Humans in the Context of Life
Humans appear to be part of existence itself. They are impermanent objects, temporary occupants of various small areas of space-time, meaning they have extent and duration. They are part of the same electromagnetic field that encompasses the entire Universe, and yet they are differentiated from other objects. They are a set of physical processes and patterns that owe their existence to a tremendously long series of events. They are the product of the world itself. They may or may not have souls, but they are first and foremost real, at least in their own frame of reference. So in that frame of reference, how might we best describe the physical nature of a human and the human place in this world?
Humans, by definition, are (for a limited period) life forms, descendants of the first metabolic processes and self-replicating molecules that existed on this insignificant planet, a biological heritage that goes back perhaps 3.8 billion years. This means that our variety of energy-matter is different from the kind that does not have the capacity (in general) to make copies of itself and does not “try” to delay the effects of entropy. We are in the world of the living or once living as opposed to that which has never lived. When our atoms are arranged in a particular way, we are alive. We are organic rather than inorganic. This distinction immediately separates us from most of the Universe. The stark fact is that life makes up a vanishingly small percentage of all energy-matter, even under the most generous assumptions about its distribution throughout space-time. The vast majorities of the subatomic particles, atoms, and molecules in the Universe are parts of inorganic entities, and will never comprise any living thing.
But even granted the rarity of our kind of energy-matter in the Universe, in our own planet’s biosphere we find ourselves immersed in it. Where do we fit within this web of life? In order to answer this, we will put humans in categories, starting with the broadest and then making finer and finer distinctions, as if we were vetting someone for a specific job or putting together a team whose members had to fit very specific criteria. We will, in short, classify them. We have looked at these matters before, but here we will gather all the definitions into one place so that we might speak with as much precision as possible. The first classification system was based almost exclusively on body type, straightforward morphology. Now, classifications follow the principles of cladistics, which we have already looked at briefly. In cladistics, scientists use genetic, morphological, and embryological data in combination to describe the various clades into which life forms fall. The calculation of genetic distance, which we have also already discussed, is the dominant method used to sort out the distinctions among living things.
However, different scientists subdivide life forms in somewhat different ways. There is some degree of disagreement about the most accurate means by which to construct a taxonomy. There are also differences about nomenclature. So, after considering various classification schemes (and trying to compromise among them), this is a taxonomic classification of a present day, modern human that seems reasonably complete to me:
This means that humans are composed of cells that have nuclei. They are thus part of a group differentiated from the bacteria and the archaea. In every cell of their bodies, humans carry echoes of the ancient eukaryotes. In the mitochondria within those cells, they have preserved an element of the prokaryotic past. This domain may have emerged as early as 3.2 billion ybp, although the matter of when eukaryotes first evolved is still an open question.
This means that humans are multicellular heterotrophs that engage in sexual reproduction. They are thus part of a group differentiated from the members of the other eukaryotic kingdoms: the plants, the protists, and the fungi. When the first animals evolved cannot be known with precision, but as we have seen, we have fossil evidence of forms of animal life that goes back at least 650 million years, and we must assume that the original animals evolved many millions of years prior to those preserved in fossils. In this and all of the subsequent categories, molecular evidence usually shows a much greater age than fossil evidence.
SUBKINGDOM (1): Eumetazoa
This simply means that humans are part of a group of animals, a group that constitutes the vast majority of the animal kingdom, which is differentiated from such utterly simple animals as placozoans and sponges. This group probably goes back more than 600 million years.
SUBKINGDOM (2): Bilateria
This means that humans are part of that enormous group of animals that are roughly bilaterally symmetrical. They are thus differentiated from the cnidarians, which are radially symmetrical. This group probably goes back well over 550 million years.
This means that humans are part of a very wide group of animals that share a common trait when in the embryonic stage: the anus develops prior to the mouth. The term Deuterostomia literally means “second mouth”. Perhaps 550 million years (perhaps more) have passed since the emergence of the deuterostomes.
Chordates comprise an enormous number of animals. They have a dorsal nerve cord, a notochord, and in the embryonic stage they possess pharyngeal slits. Humans are therefore part of a group stretching back at least 530 million years, and probably longer.
