. If it were possible to depict all phylogenetic relationships over the last 3.0-3.5 billion years, beginning with the Last Universal Common Ancestor, (or even from the first metabolic reactions that perhaps helped bring about self-reproducing molecules at 3.8 billion ybp), the bush/tree would be incredibly dense with branches and nodes (the points from which common ancestry begins to branch) and it would be of immense size. In this huge, bushy tree of life, primates are a clade (specifically an order) nested within the larger clade of mammals (a class). It is this clade upon which we are now focused. I must stress that it is genetic analysis that establishes the definition of a clade. So in what animals do we begin to see primate tendencies, and what animals seem to be genetic forebears of the primate line? Further, in what era of prehistory did the momentous events of primate evolution occur?

Approaches to Determining Primate Origins

Two distinct but interdependent methods are used by paleontologists to try to establish the phylogenetic history of a clade. There is the traditional method of evaluating fossil evidence and constructing phylogenetic trees accordingly. Then there are the newer techniques of molecular genetics, which estimate the evolutionary emergence of organisms based on assessments of the genomes of living animals and estimates of the rate of genetic change within a clade over time. Both approaches have their limitations. As we have noted, less than 0.1% of all organic material escapes being recycled into other living matter, making the incidence of fossilization exceedingly rare. In the case of primates, many of the earliest animals were small and delicately built, which compounds our difficulties greatly. When molecular techniques are used, a great deal depends on how the estimate of the genetic distance between types of organisms is calculated. These estimates rest, as we have seen, on calibration rates which start with known points of divergence in the fossil record and other types of data (see below). From these data, rates of genetic change are then extrapolated. As I noted in the chapter on the earliest animal life, there has been controversy surrounding some of the results of this methodology, most notably in estimating the time of the emergence of the first vertebrates. But I also noted, in the chapter on the evolution of mammals, that molecular techniques accurately predicted the period in which the first eutherian mammal had to have evolved, even when the oldest fossil evidence in our possession was from a time tens of millions of years after the estimated point of divergence. So, when used judiciously and with a fossil record that provides crucial evidence of known divergences, molecular techniques can be an effective tool.

In using molecular dating, scientists bear in mind that not all rates of genetic change are identical. In addition to using known divergences from the fossil record, therefore, scientists take into account such variables as the geographic distribution of a taxon (its biogeography), geographic barriers that split members of a taxon from each other and influence their biogeography (a phenomenon known as vicariance), and data from geology on the position of a given landmass in the past. Researchers must also take care to not use calibration rates calculated for one line of animals to date other lines which may be only distantly related. They must also take into account such factors as fossil samples which may be incomplete, in poor shape, or misidentified.⁸

So we will first examine the fossil record we have in hand, and then examine the molecular estimates, some of which extend the beginning of the primate lineage much deeper into the past than we might expect.

Fossil Evidence of Primate Origins

There is some uncertainty among primatologists about which of the mammals found in the fossil record was the earliest true primate, and there have been vigorous disputes about whether certain animals were or were not part of the primate line. The problem has been a paucity of evidence, and naturally, as we have seen with the other taxa we have examined, new discoveries alter our picture, sometimes dramatically. Based on the fossil record we possess, it appears to many paleontologists that primates evolved out of a line of insectivorous mammals that lived in the late Cretaceous Period, a line that probably also gave rise to flying lemurs (which do not fly and are not true lemurs) and tree shrews. Collectively, the primates, the tree shrews, and flying lemurs all form a clade called Euarchonta, and are descendants of an ancestral or basal euarchontan. As far as which primates came first, the prominent paleontologist Frederick Szalay contends that the paromomyids were the oldest known primate family, on the basis of specimens of the lower jaw, teeth, and basicranium (the bottom part of the skull). The paromomyids  were small animals, none of them much bigger than a modern rat. They evolved as a branch of Mammalia during the Paleocene Epoch. Perhaps the best known of the paromomyids was Purgatorius, specimens of which have been discovered in Montana, in the United States. Many species within the paromomyids have been identified, primarily by their dentition, although two skulls have been uncovered. Paromomyids have been discovered in Europe as well as North America, and they extended into the Eocene Epoch.⁹

There is particular interest in animals known as plesiadapiforms. As late as the 1990s they were being written out of the primate family by some, being consigned to the status of sister taxon.¹⁰ But Szalay believes that one particular family, the plesiadapidae were among the earliest primates. The best known genus in this family is Plesiadapis. It is now thought that these primates had diverged from the paromomyids by the early Paleocene at the latest.¹¹ In 2007, the position of the plesiadapiforms within the primate line was strengthened. A team of scientists at the Florida Museum of Natural History, led by paleontologist Jonathan Bloch, announced the discovery of the earliest mostly intact primate skeletons yet discovered, a pair of plesiadapiforms which have been designated Dryomomys szalayi and Ignacius clarkforkensis. On the basis of their exhaustive analysis of these specimens, the researchers believe that the ancestral euarchontan from which primates evolved was insectivorous, arboreal, similar in form to the modern tree shrew, and small in size, perhaps no heavier than 30 grams. They further contend that the radiation of the earliest primates was facilitated by and concomitant with the rapid spread of flowering plants across the continents, which would have provided ample food sources. [These researchers would appear to support the angiosperm radiation hypothesis.] The exploitation of these resources (as we have seen) would have encouraged selection for prehensile hands and feet and the development of locomotor skills. According to Bloch and his partners, diverse kinds of plesiadapiforms radiated through forests for ten million years, and the first euprimates evolved from this radiation by about 62 million ybp, some 7 million years before their first appearance in the fossil record.¹²

Molecular Estimates of Primate Origins

In the early 2000s a team of scientists offered an estimate of the chronological emergence of primates based on a wide array of molecular analyses of primate phylogeny. It was their opinion that the best estimate was that the primates diverged from other mammals approximately 85 million ybp, some 20 million years before the conventional estimates. These researchers were careful to point out, however, that probably only about 5% to 7% of all the primate species that have ever existed have been discovered, and that the assumption that we have unearthed all the significant finds is an unfounded one.¹³ Other estimates give highly variable results. One places the earliest possible date of primate divergence from the rest of the mammals at 110 million ybp.¹⁴ Another gives an estimated date of around 77 million ybp,¹⁵ and another team of researchers puts the range of primate divergence dates at 65-73 mya.¹⁶  Several other sources generally support dates in the range of 80-90 million ybp.

Perhaps the most radical hypothesis—and its radicalness does not rule out the possibility that it is accurate—asserts that the best way to ascertain when the primate lineage evolved and then radiated is to calibrate these events to the tectonic record rather than the admittedly very incomplete fossil record. A scientist taking this approach contends that the clade of basal archontans split into the plesiadapiforms, primates, tree shrews, and flying lemurs 185 million years before the present.  He argues that it was the break-up of Pangea that triggered these splits. Further, he contends that the two major primate suborders (see below) split from each other in the Early Jurassic Period and the Old World Monkeys and New World Monkeys split as early as 130 mya. He argues that the current fossil and molecular estimates greatly underestimate the antiquity of the primate line, and that a phylogeny based on a widely-dispersed common ancestor makes more sense than the current models. Moreover, this new model solves the thorny problems of primate dispersal across bodies of water.¹⁷ If this researcher is correct, the primate line began to diverge into separate clades before the oldest known eutherian mammal, and it began to emerge as an order not too long after (in geological terms) the mammals themselves evolved out of the lineage of non-mammalian synapsids. This hypothesis is still fairly recent, and it remains to be seen whether significant tangible evidence to support it more strongly will be discovered.

The Euprimates Emerge

When paleontologists and primatologists speak of euprimates, they mean animals that resembled what we think of as “true” primates, or, as the term is used, primates of the modern aspect. This means animals that are adapted for leaping (although this adaptation has vanished in humans), eyes that are completely forward-facing, more complex brains, hands and feet that are definitely adapted for grasping (indicated by such features as longer fingers and toes, divergent thumbs on the forelimbs, and divergent big toes on the hindlimbs), and teeth that in the early euprimates were adapted for herbivory rather than insectivory (a key indicator).¹⁸ The oldest fossil records we have of the euprimates date back to the late Paleocene/early Eocene, about 56-55 mya. The earliest probable euprimate yet discovered is Altiatlasius koulchii. The samples of this animal, discovered in North Africa, have chiefly been teeth, and its exact relationship to other primates is as yet unresolved. Another North African group known as the azibiids could be in the euprimate clade but the physical evidence we have of them is sparse, and no definitive judgments can be made.¹⁹

Despite the finds of possible euprimates in Africa, the place of origin of the euprimates is still a matter of intense research and debate. Their sudden appearance in the fossil records of North America and Europe at the beginning of the Eocene, coupled with the dearth of transitional forms in those areas, suggests to many researchers that euprimates did not originate in those regions. And yet, the very oldest primates, the ones that everyone puts at the base of the primate tree, are of North American origin, which complicates matters. There is evidence to suggest that euprimates might have appeared in Asia at the Paleocene-Eocene boundary [and as we will see below, there are strong advocates for an Asian origin of primates]. But many uncertainties surround the issue of euprimate origins. The difficulty, of course, lies in the enormity of the territory that needs to be explored, and the inherent difficulties associated with discovering fossils, especially ones which are oftentimes so small and delicate in nature. There are many, many areas that have yet to be sampled. The situation is complicated even further by the fact that geographic barriers to the dispersal of euprimate species may have shifted considerably over the many centuries, making their travels hard to trace. There is also disagreement among paleontologists in the matter of how the specimens that have been uncovered should be organized into clades. Since there are promising finds that have been made in Asia, Europe, North America, and Africa, we just cannot yet say on which continent euprimates first evolved, as unsatisfying as that might seem.²⁰ 

