Creation by Evolution/The Lineage of Man
THE LINEAGE OF MAN
By William King Gregory
Professor of Vertebrate Palaeontology, Columbia University
Early Stages of Life
The story of the evolution of man may at some distant date come to an end, but apparently it never had a definite beginning. In order to get a reasonable historical perspective let us open the story at a period which, for the sake of illustration, may be thought of as a billion years ago. At that time by far the greatest advance toward the human type had already been made, for living matter, according to our present evidence, was already in existence. All the myriads of years before that in which the components of living matter had gradually been built up had passed. The birth-throes of the central sun, which under the gravitational attraction of another passing sun had whirled out great tidal arms (somewhat like the streamers of a pin-wheel) and given rise to the planets, had long since been forgotten. The earth and the other planets had settled down nearly into their present orbits, and the surface temperature of the earth was not materially, if at all, higher than it is to-day. Moreover, the waters of the earth’s surface had long since been gathered together into oceans, the continents (whatever their outlines) were already in balance with the oceans, and the well-nigh eternal round of rock erosion, deposition, consolidation, sinking, and uplifting had gone on for hundreds of millions of years. Occasionally there were terrific disturbances of the earth’s crust, the older sedimentary rocks were soaked and honeycombed with the molten rock coming from below and were squeezed, mashed, folded, contorted, baked, and partly melted, so that if they ever contained any traces of life, such traces would have been hopelessly obliterated, the only possible signs of organic life being the beds of graphite occasionally found in the oldest known rock formations.
Such is the picture suggested to the geologist by the study of the oldest known rocks, now exposed again in eastern Canada, the Lake Superior region, and the Adirondacks as a result of hundreds of millions of years of erosion, but in former ages lying as the “basement complex,” beneath tens of thousands of feet of later rocks.
Passing over various doubtful traces of living organisms in the oldest sedimentary rocks (such as the famous “Eozoön canadense,” which may be a mineral formation), we come to certain markings occurring in the Proterozoic limestone formations, which were determined by the great palaeontologist C. D. Walcott as the calcareous secretions of algae. In the upper levels of the “Belt series” of formations in Montana; exposed on the side of a mountain and lying nearly eight thousand feet below the Cambrian, or lower part of the Palaeozoic, Walcott found the fossilized traces of worm burrows and trails, seemingly of segmented annelid worms.
Hence even far below the bottom of the Palaeozoic system, which to the earlier geologists marked the utmost lower limits of the record of fossil life, we come upon animals of marvelous complexity compared to their one-celled starting point. But where the fossil record fails, the “chain of beings” still in existence apparently preserves some of the main steps in the elaboration of higher living types out of single living cells. For even to-day there are forms of life that afford strong evidence for the following outline.
The single living cells from which higher life started were for a long time independent creatures, capable of assimilation, growth, and subdivision. After a time, when the daughter cells adhered together, a more or less spherical colonial type appeared, finally attaining regional subdivision and division of labor. Then one side of the sphere grew faster and a pushed-in ball appeared. Hereafter the inner layer served for the digestion of the food and became the primitive gut, while the outer layer not only held the bag together but developed sensory and contractile powers, as in the lower jellyfishes. Meanwhile the puckered skin around the mouth grew out into feelers and stinging tentacles. All this looks simple, but the organization of each individual cell was an affair of unimaginable complexity.
Certain jellyfishes began to give up their free-swimming habits and to squirm or crawl on the muddy bottom. Presently the diffuse “nerve net” throughout the body began to be drawn together into definite tracts, the squirming movements finally became more prominent in one direction, and locomotion in a head-and-tail direction was already begun.
The Origin of the Vertebrates
The vertebrate animals originated millions of years later, and there is as yet no general agreement as to what group of invertebrates gave rise to the vertebrates. Professor E. B. Wilson teaches that the vertebrates (or chordates) belong to that great branch of the animal kingdom in which the mesoderm, or middle layer of the three primary cell-layers, arises from outpockets from the primitive gut, as it does also in the echinoderms (the starfish group) and that all the articulated animals, such as arthropods (crustaceans, insects, and arachnids), annelids (worms), mollusks and other groups belong to a series in which the mesoderm buds off from a single cell or pole cell. Professor William Patten, on the contrary, holds that this distinction is not a fundamental one and that the vertebrates have been derived from some early member of the arthropod series, such as the fossil eurypterids.