SUBPHYLUM: Craniata or, Alternatively, Vertebrata
The terms Craniata and Vertebrata are sometimes used interchangeably, although technically the vertebrates are a subgroup of the craniates. Craniates have skulls. All craniates except the hagfishes have vertebrae. Humans are craniates, and as vertebrates, they have bones that provide substantial (but not completely effective) protection for the spinal cord. Thus humans are distinguished from the entire vast world of the invertebrates (the insects, the crustaceans, the arachnids, and so on). In their skeletal systems, humans possess an inheritance that can be traced back more than half a billion years, a structure so reproductively advantageous that it has characterized more animals than anyone can calculate. Now the distinction has grown even finer, and only a relatively small part of the Animal Kingdom is still being considered.
Sarcopterygii is a subclass of the vertebrates. It consists of the lobe-finned fishes and the tetrapods. The fish have strong pelvic and pectoral fins, and were the first kind to begin exploring the land, however tentatively. This group may be over 400 million years old.
The tetrapods, the immense group that contains all four-limbed animals. Tetrapods may have evolved as long ago as 395 million years.
Among the amphibian-like tetrapods known as reptilomorpha, a distinctive kind of egg evolved, one that did not have to be laid in water, and thus one differentiated from the rest of the amphibian-like animals. The vast majority of land animals are amniotes, and this reproductive strategy evolved no later than 320 million years ago, and quite possibly much earlier.
CLASS (1): Synapsida
The group from which mammals ultimately evolved, they may have diverged from the stem amniote about 315 million ybp, possibly earlier. At first resembling reptiles, they were differentiated from them by distinct structures in the interior of the skull and by their dentition. Some paleobiologists insert a suborder named Pelycosauria within Synapsida, a group thought by many to have given rise to the therapsids, although there are uncertainties in this regard.
The group within Synapsida that contains the mammals and all of their extinct relatives. Therapsids evolved more than 260 million ybp.
Often scientists prefer to be even more specific and classify mammals as the descendants of the first cynodonts. Cynodonts appeared perhaps 250 million ybp.
CLASS (2): Mammalia
Evolving out of the therapsid cynodonts, these are the animals that combine the following traits: endothermy (the ability to maintain a constant internal body temperature), hair, milk-producing mammary glands in females that have given birth, a four-chambered heart that facilitates the reoxygenation of the blood, a diaphragm to assist in breathing, a relatively high metabolic rate, well-developed sensory apparatuses, brains generally larger and more complex than those of other vertebrates, a distinct kind of lower jaw (all one bone), the possession of a variety of kinds of teeth, the ossicles (small bones) of the inner ear, five distinct sections of the vertebral column, a distinct kind of pelvic girdle, and a distinct kind of nasal structure, among other features. This group traces its existence back at least 200 million years, perhaps 225 million.
Humans are a kind of mammal called a therian. All this means is they are a kind of mammal that does not lay eggs. Only the most evolutionarily primitive mammals do so (such as the platypus). Well over 99% of all mammals are members of Theria. The split between protherian mammals (egg-layers) and therian mammals may have taken place as early as 220 million years ago.
In the group of non-egg laying mammals, humans are called eutherians. This means that they are placental mammals, although the term is somewhat misleading. They are therefore distinguished from the metatherians, otherwise known as the marsupials. (Marsupial females do develop a temporary placenta when pregnant.) This means humans are part of a group that goes back at least 160 million years.
There are four different groups (clades) of placental mammals. Pregnant females within the primates, rodents, tree shrews, flying lemurs, and rabbits all develop a kind of placenta that is distinct from the other three placental mammalian groups. This group emerged from the basal euarchontan perhaps as long ago as 130 million years.1
As we saw, these animals possess clavicles, specialized bone structures around the eye, 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, a petrosal bulla, nails or claws (tending toward the possession of nails in more “advanced” primates), a distinct big toe 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, a tendency toward opposability in at least one digit on the hands and/or feet, 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 greatly enhanced visual sense with a tendency toward stereoscopic and chromatic vision, a relatively poor sense of smell, long gestation periods compared to maternal size, relatively slow growth of offspring in relation to maternal size, relatively late sexual maturation, and relatively long life-spans. Humans are therefore part of a group stretching back at least 65 million years, and perhaps many tens of millions of years earlier. (By the way, when used as a scientific designation, the name of the order is pronounced pry-may-teez.)