Many scientists have believed that all Eocene euprimates fell into two categories: the adapoids, which traditionally have been called lemur-like animals, and the omomyoids, which have traditionally been called tarsier-like animals.²¹ Lemurs are “primitive” primates living in Madagascar (see below) while tarsiers are small nocturnal primates with disproportionately large eyes, which live only in Malaysia, Brunei, the Philippines and Indonesia.²² The designations “lemur-like” and “tarsier-like” are somewhat misleading, inasmuch as lemurs are highly diversified animals and a great majority of omomyoids were not really tarsier-like.²³ Most scientists now think that the adapoids were the first radiation of one of the two primate suborders, the one known as the Strepsirhini. Living on the margins of the biosphere in generally isolated habitats, the primates of this suborder have an appearance that sets them off from most other primates. Sometimes known as the “lower” primates, they are also frequently referred to as the prosimians. The suborder includes not only lemurs but also the lorises and the galagos. The suborder’s members have smaller and less complex brains than other primates. These animals can possess both claws and nails, and they mark their territory with either scent glands or urine. The females possess a two-chambered uterus (also known as a bicornuate uterus), and they have more than two mammary glands. Many prosimians have a somewhat dog-like facial appearance because of their prominent, often elongated snouts. They have facial and eyebrow whiskers, reminiscent of felines. Their eyes are large, adapted for nocturnal predation, and have a light-reflecting structure at the rear of the eye known as a tapetum lucidum that makes their eyes glow when a light source strikes them. Prosimians tend to have big, flexible ears and a better sense of smell than other primates. All of them have tails, although not always prominent ones.²⁴ If one wanted a glimpse of some of the earliest primates, these animals would give hints of them, because the lemurs and lorises still show marked similarities to the adapiforms, lemurs especially so.²⁵ (Lemurs, by the way, are confined entirely to the island of Madagascar, and the means by which their ancestors reached the island are unclear, although interesting hypotheses are being offered. Lemurs are assumed to be of monophyletic origin and they live in a fairly unique ecological setting, among a group of mammals that are markedly distinct from those of continental Africa.)²⁶

In general, the adapoids were larger than the plesiadapiforms and the omomyoids. Their dentition was primitive compared to that of modern primates. They had long, broad snouts, their eye orbits were encased in bone, and their braincases were larger than those of the earliest primates but smaller than modern lemurs. Their limbs were similar to modern strepsirhines but more strongly built. They had long legs, tails, and torsos, and hands that had nails instead of claws. The adapoids possessed hands with divergent thumbs and prehensile feet. Some of the better known adapoid genera are Cantius, from the early Eocene of Europe and North America,  Notharctus from the middle Eocene of North America and Smilodectes, from the early-middle Eocene of North America. There are excellent fossil specimens for these animals, which appear to have been diurnal. Adapis comes from the late Eocene of Europe, and Sivaladapis, from India, was an animal of the late Miocene Epoch [23 million to about 5.3 million ybp]. In addition, among the many other genera of adapoids, there are three from Africa, two of which are from the Oligocene Epoch [33.9 million to 23 million ybp].²⁷ The adapoids and their descendants, despite the marginal niches the strepsirhines occupy today, are a remarkable success story. It is still possible to see in the lemurs and lorises the genetic echoes of the early Paleocene Epoch. There are, in fact, some prominent scientists who suspect the adapoids may have ultimately given rise to the “higher” primates.

The majority of paleontologists have tended to believe that the humble little omomyoids are ancestral to the modern large primates. But a group of scientists working in Asia argues that modern “higher” primates descended from neither adapoids nor omomyoids. If they are right, then much of what we have thought about our deep origins needs to be reassessed. (See below)

Suborder Haplorhini—the Anthropoids

The suborder Haplorhini contains the vast majority of the world’s primates. These primates are more widely known as anthropoids, the “man-like” primates. What are their distinctive traits, and in what ways do these differ from the prosimians?

The senses and sensory organs: Anthropoids do not have a rhinarium, an area of wet, naked skin around or above the nostrils, as do the prosimians. There is a section of bone known as the postorbital septum that separates the eye orbit from the lateral part of the skull, and the eyes themselves face more directly forward than those of prosimians. The retinas of anthropoids possess a macula lutea, in the center of which is a fovea [that section of the retina that has the highest visual acuity and which contains nerve cells called cones that allow for the perception of color]. It is believed that these retinal structures are adaptations that allowed for greater visual acuity during diurnal activities, although anthropoids differ in the degree of their color perception. The visual cortex takes up a larger proportion of the brain than that of the typical prosimian. There is no tapetum lucidum, either. Vision is the dominant means by which the world is examined, with less emphasis on the senses of hearing and smell than is the case with prosimians. The anthropoid inner ear structure is distinctive, as are the blood vessels which supply it.²⁸

The appendages and general bony anatomy: Anthropoid heads tend to be more rounded than those of the “lower” primates. The structure of the foot is distinctive. The great majority of anthropoids have highly flexible hands and feet. Opposability of the thumb (pollex) is very widespread, although the degree of this ability varies significantly, and divergence of the thumb and the hallux is widespread, although not universal. Precision grip is also very common. The arms and legs tend to be more equal in length than in prosimians [although in humans the leg to arm ratio tends to be greater]. The structures of the femur, tibia, and knee differ from those of strepsirhines. The structure of the humerus is distinct in anthropoids.²⁹

The reproductive system: Anthropoid females go through a periodic menstrual cycle. Sexual receptivity in anthropoid females is not tied to the estrus cycle, as in prosimians. Hence, there is more variable sexual receptivity in anthropoid females. There are, however, fluctuations in receptivity based on the period of ovulation. The anthropoid clitoris is relatively small, although there are exceptions to this. Anthropoid females have a single-chambered uterus and the placenta is disc-like in form. Most prosimian males possess spines on the penis; the males of relatively few anthropoid species do. There are two mammary glands only.³⁰

Neuroanatomy: Most important of all, there is a significantly greater relative brain size and complexity in anthropoids compared to that of prosimians. Brain neurons important to facial recognition are linked with the amygdala, which assigns emotional responses to perceptions. It is this connection which may explain the significance of facial expressions in anthropoid social interactions. [It should be pointed out that many anthropoids possess a complex musculature in the face.] The anthropoid encephalization quotient (EQ) [a measure of expected brain mass to body mass] is 2.1, meaning that the anthropoid brain is twice as large on average as that of a given placental mammal of about the same size, although there is great variation in this EQ from species to species. In certain monkeys the EQ is only about 1.05, while in humans it is close to 6. The anthropoid neocortex, the most advanced part of the brain, seems to correlate in size with the size of the social groups typical of various species, although there are many other factors that influence neocortical development, and social group size may be a consequence of neocortical complexity. Brain growth also seems to correlate with the age of the first reproduction and the length of time it takes for an anthropoid to reach full maturation, although it is quite possible that brain complexity and the length of anthropoid juvenility have a reciprocal relationship. Bigger and more complex brains take a longer time to educate, after all. Further, most neuronal development in anthropoids takes place either prenatally or in infancy. As we will see in much greater detail elsewhere, the evolution of the anthropoid brain was not a simple matter.³¹

Suborder Haplorhini is divided into three infraorders: Tarsiiformes (the tarsiers), Catarrhini (which consists of Superfamily Cercopithecoidea, the Old World monkeys, and Superfamily Hominoidea, the apes and humans,) and Platyrrhini (more commonly called the New World monkeys). The Old World primates are differentiated from those of the New World by the structure of the nose, some differences in cranial structure, and a different dental formula (how many of each kind of tooth are in each quadrant of the mouth). All Old World monkeys, apes, and humans have a 2.1.2.3. formula, meaning that in each quadrant they have two incisors, one canine, two premolars, and three molars—32 teeth.

(A small linguistic note before we go any farther: the names of the genera of the animals ancestral to modern anthropoids very often include the root word pithecus. This root originates from the Greek word pithekos, which simply means ape or monkey.  The first part of the animal’s name generally refers to the locale in which it was thought to live, either a particular region or a particular environment. For example, as we will see below, the genus name Australopithecus simply means “southern ape”.)

Evolution of the Anthropoids

From where did Suborder Haplorhini emerge, and when did this happen? The Americas, Europe, Antarctica, and Australia have been definitively ruled out as places of anthropoid origin. For many years the dominant view has been that Africa was the birthplace of anthropoids. Given the many spectacular anthropoid fossils discovered in that continent, and the fact that the first representatives of the human genus were apparently of African origin, such a view is understandable. However, a significant number of paleontologists argue that Asia, not Africa, gave rise to the haplorhines, and they cite highly significant finds in many locations, particularly China, Burma, and India to support their contentions. These scientists argue that the presence of anthropoids in Africa was the result of Asian species that made their way to the African continent and eventually gave rise to all the lineages of African anthropoids, including the one that ultimately produced us. Moreover, the advocates of an Asian origin of the haplorhines postulate a much earlier origin of the suborder than do those scientists supporting an African birthplace.