Both the vertebrates and the arthropods are many-jointed animals provided with an elaborate locomotor apparatus, and both have a highly complex head, which has apparently developed through the growing together of a number of originally independent segments. According to what may be called the orthodox view, all the resemblances between vertebrates and arthropods have been independently acquired in these two great groups, for both had to solve many similar mechanical problems in the perception, pursuit, ingestion, and digestion of their food. According to Professor Patten, on the other hand, the arthropod mechanisms were attained first and afterward were changed to form the vertebrate ground-plan of organization along lines which he has inferred, but which the orthodox reject as requiring too many hypothetical stages between arthropods and the oldest vertebrates known.
The Earliest Chordates
Recently Professor Johan Kiaer has described many beautifully preserved fossil fish-like forms from the Silurian rocks of Norway. These fossils belong to a group of animals, hitherto known chiefly from the Silurian and Devonian rocks of Scotland and Russia, which are commonly called ostracoderms. Some (including Cephalaspis) were flat-bodied like skates; others were shaped more like ordinary fishes. The modern lampreys appear to be degenerate descendants of this group, which is also remotely related to the sharks and higher fishes. Professor Stensiö has collected very perfectly preserved fossil specimens of Cephalaspis from the Devonian rocks of Spitzbergen. Serial sections of these specimens have been studied by Professor Patten, who states that the radiating bony channels for the cranial nerves and many other architectural features of the anatomy of the head conform to the general plan seen in the head of the fossil eurypterids and other arthropods. Patten therefore argues that this new material has proved his theory that the vertebrates have been derived from the arthropod stock.
Whether the vertebrates came from very early arthropods or whether they were derived from unknown cigar-shaped forms that preceded both the ostracoderms and the existing lancelets (Amphioxus), it is at least certain that the earliest known ostracoderms already foreshadowed the higher vertebrates, including man, in the ground plan of their organization. Already they had the main chordate characters that are displayed in the human embryonic and foetal stages but that are masked in the adult human stage, namely, a notochord or elastic axis, above which is the central nervous system and below which is the primitive gut and heart.
Like all primitive chordates the ostracoderms swam head on, by throwing the long body into waves proceeding from in front backward. This undulating motion is produced by the rhythmic contractions of a series of zigzagging muscle plates ranged along each side of the body from behind the head to the base of the tail. Each zigzag is separated from the next by a partition of connective tissue which runs inward toward the notochord.
The ultimate unit of locomotion is not the zigzag muscle segment but the short red muscle fibre. Thousands of these little fibres are placed along the zigzag path of the muscle segment, each fibre being attached at its front and rear ends to the connective tissue partitions between the segments. Each little red muscle fibre is a tiny “gas engine,” consuming the oxygen of the blood stream; it is touched off, so to speak, by the nerve current which is conveyed through a nerve fibril from the larger nerves that pass down from the spinal nerve cord to the muscle.
All the complicated locomotor apparatus of the vertebrates, including man, has evolved according to clearly discernible stages out of this relatively simple ground plan. In support of this statement Nature supplies us with hundreds of different variations of this simple theme. The ultimate causes of evolution may be as mysterious as you like, the origin of the vertebrates from invertebrates may be obscure if you will, but the main stages in the evolution of the locomotor apparatus, from fish to man, are now a matter of record, and the same is true as to the evolution of the human skull, brain, and spinal cord.
The following story of evolution has not been built up like a system of metaphysics or philosophy out of abstruse untested reasoning; it is the plain result of many more or less independent lines of research and discovery pursued by geologists, palaeontologists, zoologists, embryologists, and other scientists for more than a century. Naturally, within the limits of the space available in this book I cannot review the evidences that have led to this general picture of vertebrate evolution.
Early Evolution of the Fishes
Whatever may have been the origin of vertebrates, by the time the Devonian period was reached a very great advance had been made toward the higher forms, for at that very distant time, probably half a billion years ago, there were already in existence shark-like fishes that resembled man in possessing the following important structural characters: They were already true vertebrates, with the same main divisions of the whole body as in man, the same ground plan of the brain and spinal cord, the same type of segmentation of the vertebral column, the same general type of complex skull (to be described later); moreover, these ancient Devonian fishes and their modern representatives agree with early stages of the human foetus in the general plan of the jaws and gill arches and in the basic features of the digestive, circulatory, respiratory and reproductive systems. Hence the humble dogfish or shark, which is a relatively little modified survivor of the early vertebrates, is universally recognized by biologists as affording a true ground plan of human anatomy and physiology.