These are the primates that lack the rhinarium of the more “primitive” strepsirhines. Their eyes face more forward and possess very great acuity. They detect the world through vision more than smell. They have more rounded heads than the prosimians. Their hands (and usually) their feet are highly flexible. Their hands possess precision grip and a high degree of opposability in the thumb. Haplorhine females in their reproductive years have menstrual cycles and have only two mammary glands. Haplorhines have genitalia distinct from those of strepsirhines. Most haplorhines have a much greater degree of encephalization than strepsirhines. There are anthropologists and primatologists who prefer to use the term anthropoid to describe most of these primates, making a distinction between “true” anthropoids (the Old and New World monkeys) and tarsiers, who occupy a middle ground that might be characterized as “prosimian haplorhine”. I simply divide the primates between strepsirhines and haplorhines. The haplorhines may go back 55 million years.
Humans belong in the huge group known as catarrhines, otherwise known as the Old World monkeys. The monkeys of the eastern hemisphere have nasal and cranial structures generally distinct from those of the New World monkeys. They also possess a distinct 184.108.40.206. dental formula. This group may be more than 30 million years old.
The Old World monkeys are split into two large groups. One, the modern-day monkeys of the eastern hemisphere, are known as Superfamily Cercopithecoidea. The other, the apes and humans, are the hominoids. This group may have split off as long ago as 29 million years, although the evidence is not conclusive. The ape-like primates lack tails, are generally terrestrial, are usually larger than monkeys, and have a higher level of encephalization.
This group represents all the Great Apes of Asia and Africa, and excludes the gibbons.
This group consists of only the African Great Apes, therefore excluding the large Asian apes known as orangutans. Homininae’s first members may have evolved 23 million years ago.
TRIBE (?): Hominini
I have placed a question mark after this to emphasize that this usage is not yet universal. However, this designation is useful because it emphasizes the close relationship humans have with chimpanzees. In a system that uses this scheme, Panini (containing the genus Pan, the chimpanzees) and Hominini are the two tribes emerging out of Homininae. As we saw, the ancestors of the modern chimpanzees and the ancestors of the humans genetically diverged about 5-7 million ybp.
SUBTRIBE (?): Hominina
If Hominini is recognized as one tribe stemming out of Homininae, it has two subtribes. One is thought to consist of the ardipithecines, australopithecines, Orrorin tugenensis, perhaps Kenyanthropus, and the members of Paranthropus. The other consists of the genus Homo.2 As we noted before, the term australopithecine can be interpreted in a number of ways, and often animals designated as australopithecines by some researchers are not labeled in the same way by others. Bipedalism, either facultative or obligate, and a high degree of encephalization are the hallmarks of this group.
The true humans. As we saw, there is a vigorous debate among paleoanthropologists over such issues as the status of habilis and rudolfensis. At the outermost, this genus is 2.5 million years old. It is perhaps more defensible to say that we are reasonably certain it extends back to about 1.8 million ybp. The earliest of them are the animals that explored much of the eastern hemisphere, and devised a useful set of tools. They underwent extensive speciation. Later varieties had large brains, mastered the use of fire and learned how to kill other animals for food. They presumably learned how to clothe themselves as well, and eventually began to build artificial shelters.
The true modern humans, the earliest examples of which have been found in Ethiopia, dated to 200,000 ybp. The final distinctions are now made. With the arrogance that sometimes exhibits itself in our kind, our species, the “modern” kind of human is known as Homo sapiens—“Wise Man”.
To compound our hubris, the contemporary human is known as Homo sapiens sapiens—“The Wisest of the Wise Men”. Modern sapiens are distinguished from earlier versions by a somewhat flatter brow ridge and other minor cranial features. This is the “modern” form, African in heritage but with a mixture of other human species’ genes in many. The subspecies emerged perhaps 100,000 to 130,000 ybp. Its members have built the advanced societies and cultures of the human world. They have created, through the accumulation and cultural transmission of knowledge, a vastly powerful technology, the effects of which they cannot always control. They have invented complex laws, religions, philosophies, and ideologies to organize themselves. They have begun to use methods of acquiring knowledge, both empirical and deductive, that are revealing the inner workings of the Universe, their planet, life itself, and their own existence.