EVIDENCE OF ANTHROPOIDS IN ASIA

In 1994, a group of paleontologists working in China announced the discovery of what they believe to be a basal anthropoid, which they named Eosimias—“dawn monkey”. Eosimias had several traits that seem primitive in combination with others that appear to be unusually advanced. It is the opinion of paleontologist Chris Beard, who has worked extensively with the remains of Eosimias, that this primate represents an animal distinct from both omomyoids and adapoids, and that, in fact, it represents the true base of the anthropoid tree. The specimens uncovered by Beard and his colleagues have been dated at 45 million ybp, and consist of several significant parts of the anatomy: complete lower jaws with dentition, upper canine teeth and sections of maxilla (upper jaw) that give us important clues about the face and eyes, and parts of the ankle that give clues to Eosimias’s monkey-like way of moving through branches.³² Some eight examples of Eosimias have been uncovered, five of them in China, and together they are part of a family known as the Eosimiidae.³³

In 2005  a team of paleontologists announced that several dozen fossils, mostly teeth, representing two previously unknown genera from the anthropoid families Amphipithecidae and Eosimiidae, had been uncovered in Pakistan. These specimens are from the Oligocene Epoch, and have been dated at around 31 million ybp. The significance of these finds, which at the time were the oldest of their kind yet discovered in South Asia, is expressed by these researchers in this statement, which makes reference both to various families (all of which have “dae” or “pithecidae” in their names) and genera (with the roots “simias” or “pithecus”): 

The results of our various phylogenetic analyses…primarily based on morphological characters…consistently point toward the monophyly of a large clade, including Asian Eosimiidae, Amphipithecidae, Arabo–African Oligopithecidae, Propliopithecidae, African Proteopithecidae, Parapithecidae, and South American platyrrhine primates. Assuming this clade to be the Anthropoidea clade, from the present evidence, eosimiids and amphipithecids (and by extension Phileosimias and Bugtipithecus, respectively) are stem anthropoids and, as such, support the hypothesis that Asia was the ancestral homeland of the Anthropoidea clade.³⁴

In 2008 a team of researchers announced the discovery in India of an eosimiid of very small size, roughly the dimensions of a mouse lemur with a weight of perhaps no more than 75 grams. Its genus has been named Anthrasimias by its discoverers, and it is estimated to have lived 55 million ybp. This pushes anthropoid origins in Asia back 10 million years. The discoverers of Anthrasimias contend that the eosimiid line in Asia means that the Omomyoidea and Adapoidea were sister clades of Anthropoidea, and that neither therefore could have given rise to it.³⁵ The implication here is clear: Anthropoidea emerged very soon (in geological terms) after most scientists believe the primate order itself evolved.

There has been a long debate in paleontology surrounding the family Amphipithecidae, mentioned above. Specifically, scientists have argued about its true phylogenetic status, and whether it was an anthropoid family. Based on recent evidence from the Pondaung Formation of Burma, the researchers involved in analyzing these specimens (including Beard) contend that the remains are of an animal of a new genus and species, Ganlea megacanina. It appears that Ganlea had very large lower canine teeth, ones that showed a pattern of wear typical of an animal that is using its teeth to break open hard objects to get the food, such as seeds and fruits with tough skins, inside of them, The dentition in general is that of a basal anthropoid, and according to the authors of the study announcing it, Ganlea demonstrates that the amphipithecids were indeed anthropoids. Perhaps just as significantly, its feeding behavior and ecological setting were highly similar to those of the New World Monkeys we see in the Amazon basin today.³⁶ So somewhere around 37-38 million years ago in the late middle Eocene of southeast Asia, a primate was moving about the tree branches living like a monkey—a member of the Suborder Haplorhini.

In general, scientists favoring the Asian hypothesis of anthropoid origins can point to the antiquity of such crucial finds, and the monkey-like traits of these specimens, to bolster their argument that Anthropoidea’s earliest members emerged in southern and eastern Asia and that the African lineages are the result of migrations into Africa, not the evolution within Africa of indigenous types.

EVIDENCE OF ANTHROPOIDS IN THE AFRO-ARABIAN REGION

The African area that has been explored most extensively in the search for anthropoids is the Fayum Depression of northeastern Egypt. This is the richest source of mammalian fossils in Africa. Fayum was tropical in the Eocene/Oligocene Epochs. In this warm and wet climate lived a variety of anthropoids. The most significant families and genera that have been found are:

The Parapithecids. Relatively primitive in dentition and limb features in comparison to other anthropoids, these animals ranged in size from small primates estimated to be no more than about 300 grams in mass such as Qatrania wingi to a larger specimen ranging up to 3000 grams, Parapithecus grangeri. Certain parapithecids are thought to have had large olfactory bulbs in their brains (denoting, perhaps, a keener sense of smell than other primates), and members of the genus Apidium appear to have been excellent leapers.³⁷

The Propliopithecids. More advanced in their dental formula than the parapithecids but more primitive in their cranial and postcranial anatomy than modern Old World monkeys, their most famous representative was Aegyptopithecus zeuxis, which lived about 30 million years ago. Aegyptopithecus was a relatively large animal, estimated to have been around 6700 grams in mass. Despite its small brain, it was probably one of the most intelligent animals on the planet at the time it existed. Many areas of its body are represented in the fossil record. Another major genus of propliopithecids was Propliopithecus, of which only teeth and parts of limbs have been uncovered. Members of this genus appear to have been frugivorous, arboreal, and characterized by prehensile feet. This family is quite possibly very phylogenetically significant, as it may have been at the base of the African primates that produced the modern great apes and humans.³⁸

The Oligopithecids.  This includes the genera Catopithecus and Oligopithecus. These primates possessed the modern anthropoid dental formula. They probably ate both insects and fruit (based on an analysis of their teeth). Judging from its eye orbits, Catopithecus appears to have been diurnal, and its limb bones suggest an animal that was an arboreal quadruped.³⁹

The preeminent researcher of African anthropoids is Elwyn Simons. In 2005 Simons and a number of his colleagues announced the discovery of specimens in Egypt of an anthropoid genus known as Biretia. The find was dated at 37 million ybp, and its discoverers and researchers considered it definitive proof that the Haplorhines originated in Africa. In fact, it was their contention that Biretia and a genus designated Algeripithecus represented an ancient African anthropoid clade.⁴⁰

However, this finding was later dealt a substantial blow. Advocates of an African origin of anthropoids have often pointed to Algeripithecus minutus, which lived about 45 mya in what is now Algeria, to buttress their case. But in 2009 it was announced that on the basis of the discovery of more complete fossils of Algeripithecus, it must be concluded that the animal was definitely not an anthropoid. Algeripithecus and its sister genus Azibius were actually strepsirhines, and the dentition and jaws of these animals show this clearly.⁴¹

The evidence, therefore, seems to be shifting in favor of an Asian origin of the anthropoids. However, no conclusive judgment in this matter is yet possible. It would appear that the greatest likelihood is that anthropoids first evolved in Asia relatively soon after the evolution of the primate order itself. Several families of them appear to have colonized Africa, and from these African “immigrant groups” the line of animals that ultimately produced the African great apes emerged.

As far as when specific anthropoid characteristics evolved, a group of researchers working in the area of Haplorhine evolution has put it this way:

We do not know the order in which most of [the] crown anthropoid features evolved. Most features probably appeared in a mosaic fashion in stem anthropoids. Some features may be primitive for Anthropoidea or Primates as a whole and others may have evolved in parallel in multiple crown anthropoid clades. We do know that most of the hard tissue features…are evident in Afro-Arabian fossils of late Eocene age, although several taxa lack some derived features found in living anthropoids... We do not know the character states for many of the presumed Eocene anthropoids from Asia because they are not yet known from adequate cranial materials. Postcranial materials of putative Asian stem anthropoids have been found in isolation, complicating their specific attribution.⁴²

The concept of a developmental mosaic is key because it emphasizes again that evolution and adaptation are not nice, neat, linear, straightforward processes. There are a great many environments exerting a great many selection pressures over a very wide span of time and a huge stretch of geography. It took many millions of years to bring about the advent of the “advanced” anthropoid types. Traits emerge and disappear, the same trait emerges (although not in an identical fashion) in different animals living in similar settings, and behavioral routines evolve that either enhance reproductive fitness or detract from it. The story is only now becoming clearer, and many, many tiles in the mosaic have yet to be found.

The Evolution of Hominoidea

It was these first anthropoids that formed the base of the huge primate clade of the eastern hemisphere, the Catarrhines. It is not yet clear when these earliest anthropoids began to differentiate into animals we would recognize as true monkeys (or monkey-like animals), although the evidence for such primates as Eosimias suggests monkey-like anatomy and behavioral adaptations. Catopithecus browni, a somewhat primitive Fayum primate, is thought to have been a catarrhine, quite possibly among the very oldest. Aegyptopithecus zeuxis has been described as an early catarrhine as well, a mixture of primitive and more specialized features. It is the features of the cranium and teeth that differentiate these animals from the earliest known anthropoids, although the differences can be subtle. There seems to have been a great deal of evolutionary development between the time of Catopithecus and the appearance of Aegyptopithecus.⁴³

There is a significant gap in the African primate fossil record, a seven million year stretch between the late Oligocene Epoch (around 30 mya) and the early Miocene Epoch (around 23 mya). This is particularly frustrating to paleontologists because they wish to identify the earliest true Old World monkey of modern aspect (currently a primate known as Victoriapithecus macinnesi holds that title at 19 million ybp)⁴⁴ and they want to identify the point at which a tremendously significant genetic split took place: the divergence between Cercopithecoidea, the Old World monkeys, and Hominoidea, the great apes and the humans. The discovery of a fossilized partial cranium of a catarrhine, Saadanius hijazensis, in what is now Saudi Arabia, has triggered a major debate about the timing of this split. Saadanius, dated at between 28 and 29 million ybp, appears to possess a combination of cercopithecoid and hominoid features, suggesting that the split of the great cercopithecoid-hominoid clade took place between 29-28 and 24 million ybp. One of Saadanius’s discoverers characterizes this animal as an ape-monkey intermediate.⁴⁵ Other scientists, however, strongly dispute this interpretation. One critic has contended that an ancestor-descendent relationship between Saadanius and extant catarrhines has not been demonstrated definitively, and that this specimen could very well represent a sister taxon to the modern catarrhines, a detour on the road to the split between monkeys and apes, not part of the main highway.⁴⁶ As we have already noted so often, more physical evidence is absolutely necessary to settle these questions beyond dispute.

At this juncture, it may be useful to explain the distinction between a monkey and an ape. Monkeys very often have tails. Apes lack this feature completely. Most monkeys tend to be arboreal in habitat. Apes, while having the physical capacity for climbing, are generally (but not always) terrestrial animals. Monkeys have molars of a different shape than those of apes. With the exception of gibbons, apes tend to be larger than monkeys. Most importantly, perhaps, apes tend to have a higher brain-to-body mass ratio than monkeys. What can molecular evolution tell us about when they branched off from each other?

In 2009 a team of molecular biologists did a broad study of divergences within the primate order. They noted that estimates of the Cercopithecoid-Hominoid divergence time have yielded a very wide range of dates. On the basis of their research, they put the dividing line at about 29.3 million ybp, which, they say, accords with other recent estimates. They further estimate that the split between the Hylobatidae, the lineage containing the gibbons and siamangs, and the Hominidae, the lineage that contains the chimpanzees, bonobos, orangutans, gorillas, and humans, took place around 21.5 million ybp. The authors point out that their calibration points are based on a wide array of fossil evidence representing as much of primate phylogeny as can be accounted for at present.⁴⁷

The earliest primate thought to have been a possible hominoid was Kamoyapithecus, found in Kenya, and dated from 27.8 to 23.9 million ybp, in the late Oligocene. Paleontologists feel they are on firmer ground with the group of Miocene primates known as proconsulids.⁴⁸ (See below.) The first primate considered to be a probable true hominoid, and one of the specimens thought to be of key importance in the ultimate evolution of hominids, was Morotopithecus bishopi, the remains of which were first discovered in Uganda in the 1960s. It has been dated at a minimum age of 20.6 million ybp. Analysis of these finds, along with the discovery of additional material in the 1990s, has shed new light on  the development of hominoid locomotor abilities and the evolution of hominoid morphology. Parts of the animal’s cranium, dentition, vertebral column, femur, and scapula have been uncovered. Morotopithecus appears to have been arboreal and quadrupedal. It apparently had prominent forelimbs, strong climbing abilities, and the ability to hang off of branches. If further research confirms its similarities to modern apes, Morotopithecus might be seen as a true ancestor of humans.⁴⁹

The Hominidae and Homininae

As we noted above, the primates classified as members of Hominidae are the great apes, both Asian and African, and humans. In addition to trying to elucidate their ancestry, we are going to look in particular for the ancestors of a subfamily of Hominidae known as the Homininae—the African apes (the gorillas, chimpanzees, and bonobos and their extinct relatives) and the humans (and their extinct relatives). In order to do this, we must focus on the primates of the Miocene Epoch, Morotopithecus being an early example. It was during the Miocene Epoch, from about 23 million to 5 million ybp, that many of the specific features that would ultimately become part of our genus evolved. It was from certain lineages of Miocene hominoids that the direct ancestors of the genus Homo emerged. The challenge for paleontologists and paleoanthropologists has been to place the discoveries that have been made in their proper context. This challenge is a daunting one, to say the least.

A very well-known genus of Miocene ape is known as Proconsul. First discovered in Kenya in 1909, the largest number of specimens has been found on Rusinga Island, in the eastern, Kenyan part of Lake Victoria. There were, by the best calculation, four species of them, ranging in size from that of a large monkey (perhaps 22-25 pounds) to almost as large as a gorilla (approaching 190 pounds in size). All of the specimens recovered existed between 21 and 14 million ybp in what are now Kenya and Uganda. They possessed a 2.1.2.3 dental formula, as all Miocene apes and modern Old World primates do, and surprisingly, their brain-to-body mass ratio was not very much different from that of modern African apes and monkeys of comparable size.⁵⁰  There is intense debate about Proconsul’s phylogenetic relationship to both the modern apes and humans. Many paleontologists believe Proconsul to be a stem hominoid genus, while others point out that its anatomical features do not show significant synapomorphies [derived traits in two or more taxa that appear to demonstrate common ancestry] with modern hominoids.⁵¹

Other significant Miocene ape genera include, but are not limited to:

Dryopithecus. A hominoid genus the remains of which have chiefly been found in Europe, these primates lived from 12-13 mya to about 9 mya. Four species of them are known. They appear to have been frugivorous, suspensory [meaning simply that they could hang from branches and move about by doing so], arboreal, and capable of living in a wide variety of environments. In certain ways these animals displayed crucial hominoid-like traits. They had many cranial and dental similarities to modern African apes, and may in fact be ancestral to them.⁵²

Pierolapithecus. Another primate from the Middle Miocene, about 12-13 mya. This animal was a mixture of primitive and derived traits, the more modern features being found in the thorax and the vertebrae. It seems to have had an ape-like facial structure. Although there are paleontologists who postulate that Pierolapithecus may be ancestral to both apes and humans, their arguments are not yet widely accepted. Pierolapithecus’s remains were found in Spain, but it is thought to have lived in Africa as well.⁵³ It should be noted here that in the Middle Miocene there were still widespread hominoid populations in Eurasia which are thought to have been the product of the travels of African populations.

Kenyapithecus/Nacholapithecus. Paleontologists and paleoanthropologists differ about the role of these primates in hominoid evolution. The first specimen of Kenyapithecus was discovered in the 1960s, and examples were still being unearthed in the 1990s. The oldest members of this clade go back to around 15 million ybp. Kenyapithecus seems to have had the kind of facial features, dentition, and, possibly, method of locomotion common to modern African apes. The discovery of an animal similar in certain ways to Kenyapithecus and yet displaying crucial derived traits has led to the naming of a new genus of Miocene ape, Nacholapithecus. Interestingly, this genus of primate may have had the ability to move itself on the ground, and it may have been capable of something which is of the greatest significance: the ability to assume an orthograde posture—the ability to hold itself upright.⁵⁴

There are many other Miocene primate genera that have been identified, but gaps in the record (slowly being filled in—see below) have impeded our ability to trace a definitive line of descent leading to true hominins—the animals directly ancestral to humans, extinct varieties of humans, and modern humans themselves. Paradoxically, the more specimens that are unearthed, the more intense the debate seems to be getting. What we can say is that certain primates living in the latter part of the Miocene Epoch of Africa became increasingly terrestrial in their living habits, increasingly able to move themselves along the ground, and increasingly able to hold themselves upright for certain lengths of time.

The Evolution of Bipedalism

It is necessary at this point to examine some of the hypotheses concerning the development of bipedalism—the ability to walk on two feet. There were reptiles that were bipedal in the age of the dinosaurs, and of course the descendants of dinosaurs, the birds, very often move on the ground through bipedal walking. But habitual obligate bipedalism is (or was) found only in the hominins, and it is now one of the distinguishing traits of the genus Homo itself. Some researchers are convinced that it had its origin in a mode of locomotion known as knuckle-walking. [This method, which is employed by contemporary gorillas and chimpanzees, involves using the forelimbs to help stabilize an animal on the ground in conjunction with the movement of the hind limbs.] In this hypothesis, humans were descended from a line of primates that had been both terrestrial and quadrupedal for a very long time.⁵⁵ Other data indicate, however, that knuckle-walking evolved in the African apes that are sister taxa to humans but not in the line of primates leading to humans. Gorillas and chimpanzees do not knuckle-walk in the same way biomechanically, and there are key differences in the anatomies of gorilla and chimpanzee forelimb structures, particularly those of the metacarpals. It would seem, therefore, that knuckle-walking evolved independently in each lineage. Further, some of the features that previous researchers had said were necessary for knuckle-walking are absent in modern African apes. The conclusion of the authors presenting this research:

The results of this study show that researchers need to reevaluate all posited knuckle-walking features and reconsider their efficacy as indicators of knuckle-walking behavior in extant and extinct primates. In this context, the absence of several posited knuckle-walking features in extant knuckle-walkers (and the presence of some of these features in nonknuckle-walkers) makes it difficult to argue that there is unambiguous evidence that bipedalism evolved from a terrestrial knuckle-walking ancestor. Instead, our data support the opposite notion, that features of the hand and wrist found in the human fossil record that have traditionally been treated as indicators of knuckle-walking behavior are in fact evidence of arboreality and not terrestriality.

The study’s authors do not necessarily reject the hypothesis that knuckle-walking may have evolved in the primates ancestral to humans, gorillas, and chimpanzees, and the differences we see in gorilla and chimpanzee knuckle-walking are adaptations that came after the split with the human lineage. Nevertheless, they think it likely that bipedality evolved from an arboreal, not a terrestrial ancestor.⁵⁶  The implications are huge: it means that our ancestors may have stood up regularly before they adapted themselves to ground living. Bipedalism may have had its uses in the trees. It survived on the ground because it was probably even more useful there.

It is worth noting that bipedalism may have evolved independently in several species, (see below) and that its evolution probably did not proceed in a simple, linear manner.  It is further worth noting that the earliest bipedal primates may not have been able to hold themselves fully upright, nor may they have been able to hold themselves upright for prolonged periods of time. We must not assume that our ancestors’ bipedalism was as developed as our own.

As we have seen before, major changes in an animal lineage’s physical adaptations tend to involve a number of factors working in combination and/or influencing each other in a synergistic way. Animals that walk upright have identifiable and distinct anatomical features. Those associated with bipedalism include:

--an increase in the relative length of the legs

--a femur that angles toward the midline of the body and a slightly outward bowing of the knee

--the evolution of a long and curved lumbar (lower) spine

--big toes in line with the midpoint of the body

--a large heel and a stable ankle

--a change in the structure of the ilium and the musculature associated with it, specifically the role of the gluteus maximus as an extensor (straightening muscle) of the thigh and the gluteus medius and gluteus minimus as abductors (extending muscles) of the femur. These abductors act to balance the body during the stride.⁵⁷ Another key structural feature indicative of bipedalism is the position of the foramen magnum, the opening in the skull through which the spinal cord passes in order to connect to the brain. If the foramen magnum is located at the base of the skull rather than the back of it, there’s a pretty good chance the animal is bipedal.⁵⁸ We must assume that the evolution of these features happened in different ways in different ancestral primates, in a mosaic-like fashion, and that these features, once they began to come together in certain lineages, must have been mutually reinforcing ones.