In summary form, therefore, a modern human is a member of:
SUBKINGDOM (1): Eumetazoa
CLASS (1): Synapsida
CLASS (2): Mammalia
TRIBE (?): Hominini
SUBTRIBE (?): Hominina
In a sense, a human carries within himself or herself a very narrow and specialized version of the history of life, somewhat like a single volume in a huge library. Every member of the animal kingdom alive today carries a similarly impressive heritage. Yet, its commonness should not make it any less remarkable to us. Every single human in the world is, as some observers have put it, the product of a long string of “winners”—the winners of the battle for survival, the winners of the struggle to perpetuate sets of genes. When one considers the many mass extinction events in the history of the biosphere, episodes that wiped out countless life forms (and opened doors of opportunity for others), or the lowly status of the mammals 200 million years ago, or the times in the last several hundred thousand years when our genus was itself on the verge of extinction, we may count ourselves fortunate—if we consider the evolution of the humans to be a fortunate event—that our form of life came to be.
The categories of living beings into which humans fall link us to progressively smaller, more specialized, and more recent groups as we go from the broadest category, life itself, to the narrowest, Homo sapiens sapiens. But if we mentally “reverse” the process, and go back farther and farther in time, we see ourselves connected to the whole of life on the planet Earth. We are an intrinsic part of this life, the product of its processes. We are not apart from the natural world; we are a part of the natural world. And as far as we know, we are the only life form on the planet that knows there is such a thing as the natural world. At every turn, we are defined, to some extent, by the categories into which we fall, and our life experiences are as they are because of this. We hunger, thirst, hurt, desire sex, and feel the world because we are animals. Our bodily warmth, our hair, and the infants who suckle at the breasts of mothers remind us of our mammalian forebears. Our color vision and our hands echo our primate ancestors in the forests of the world 60 million years ago. We stand and walk because several million years ago ape-like creatures found it useful to do so. And we speak, think, dream, imagine, and search for our past because 200,000 years ago an animal capable of all those things, the inheritor of almost four billion years of ceaseless change, took its place in the world—and against all odds, survived.
Humans are the most numerous large land animal on the planet. Very few living things are larger than humans, although Arthur C. Clarke put the size and numbers of the human species in startling perspective when he pointed out that the whole of humanity would fit inside of a cubic mile. They are the only animal that combines relatively high intelligence with prehensile hands, and thus their technology is overwhelmingly superior to that of the other advanced animals. In the diversity of habitats they occupy, the sweep of their geographic distribution, and the extent to which they exploit the natural environment and other life forms, they have no equals in the animal kingdom. They have more control over the world than any other kind of animal, and yet they have less control of it than they generally believe.
The evolution of the genus Homo’s brain and the advanced consciousness that is its byproduct have given humans an invaluable advantage over all life forms. Humans have proven that brains really are superior to brawn, and they have even driven many of the biggest, strongest, toughest, deadliest animals to the point of extinction. Yet, humans are vulnerable to the predations of the viruses and bacteria, and being animals, humans are highly vulnerable to illness, injury, and physical suffering of all kinds. The complexity and fragility of their brains makes them vulnerable to a great many affective illnesses as well. Their numbers could be severely reduced by mass epidemic, environmental disaster, the striking of the Earth by large objects crashing in from outer space, or the kinds of warfare human technology has made possible. They are the only life form that possesses the conscious ability to exterminate both itself and most of the other life forms. They are the only animals capable of deliberate, premeditated cruelty. They are also the only animals capable of sustained acts of compassion and generosity. They are probably the only animals aware of the inevitability of their deaths.
They have altered the general nature of the biosphere, not always to good effect. They have disrupted the breeding and migratory patterns of many animal species and ruined many ecological niches. Their actions may in fact be creating another mass extinction event. Through selective breeding they have modified a great many other species of animal and plant, and have in fact made many of these species dependent on them. Humans have organized the systematic breeding, raising, slaughtering, and butchering of other animals for food. They have also attracted animals, such as rats, which make their living from stealing the food humans have raised.
To a certain extent, humans have begun to influence their own evolution, and are now engaged in what might be called artificial selection. In their biotechnology and artificial intelligence laboratories they may be creating their own successors. They are probably the only animals that regularly experience feelings about the world and reality that they call spiritual. They are the only animals who believe they can sense the divine. They are the only animals capable of believing in a transcendent world beyond this life. They are capable of the most harrowing fears and the most glorious hopes. And they are the only animal that may ever leave this world to move to another, never to return.
They do not have bodies; they are bodies, despite their delusions that mind and body are separate. It is to the composition, structure, function, capacities, and limitations of the modern human body that we now turn, aware that its features are the sum total of its evolutionary history and the categories of life into which it falls. In examining ourselves as physical entities, we will hope to gain insight into the kind of animal that holds such an unusual and unique place in the context of life on this planet.