Many researchers have naturally focused in particular on the evolution of the foot. The discovery of hominin foot bones by paleontologists and paleoanthropologists gives us major clues about the nature of bipedalism in different primates. The investigators of fossil foot remains believe that there was a great diversity of hominin foot types. It also appears that the feet of several early hominins show a mixture of ape-like and human features, but not always the same mixture. In fact, these differences seem to buttress the belief that bipedalism evolved independently in different lineages of primates, and that there were different modes of primate bipedalism. Different varieties of hominin may have used bipedal locomotion in different kinds of environments, perhaps adapting to a mixture of arboreal and terrestrial settings.⁵⁹

Of great interest to scientists studying bipedalism’s evolution are the footprints found at the Laetoli Formation in Tanzania. The formation has been dated at 3.6 million ybp. The first footprints at Laetoli were recognized in 1976, and since that time they have been the subjects of painstaking analysis. There are many types of animal footprints in the formation, but of particular interest are footprints that appear to have made by a hominin. There is a difference of opinion concerning the number of individuals who made the prints, and naturally other aspects of the finds have been the subject of debate. A primate  known as Australopithecus afarensis (see below) existed at the time the footprints were made, and many  investigators believe these prints to be theirs. If they were indeed made by A. afarensis, the evidence indicates these animals may have been fully bipedal, with more flexible, ape-like feet than humans. Other researchers investigating Laetoli suggest that the animal that made the footprints may have been both bipedal and arboreal. Many researchers agree that the animals that made the prints tended to walk with a gait that was similar but not identical to that of modern humans.⁶⁰ The Laetoli footprints are the earliest direct evidence we have of bipedalism, but most researchers assume its origins go back farther in time.

If we assume that bipedalism, at least in its rudimentary form, existed in arboreal primates, then obviously terrestrial life did not give rise to it. But terrestrial life was tremendously facilitated by it, and the adaptation of bipedalism to terrestrial uses was of enormous significance. What might have been the selection pressures that encouraged the use and development of the bipedal posture?

--Food acquisition. A bipedal animal is a more effective forager, and can transport food more readily over long distances. Early bipeds may have also been scavengers, and the availability of grasping hands for this task may have been crucial. If the first bipeds on land were omnivorous, these capabilities would have been especially advantageous.

--Predator avoidance. The ability to stand upright and see over tall grass to spot potential threats would have been a strong selector for bipedalism.

--Enhanced reproductive success. The ability to transport food by males may have made it possible for females to stay sequestered at “camp” sites and care for infants, thus improving the chances that those infants would survive. It should be pointed out here that bipedalism would have been part of a whole complex of traits enhancing sexual bonding and reproduction.⁶¹

We do not think that tool-making was one of the original survival skills arising from bipedalism, since bipedal walking preceded tool-making by at least a million years. But it was the freeing of the hands, in conjunction with a brain capable of conceptualization, that was to be the great builder of human technology-based societies. Bipedalism may not have arisen on the ground, but it was in a terrestrial setting that it found its greatest expression. Its tremendous usefulness made survival possible for the species that put it to its best uses and developed it most fully. Without it, human culture would have been inconceivable.

African Great Apes of the Mid-Late Miocene

We briefly examined some of the important lineages of Miocene apes, and we noted that gaps in the record had impeded our understanding of the evolution of hominins. In the late 20^th and early 21^st centuries, there was some progress made in filling these gaps, although a definitive picture has yet to emerge. The most important discoveries have included:

--Chororapithecus abyssinicus, a new species claimed on the basis of nine teeth from at least three individuals, discovered in Ethiopia. This find, dated at 10-10.5 mya, appears to be from animals ancestral to the modern gorilla line, although the exact phylogenetic relationship of abyssinicus to gorillas has yet to be determined.⁶²

--Nakalipithecus nakayamai, a new genus and species discovered in Kenya, dated from 9.9-9.8 mya. Based on the animal’s mandible, incisors, and molars, it is also thought to have been gorilla-like in anatomy. It is similar in certain respects to Ouranopithecus, an animal discovered in Greece and thought to be a possible ancestor of the African great apes and humans.⁶³

--Samburupithecus kiptalami, discovered in Kenya and dated to 9.5 mya. This primate is known from its maxilla and dentition, which indicate, as the other finds do, an animal of gorilla-like size. It may have a phylogenetic relationship to the later African apes.⁶⁴

Obviously, scientists will press the search for possible ancestral hominids further. But these finds are indications that there was a continuous African ape lineage throughout the Miocene Epoch.

The Splitting  of the African Ape Lineage

Scientists have sought to determine, through the techniques of molecular genetic analysis used in conjunction with fossil evidence, whether the gorillas or the chimpanzees are the closest relatives to us. The preponderance of evidence is that the gorillas went in their own direction before the chimpanzee-human split. However, the timing of the human-chimp split (or more precisely, the split between hominins and the ancestors of the modern chimpanzees) is a matter of debate. Most estimates have put the split between 7 and 5 million ybp. A study announced in 2005 made an estimate based on two factors: first, an assessment of when the Old World monkeys and apes diverged, and second, the possibility that hominids existed at 6 million ybp. Based on the various assessments of the Old World monkey-ape split, and the effect the timing of this divergence would have had on subsequent branching, these researchers estimated that the human-chimp divergence occurred between 4.9 and 6.6 mya.⁶⁵ Another group of researchers, comparing sequences of DNA base pairs from four different primate lineages (orangutans, gorillas, chimpanzees, and humans) puts the split between humans and chimpanzees at only about 4 million ybp. They emphasize that this date indicates the complete divergence of the two lineages, implying (if I am interpreting this correctly) that the human-chimp divergence may have begun much earlier than 4 mya:

Our molecular dating estimates are generally in agreement with a large number of studies using different calibration points…[which] found a molecular divergence of HC [human-chimp lineage] at 5–7 Myr, 6 Myr, and 5 Myr, respectively. Speciation, defined as the total cessation of gene flow, is necessarily more recent than these molecular dates, and our value of approximately 4 Myr agrees very well with the time suggested by Patterson et al. for complete cessation of gene flow. It is also in agreement with the oldest fossils generally accepted to belong to the human lineage after the HC split. The autosomal analysis alone cannot be used to determine if the large variance in coalescence times of human and chimp along the genome is due to a large ancestral effective population size or due to prolonged speciation.⁶⁶

Controversy has arisen over the contention that not only was the split between the human and chimpanzee lineages prolonged, it was marked by interbreeding between the two emerging (but not yet fully diverged) species. The researchers who put forth this hypothesis believe that hominins and ancestral chimpanzees diverged from each other in two stages, and that gene flow between them did not end until the final split occurred, an event, in their view, which occurred no later than 6.3 mya, and probably more recently. These scientists contend that the low rate of divergence of Chromosome X [one of the sex-related chromosomes] and the high rate of divergence of autosomes [non-sex related chromosomes] between humans and chimpanzees is suggestive, and could be an indicator of interbreeding, or as they also put it, hybridization.⁶⁷ This hypothesis has been vigorously contested, and the authors of a recent study argue that the difference between the divergence of autosomes and X chromosomes in chimpanzee and human populations can be accounted for simply by the coalescent process [the way in which, going back in time, lineages increasingly narrow as they approach common ancestors] in large ancestral populations of hominins and ancestral chimpanzees. In their view there is no need to postulate a complex hybridization process, and indeed the model of human-chimp speciation that assumes no hybridization occurred best conforms to experimental data.⁶⁸

It should be noted here that whether there was interbreeding between ancestral hominins and ancestral chimpanzees or not, the genetic evidence of the relationship between humans and chimpanzees is very strong. (It needs to be stressed, by the way, that chimpanzees are part of a modern genus that has itself undergone changes over the last several million years.) A systematic comparison of the human and chimpanzee protein sets—their proteomes—shows a very marked similarity in orthologous proteins (those that show descent from a common ancestor). In the genomes of the two genera, the total divergence of nucleotides between them was found to be about 1.2%, but of course that average divergence masks greater or lesser divergences in specific areas, and it does not mean that humans and chimps are “98.8% similar.” There are, of course, obvious differences in general appearance between the two, but there are other, less obvious differences. Chimpanzees, for example, do not appear to be vulnerable to Alzheimer’s disease. Chimps and humans have different immune and inflammatory responses, and they have differences in their resistance to parasites. Still, the genetic similarities are striking, and there can be no doubt that chimpanzees are the sister taxon to humans.⁶⁹

The Geography and Climate of the Miocene Epoch

By the middle part of the Miocene Epoch, about 14 million ybp, the world map had largely assumed its present form, although Florida, sections of Europe and northeast Africa, and parts of southeast Asia were submerged [indicative of a warm climate].⁷⁰ A team of scientists studying the Miocene Epoch has ascertained that there were major fluctuations in atmospheric CO² concentrations over its course, with high levels corresponding to warmer periods and low levels corresponding to greater ice cover, especially in Antarctica. Moreover, these climatic shifts had discernible effects on the animal species of the time. Most intriguingly, the shifts in CO² seem to have affected the respective growth and recession of forest areas and grasslands, with cooler periods reducing forested areas and stimulating the spread of grasses, although the paleobotanical evidence is incomplete on this point. Herbivorous animals seem to have coevolved with the spread of grasslands, especially the ungulates (hoofed animals). The kind of grasses that dominated in the cooler, drier, lower CO² environments are known to botanists as C₄ grasses. C₄ grasses evolved a more efficient way of acquiring and using CO². In areas that were now saturated with sunlight (because of the reduction of forest canopies), they flourished. The data seem to point to a major spread of C₄ grasses in the late Miocene, about 8 mya.⁷¹ If there was indeed such a reduction in the forested areas of the world, including Africa, could this have influenced the evolution of animals capable of using their bipedal abilities in both arboreal and terrestrial settings? Would a premium now be put on the ability of a primate to move with ease through both kinds of landscapes, albeit not necessarily with equal fluidity? The research suggests that the increasing encroachment of grasslands on the forested areas of the world may have had just such an impact.

At the Threshold of Humankind 

Sometime in the late Miocene Epoch the first hominins evolved, and over the course of several million years an animal that can be called human arose out of them. It is impossible for us to draw a sharp distinction between our most advanced non-human ancestor and our earliest human ancestor. As we will see in the next chapter, our definitions of the boundary between them will always be, to some degree, arbitrary.

We should also keep in mind, as we examine the various discoveries of possible hominids and hominins that have been made, that we cannot infer that they all have a phylogenetic relationship with the others. In other words, we cannot assume that just because we can present these finds in chronological order that we have established a genuine lineage. That job is still very much a work in progress.

Sahelanthropus tschadensis

Discovered in Chad, six different specimens have been found. Based on a comparison with the remains of other animals in the area, these specimens are believed to be at least 6 million, and possibly 7 million years old. Those fragments recovered include a cranium, with a relatively small brain case, a narrow basicranium, and a prominent supraorbital torus (a bony ridge that forms a prominent brow above the eyes), among other features. The animal appears to have had a small chin, as well. Several of the features of Sahelanthropus appear to differ from those of modern apes, particularly the smaller canines, and various structures of the face (with different features standing in contrast to those of both particular modern apes and extinct hominoids). Sahelanthropus appears to have marked similarities to other primates thought to be hominid, and its discoverers claim hominid status for it. If the analysis of these remains is correct, it is the earliest hominid yet discovered, a mixture of both primitive and derived features. The discovery is noteworthy for another reason as well: it was made more than 2,500 kilometers from the Rift Valley of East Africa, the region believed by many to have given rise to the hominid line. A hominid that long ago and that far west may cause us to reevaluate our conceptions of the path that led to human evolution.⁷² This animal is also known as Toumaï'.

Orrorin tugenensis

Announced in 2001, prior to the publication of the Sahelanthropus find, Orrorin was declared the earliest hominid yet discovered. Found in Kenya and dated at 6 million ybp, 13 fossils from what were believed to be five individuals were recovered. The specimens include parts of the animals’ jaw, dentition, femora, and humerus. Analysis indicates that the structure of the femur was that of an animal capable of bipedal walking on the ground while the structure of the humerus indicates an animal that retained the ability to climb. Orrorin is thought to have been the size of a modern chimpanzee. Its discoverers claim that its mixture of primitive and derived features implies a split in the ape-hominin line prior to 6 mya.⁷³

The Ardipithecines

The first specimens of the genus Ardipithecus were discovered in 1992 and announced in 1994. The new genus and species, discovered at Aramis in the Middle Awash region of Ethiopia, was designated Ardipithecus ramidus. After its announcement, an extraordinary fifteen year-long project of excavation and analysis followed, necessitated by the extremely poor physical condition of the specimens. (See below.)

From 1997 to 1999 a set of primate remains discovered in the same region were identified as those of an ardipithecine. In 2001 the animals from which these specimens came were designated Ardipithecus kadabba. The fossil fragments of kadabba were dated at approximately 5.5 to 5.8 million years of age. The remains included a great many teeth and samples of mandible, ulna, toes, fingers, humerus, and clavicle. Kadabba was an animal generally more primitive in many respects than the australopithecines (see below).⁷⁴ Whether kadabba was bipedal or not, and whether it was part of the common ancestry of humans and chimpanzees or a post-split hominid, has not yet been definitively ascertained.

However interesting researchers have found kadabba, by far the greatest excitement and interest have surrounded ramidus. After an epic feat of removal, reconstruction, and analysis by means of the most sophisticated technology available, Ardipithecus ramidus was formally described in 2009. The remains are a spectacular find, and an entire issue of the journal Science was devoted to their description. Ramidus was an animal of the Pliocene Epoch [around 5.3 to 2.6 million ybp]. The fossils were found in a layer of volcanic ash dated to 4.4 million ybp. The specimens, more than 110 of them, were from numerous individuals, with extensive post-cranial remains of two. There were enough remains, in fact, to reconstruct much of an entire individual. What emerged from the findings is a possible ancestor of the genus Homo.⁷⁵

The skull of Ar. ramidus resembles that of Sahelanthropus, although somewhat smaller. The difference in size may be due to the fact that the specimen is from a female. The face was less prognathic [characterized by a jaw that jutted out past the facial plane] than that of modern African apes. The teeth of ramidus suggest that it was an omnivore, and its dentition is markedly different from that of modern apes. In particular, the upper canine teeth are less prominent.⁷⁶

The most complete set of remains is a partial skeleton designated ARA-VP-6/500. It was probably a female. Standing on two feet, its height would have been about 120 centimeters, or about 47 inches. Its body mass is estimated to have been about 50 kilograms, or around 110 pounds. Although this skeleton does not include the humerus, comparisons with other Ar. ramidus humeri indicate an animal that was not very different in size from the males of its kind, in contrast to the sexual dimorphism (differences in size) that developed in australopithecines. The individual’s hind limb was primitive, with an opposable big toe. However, other structures in the feet provided stability. Hence the foot was flexible and capable of grasping while being able to help propel the body. It was a foot adapted for bipedal movement, when required. The individual’s leg structure was typical of quadrupedal primates, but its arms were proportionally shorter and its legs proportionally longer than modern apes. ARA-VP-6/500 seems to have been an animal capable of either walking or clambering around in the branches. Tellingly, its hands lacked the adaptations characteristic of knuckle-walking. The authors of the study describing Ar. ramidus make these conclusions about it:

The adoption of bipedality and its temporal association with progressive canine reduction and loss of functional honing now constitute the principal defining characters of Hominidae. The orthograde positional behaviors of hominids and apes were thus acquired in parallel, generated by early bipedal progression in the former and suspension and vertical climbing in the latter. Overall, Ar. ramidus demonstrates that the last common ancestors of humans and African apes were morphologically far more primitive than anticipated, exhibiting numerous characters reminiscent of Middle and Early Miocene hominoids.⁷⁷

All the ardipithecines recovered so far are from Ethiopia, not very far from where significant discoveries of other hominids have been made. Ar. ramidus appears to have lived very close, chronologically, to the split between ancestral chimpanzees and hominins (by some calculations). Are the ardipithecines ancestral to the next major group we will examine, the australopithecines? And if the australopithecines were the line that gave rise to the humans—a proposition about which there is disagreement—could we say that we can now trace our heritage back at least 4.4 million years, and possibly 5.8 million?

The Australopithecines

The name Australopithecus was first suggested by anthropologist Raymond Dart. It was the name he gave to a small hominid skull that had been discovered embedded in dolomite in the Taung quarry in South Africa in 1924. The specimen was  shipped to Dart, who extracted it from the rock, analyzed it, and formally described it in a paper of 1925. Dart called it Australopithecus africanus—the “southern ape of Africa”.⁷⁸ This discovery, widely hailed at the time as the “missing link” (an unfortunate term that paleontologists try to avoid using) helped reestablish the view that Africa was the ancestral home of the hominids (as opposed to those who believed Asia to have been our place of origin). Since the 1920s, a series of major finds has convinced many researchers, but not all, that one of the many lineages of australopithecines is probably ancestral to our genus.

There is disagreement about the proper taxonomic classification of the australopithecines and the terminology associated with it. Many paleontologists and primatologists (such as John Fleagle) place the ardipithecines, australopithecines, and a genus known as Paranthropus (see below) into a broad subfamily of Hominoidea called Australopithecinae.⁷⁹ Other researchers prefer a taxonomic scheme in which the subfamily Homininae is divided between the Tribe Panini, which gave rise to the genus Pan (the chimpanzees), and the Tribe Hominini, which is first divided into a subtribe known as Australopithecina. Australopithecina, in this view is divided into the following genera: Ardipithecus, Australopithecus, Orrorin, Paranthropus. Most interestingly, in this taxonomy, the second subtribe that evolves out of Hominini is simply called Hominina—the genus Homo. The implication is that humans share a common ancestor with the australopithecines but are not evolved directly from one of the australopithecine lineages.⁸⁰ As we will see, this latter view is not universally shared.

Part of the difficulty in reconciling the various ideas that have been offered is semantic. For example, the term Australopithecus afarensis (see below) might be rejected, seemingly removing a key australopithecine from our ancestry. Yet, in a given taxonomic scheme, what is known as afarensis might be placed into a different genus. For example, the name Praeanthropus has been proposed by some, and afarensis renamed Praeanthropus afarensis—a possible ancestor to both humans and a more restricted group of australopithecines.⁸¹  Different views can lead to the use of different terms.

The australopithecines are spread out over three million years in time, and many species have been identified. But there are some general traits and tendencies that stand out. First, australopithecines were bipedal. They may not always have been fully upright, and their gait may not have been fully human-like. But the evidence is clear: when they had to move on the ground, they walked on two feet to do so. Additionally, there is evidence that over the centuries australopithecine bipedalism evolved in the direction of distinctly hominin bipedalism, an evolution indicated by changes in the hip and femur. Many of the earlier australopithecines retained the ability to move about in arboreal settings as well. Second, australopithecines had average brain capacities ranging from about 420cc to about 530cc, and encephalization quotients ranging from 2.5 to around 2.7. Moreover, the shape of the brain in certain australopithecines hints at changes in brain organization. Third, there was a general reduction in the size of the premolars and molars over time.⁸²  There seems to be, in short, a set of traits in certain australopithecines that we can recognize as being similar, albeit not identical, to those of our own genus. 

So let us examine the major australopithecine-like finds that have been made, and weigh the evidence that paleoanthropologists have gathered about their possible role in our evolution.

Australopithecus anamensis (4.2 to 3.9 million ybp)

Discovered in Kenya at Kanapoi and Allia Bay, the specimens include pieces of both maxilla and mandible with dentition, some isolated teeth, some fragments of skull, pieces of tibia, and part of the knee joint. The tibia and knee confirm anamensis’s bipedalism. Could anamensis have been descended from Ardipithecus? Given the time frame in which anamensis lived, and the significant anatomical differences between it and Ardipithecus, it does not appear likely.⁸³ There is, however, evidence that anamensis is related to another australopithecine species, afarensis (see below). The oldest specimen of afarensis yet discovered is 3.6 million years old, and until recently the 300,000 year gap between it and the most recent anamensis remains had not been bridged. The discovery of australopithecine remains in the Woranso-Mille area of the Afar triangle in northern Ethiopia, specimens dated between 3.8 and 3.6 million ybp, appears to have bridged this gap. [Not all agree.] These remains have a mixture of anamensis and afarensis dental and jaw features. Since, in the view of some researchers, there are no significant anatomical differences between anamensis and afarensis, they do not see the two as separate species at all, but just the earlier and later types of the same lineage.⁸⁴

Australopithecus afarensis  (3.6, possibly 3.7 to 3.0 million ybp)

The first specimens of this (then unnamed) species were unearthed by a team led by paleoanthropologist Donald Johanson at Hadar, in the Afar region of northern Ethiopia, in 1973. However, the most famous example of this species was unearthed by Johanson’s team in 1974, working in the same locale. A significant part of the skeleton, about 40%, was discovered at Hadar, an extraordinarily rare find. The specimen was nicknamed Lucy, and showed unmistakable evidence of upright posture. (Johanson’s team recovered more specimens the next year, representing at least 13 individuals.) The Lucy specimen, analyzed by a group led by paleoanthropologist Tim White, contained significant samples of cranium, jaw, teeth, humerus, ulna, radius, fingers, vertebrae, ribs, pelvis, femur, and tibia. Initial analysis indicated that it was in the range of 3.3-3.4 million years of age. [It has since been determined to be about 3.2 million years old.] Lucy was a complete surprise: a little upright-walking primate with a small brain, and an apparently ape-like head. She stood 3½ feet tall, and probably weighed 60 pounds. (Specimens of afarensis from Hadar and Laetoli yielded individuals up to five feet in height and probably much heavier.) Johanson became convinced that afarensis was ancestral to the human lineage.⁸⁵ In 1992 the skull of a large male afarensis was discovered at Hadar by researcher Yoel Rak. The skull was designated AL 444-2, and is approximately 3 million years of age. Discovered in about 50 fragments and reconstituted into 8 major sections, the  skull is 75% to 80% complete, with a brain size of about 500cc.⁸⁶ It seemed that afarensis’ place in the phylogeny leading to humans was secure.

However, the discovery of another largely complete skull, AL 822-1, unearthed in 2002, has cast some doubt on the proposition that afarensis is an ancestor of Homo. The part of the lower jaw in primates that ascends upward toward the temporomandibular joint is known as the ramus. The ramus of each species of “higher” primate is distinct, or species-specific, but broadly the configuration of the ramus falls into one of two groups: on one side, chimpanzees, orangutans, and humans, and on the other, gorillas. The ramus exhibited in AL 822-1 is clearly gorilla-like, and is most similar to that of a species known as Paranthropus robustus [often referred to as Australopithecus robustus]—an evolutionary dead-end. There are many other morphological similarities between afarensis and robustus, so the authors of the study announcing these findings concluded (in 2007):

Additional support for the phylogenetic hypothesis proposed here comes from another early hominin, Ardipithecus ramidus,[see above] whose ramus was recently unearthed at an Ethiopian site dated at 4.51–4.32 million years ago. In our analysis, the specimen's posterior [referring to the posterior section of the ramus]  probability is highest with chimpanzees, at 98%. In other words, the Ar. ramidus ramal morphology is almost identical to that of a chimpanzee and thus constitutes further evidence that this morphology is primitive for the chimpanzee and human clade.⁸⁷

But as is always the case in paleontology, new discoveries can change our picture rapidly. In 2011, evidence of a very important afarensis find was presented: a complete fourth metatarsal, a bone of the foot. Its structure strongly suggests that afarensis’s foot had an arch, and its features in general indicate that afarensis had a foot that was strikingly similar to that of humans. If this was the case, it indicates that the species was largely terrestrial as well.⁸⁸

In further response to the argument that afarensis has many similarities to robustus, several observers have pointed out that it has long been hypothesized that afarensis is ancestral to both Homo and  P. robustus. In regard to the issue of a gorilla-like ramus, we have no idea what the upper part of that structure looked like in the very earliest examples of Homo. Further, afarensis shows a mosaic of human-like and ape-like features in its face and cranium. (Its dentition, however, is distinctly ape-like.) The humerofemoral index, which measures the length of the arms in relation to the length of the legs, has been determined for only one specimen. In the smallest humans, the index is about 74; in the smallest great ape it is 98; in the afarensis specimen, it was 85, almost exactly intermediate (although the femur in afarensis is ape-like in nature). The afarensis humerus is Homo-like. The hand has many human-like features (although it is not identical to ours, of course). In short, we do not yet have definitive evidence about afarensis’s phylogenetic status.⁸⁹

Afarensis has been unearthed at three locations in Ethiopia, one in Kenya (Koobi Fora), and at Laetoli in Tanzania.  Some 400 specimens have been discovered. If we can answer key questions about its locomotor abilities and how they evolved (whether by the piecemeal acquisition of anatomical traits or not), its paleoenvironment, its diet, and the extent of sexual dimorphism in the species over time, we may get closer to the day when we can say whether these east African bipeds were in the line that led to us.⁹⁰

Kenyanthropus platyops (3.5 million ybp)

Discovered in 1998 by paleoanthropologist Meave Leakey’s team and announced in 2001, the name, translated literally, means “flat-faced man of Kenya”. The physical evidence consists entirely of a partial skull. It was unearthed on the shore of Lake Turkana, in northern Kenya. It was given the status of a new genus by Leakey. Its features bear a striking similarity to a skull designated KNM-ER 1470, which is now believed to have been an early variety of human, Homo rudolfensis.⁹¹ Kenyanthropus remains a controversial find. The skull which is the only specimen may have been seriously distorted by first an expansion and then a collapse of one of the sides. In particular, Tim White has been highly critical of the analysis of the find, and he sees it as another example of afarensis.⁹² Is Kenyanthropus a direct ancestor of the human line or is it a misidentified specimen? At this writing it can only be said that the question of Kenyanthropus’s phylogenetic relationship to Homo is unsettled.

Australopithecus bahrelghazali (3.5 million to 3.0 million ybp)

We have only the most limited physical evidence for this australopithecine, a single mandible and some dentition. Many researchers question whether it deserves designation as a separate species, seeing it as a variety of afarensis. The most striking fact about it is its place of discovery: Chad, well outside the east African locale of other australopithecines.⁹³ (See Sahelanthropus tschadensis above.)

Australopithecus  africanus (Perhaps as early as 3.0 million, to 2.3 million ybp)

First named by Dart (see above), all A. africanus remains have been located in South Africa. The most important sites at which examples of africanus have been located are at Sterkfontein, Taung, and Makapansgat. Anthropologist Robert Broom is notable for his extensive work on this species, especially in the 1930s and 1940s. In the literature, africanus is often described as a gracile australopithecine, as opposed to the robust variety. Gracile was originally intended to mean slender or with smaller, less pronounced teeth and facial features. The terms gracile and robust are misleading; there are some very stocky australopithecines that have been labeled “gracile” and some rather small ones labeled “robust”. In fact, the terms are outmoded, and some researchers advocate their outright discontinuance.

Africanus’s chief physical features were as follows:

--  Anatomical structures consistent with bipedalism

--  Cranial capacity: average male, 485cc, average female 428cc

--  Average height (when sex can be determined): Males, 138 cm, females 115 cm

--  Body mass (when sex can be determined): Males, 41 kg, females 30 kg⁹⁴

It should be noted that the estimated brain size of africanus, while small by our standards, is of considerable size in relation to its body mass. The face of africanus was ape-like but its dentition was unlike that of modern apes, with smaller, more human-like canines, and

premolars and molars similar in many ways to ours.⁹⁵ In the post-cranial anatomy, africanus gives indications that its arms and legs may have been of more similar length to each other than those of afarensis, a possible indication of greater arboreality. This question, however, has not been settled. Nor have questions about whether its vertebrae, forelimb bones, or hip were more or less human-like, although the ilium appears more human-like than that of afarensis.⁹⁶

The significance of africanus in the lineage leading to humans is a matter of great debate. The problem, as is the case with all of these species, is the often ambiguous nature of the physical evidence, which can be interpreted plausibly in a number of different ways. The differences between species can be very subtle as well. Africanus had an unusual mixture of traits, but such features as its human-like dentition and the human-like curve of the jaw may be of great significance. Indeed, there is a group of paleoanthropologists who have reached an intriguing conclusion. In their view, the population ancestral to humans underwent a split into two or possibly more lineages about 2.5 million years ago. Many are ready to say that the lineage that led to us was derived from one of the later variants of africanus, and that africanus is the common ancestor to three kinds of primate: P. robustus, P. boisei, (see below) and Homo habilis, the latter being the (probable) first true human. One particular paleoanthropologist has catalogued 64 features shared by habilis with either robustus or boisei, or with both. This is a very strong indicator of common ancestry. All of these features represent changes from africanus. Given the time frame and the narrow geographical area in which africanus lived, this researcher believes that a population of africanus that existed outside of South Africa is very probably the ancestor of our genus.⁹⁷

Australopithecus garhi  (2.5 million ybp)

A garhi is a yet another hominid from the Awash region of Ethiopia. In the 1990s, researchers led by Tim White recovered specimens from several locations that include mandibles, dentition, partial ulnas, partial femurs, a bone from the foot, and most significantly, part of a cranium. There are indications that it might be a descendant of afarensis, but its cranial anatomy is not similar to africanus. In other words, A. garhi is not the africanus-like animal the existence of which was postulated above. Additionally, A. garhi may have used simple stone tools to get at the marrow of animal bones. There are animal remains with cut marks on them that may indicate this. As the researchers announcing this find put it, in assessing garhi’s place:

It is in the right place, at the right time, to be the ancestor of early Homo, however defined. Nothing about its morphology would preclude it from occupying this position. The close spatial and temporal association between A. garhi and behaviors thought to characterize later Homo provide additional circumstantial support.⁹⁸

Australopithecus sediba (1.95 to 1.78 million ybp)

The debate about human origins has revolved around several key issues: Are the australopithecines ancestral to the human line, or are they a lineage that diverged from an ancestor common to both australopithecines and humans? What is the place of Homo habilis in our lineage? Was it truly the first identifiable human genus and species or was it another variety of australopith? (See the next chapter for a more extensive examination.) Did humankind evolve in east Africa, perhaps somewhere along the enormous geological feature known as the Great Rift Valley (a 3000 mile-long complex rift and tectonic plate boundary line that runs from southwestern Asia to southeastern Africa)? Or did humans first emerge deep in southern Africa? In 2010 a discovery of potentially enormous significance was announced, one that bears on all these questions. A new australopithecine species, A. sediba, was first described. Its remains were discovered in a cave at the Malapa site in South Africa. The first fossils recovered were parts of two individuals, one apparently an adolescent male and the other an adult female. From the initial stage of the investigation of these two individuals, sections of the cranium, mandible, dentition, vertebrae, humerus, ulna, radius, hips, pelvis, knee, femur, tibia, shoulder, scapula, rib cage, fingers, and toes were recovered, meaning that we have a pretty good idea of how this primate was put together.⁹⁹

The cranium, from the adolescent, is fragmented, and distorted to a small degree. It has a cranial vault capacity of at least 420cc. The cranial vault itself is ovoid in shape. The extension of the chin is just about in line with the forehead. The dental arch is in the shape of a parabola, much like ours. The teeth are small in size, even in relation to the earliest purported humans. In the post-cranial remains of these specimens, the hip, knee, and ankle are strongly indicative of an animal that was habitually bipedal. Many features of sediba set it apart from other australopithecines, most notably the face and teeth, and it resembles early examples of Homo in certain respects more than it does most other australopiths. The closest resemblance to another australopithecine is to africanus, and even here sediba is distinctive. The authors of the first major paper describing sediba did not consider it a member of Homo because of its limited brain size, a somewhat prominent brow, certain features of the cheekbones, and certain features of its dentition. But it shares more evolutionarily novel, or derived features, with Homo than any other kind of australopithecine.¹⁰⁰

Investigation of the Malapa site has continued, and it is yielding even more spectacular results. Almost all the bones of the right hand of the first female discovered there have been located and reassembled, revealing a hand that had a unique combination of australopithecine and Homo qualities. The hand of sediba is the hand of an animal able to manipulate objects, and yet it is also a hand suitable for arboreal activities. The scientists who rearticulated it believe that sediba had a good precision grip. The anatomy of the thumb (although the thumb is longer than ours) would certainly seem to suggest so. Perhaps most significantly of all, sediba’s hand may be the hand of a tool-maker. Even if sediba did not make tools, certain features of its hands are derived enough for us to say that it could have. Certainly it seems to have been just as capable in this respect as Homo habilis (albeit in a different manner), an animal associated with basic stone tool technology. Sediba may in fact change our ideas of how a tool-making hand looked, offering us a broader range of possibilities.¹⁰¹  

Since 220 specimens have already been recovered, and at least five individuals identified, there will no doubt be many more revelations about sediba to come. Sediba is not a member of Homo. But the leader of the team investigating and analyzing it, Dr. Lee Berger, had this to say about its possible role in hominin phylogeny:

The fossils have an overall body plan that is like that of other Australopiths – they have small brains, relatively small bodies and long and seemingly powerful arms. They do, however, have some features in the skull, hand and pelvis that are found in later definitive members of the genus Homo but not in other Australopiths. However, given the small brains and Australopith-like upper limbs, and features of the foot and ankle, the team has felt that keeping this species in the genus Australopithecus was the conservative thing to do.  Nevertheless, sediba is turning out to be one of the most intriguing hominins yet discovered, and it certainly shows a mosaic of features shared by both earlier and later hominins… Our study indicates that Australopithecus sediba may be a better ancestor [than habilis] of Homo erectus …¹⁰²

Finally, there is the electrifying possibility that some of the skin and hair of these primates has been preserved, meaning they may contain accessible proteins and perhaps even DNA. If this turns out to be the case, it will be the first skin of an ancient hominin ever recovered, and it may give us a deeper understanding of the evolution of our own genus.¹⁰³

The Side Branches: Paranthropus aethiopicus, P. boisei, and P. robustus

Three primates that were once placed inside the genus Australopithecus have been recategorized. They are still seen as part of the broader clade of australopithecine types, but they have been placed in their own genus: Paranthropus. As far as we know, they left no modern descendants. They appear to have been of monophyletic origin. P. aethiopicus is the most poorly documented in the fossil record, while both skulls and fragmentary examples of the post-cranial anatomy have been recovered for boisei and robustus. Some researchers believe that although they were bipedal, they retained a significant capacity for arboreal living. Aethiopicus (as one might gather from the name) and boisei have been found in east Africa, while robustus was an inhabitant of southern Africa.¹⁰⁴ There are researchers who question whether Paranthropus robustus was in fact an obligate biped, based on an examination of the (limited) samples of hip and vertebrae recovered. They surmise that robustus, living between about 2 million and 1 million ybp in a climate very much like the one found in South Africa today, may have engaged in four-legged locomotion at certain times. Intriguingly, there is some evidence that robustus may have used objects as simple tools, and may even have used fire at times.¹⁰⁵ Boisei had a large skull and a prominent jaw, with large molars and premolars. With a braincase that averaged around 500cc, its brain was apparently large relative to many australopithecine types. Boisei’s skull had a very prominent sagittal crest, a ridge of bone at the top of the skull running from front to back. Boisei remains have been found in Kenya, Tanzania, Ethiopia, and Malawi, and it appears to have flourished in those regions between 2.3 million and 1.4 million ybp.¹⁰⁶

Other Developments in Mammalian Evolution in the Cenozoic Era

Many mammalian lineages underwent major diversification and adaptive radiations in the ages following the death of the non-avian dinosaurs. Mammals, which originally had been small in size and rodent-like, ultimately evolved lineages such as that which produced Paraceratherium, an enormous, hornless rhinoceros that lived in south Asia from around 20 to around 30 million ybp, the largest land mammal that has ever lived. It is gone now, but other mammalian lines would ultimately play a major part in the story of the genus of upright primates that was evolving in eastern and southern Africa.

More than 50 million years ago, the first members of an order called Proboscidea evolved. The elephants of Africa and India are the largest carriers of their heritage today.

Some 55 million years ago, a small mammal, dog-like in size, known to scientists as Hyracotherium first evolved. Many millions of years later, its descendants lived in the open plains, and selection pressures on them began to favor size and speed. By 4 million years ago, about the time Ardipithecus ramidus existed, the most famous genus that evolved from this line had come into being—Equus. The equines were then about the size of ponies. Their path and that of the upright primates were to intersect much later—and change the world.¹⁰⁷

About 50 million years ago, along the shores of what is now Pakistan, there lived an animal about 650 pounds in weight, with huge feet, an animal capable of walking or swimming, as required. Known as Ambulocetus, its descendants now make up the whale population of the planet Earth.¹⁰⁸

Some 60 million years ago, an order of mammals evolved that gained its nourishment exclusively from the flesh of other animals. The order Carnivora, with its deadly teeth, had arrived. It spawned many distinct families. Two of them, Canidae and Felidae, were ultimately the producers of the hundreds of millions of dogs and cats that roam the world’s wild lands or inhabit the homes of its consciousness-bearing primates.¹⁰⁹ Those primates’ ancestors feared the cats (with good reason), and may have first banded together to defend themselves from these fearsome predators. The humans tamed the wolf-dogs only over a very long time, and bent them to their purposes. The humans, through breeding, were to transform the felines and canines into a multitude of diverse types.

All of the other mammals that humans ultimately domesticated, the cattle, the sheep, the camels, the pigs, and the rest, all have their roots in the tens of millions of years prior to the evolution of the hominins. It was the emergence of consciousness that was to give the humans their mastery over them. The other mammals would serve as beasts of burden, sources of milk and meat, sources of leather and wool, and sources of fuel and fertilizer for the last hominins left standing—Homo sapiens sapiens.

In sum, what we can finally say is that eventually, from one of the families of terrestrial primates, there emerged a uniquely specialized group. The primates of this group moved about the landscape by walking in it, as did other types. They possessed hands capable of grasping and examining objects, as did their relatives. In all likelihood these primates lived in groups and acted to protect each other, just as the other small packs of primates did. They were unimpressive at first, but they possessed a crucial advantage: they had the most advanced brain that had ever existed in the animal kingdom. When they looked about the world, they did not merely perceive it as just a mass of physical sensations. It was a world of causes and effects, a world of objects, a place, a place existing in time. There had emerged an animal capable of not just being shaped by the world, but of shaping it in turn. Many animals had, of course, shaped the world unintentionally. But this animal was different:

Sometimes it would shape the world because it chose to.