We do not know why the earliest fishes gave rise on the one hand to a line of forms that progressed to ever higher types and on the other hand to far more conservative ones, including the existing sharks and ganoid fishes, which represent, on the whole, an arrested or retarded evolution; but the fossil record of the vertebrates shows that in every geological age one finds the more conservative, less modified descendants of older stocks living contemporaneously with the far more progressive relatives. The science of comparative anatomy, in collaboration with allied branches of science, is able to decipher the evolutionary history of man from the vast mass of Nature’s documents precisely because the more conservative forms of each geological age enable us to visualize the anatomical characters of older periods and to reconstruct, with progressive approach to complete accuracy, the steps by which the older conditions have given rise to the newer ones.
By lower Devonian time the vertebrate stock had already split up into the following main groups: first, the ostracoderms, an excessively ancient group, of which nearly all the known lines were then near the end of their career; second, the shark group, at that time relatively small, which was to become highly diversified in the next period and then to be crowded almost to the wall by the higher fishes; third, the true ganoids (actinopts), ancestors of the sturgeons, spoon-bills, bony gars, and eventually of the higher or teleost fishes; fourth, the dipnoans or lung-fishes, and fifth, the lobe-finned or crossopterygian ganoids, to be described presently.
Among which, if any, of these groups are we to seek for the ancestors of the higher vertebrates, including man? The known ostracoderms of Devonian time were already specialized side branches, all far too late in time to be the actual ancestors of the higher vertebrates. The Devonian sharks were already giving rise either to the specialized side branches or to the ancestors of the modern sharks; the actinopt ganoids were already on the line of advance to the higher fishes; the dipnoans were already highly specialized in their dental apparatus and skull characters and well along on the line to their modern descendants. Only certain of the lobe-finned fishes seem to have the right combination of characters to be even nearly related to the direct ancestors of the land-living vertebrates.
What, then, are the broad characteristics of these interesting fossil fishes, some of which may lie relatively near to the line of our own ancestry? Some of them were long-bodied, pike-like fishes, with great, strong jaws armed with sharp teeth. Others were stout and heavy-bodied, somewhat like a sea-bass, with shorter jaws. One very specialized group that lasted for many millions of years was short-bodied, propelled by fan-shaped paddles and by a very broad tail. Of all these, only the first lot, including Osteolepis, Megalichthys, Eusthenopteron and their allies, appear to be near the line of ascent to the land-living vertebrates. In these strong-jawed, pike-like fishes each of the stout paired fins (corresponding to our arms and legs) was supported by an internal skeleton consisting of bony rods converging toward a single bone, corresponding respectively to our single upper arm bone (the humerus) or to our thigh bone (the femur). The fore paddles were supported by a complex shoulder girdle, parts of which correspond to our collar and shoulder bones; the hind paddles were supported on a bony plate corresponding to the lower bars of our pelvis.
The skull of these lobe-finned ganoids, like the human skull, was a complex of two very distinct sets of elements. The inner skull, or braincase, consisted of the bony trough surrounding the brain and of the bony shells or capsules surrounding the organs of smell, sight, and balance. The outer skull consisted of a shell of bones, derived, like the scales on the body, from the skin. In the earlier crossopts (such as Osteolepis) the outermost layer of the bony skull and scales consisted of a hard, shiny, porcelain-like substance called ganoin, but in many of the later crossopts this outer layer was lost, leaving a sculptured bony surface.
Among living fishes only the famous Polypterus, the bichir of the Nile, and a nearly related genus have any claim to be considered the modified descendants of these lobe-finned or crossopt fishes of the Devonian period. These interesting relics still retain vestiges of former air-breathing arrangementsin their lungs or swim-bladders, but they now rely chiefly on their gills for aeration.
Origin of the Amphibia
The connecting links between the lobe-finned fishes and the amphibians of the coal forests are not yet discovered, but all the known earlier land-living forms agree in so many points of structure with the lobe-finned fishes that there can be little doubt as to their relatively close relationship. The alternative possibilities, either that fishes were derived from four-footed amphibious animals, much as whales have been derived from mammals, or that air-breathing fishes and four-footed animals represent entirely distinct groups of vertebrates, have been carefully considered by those best qualified to weigh the evidence and have been rejected for excellent reasons.
We must infer, then, that some adventurous pioneers among the early air-breathing fishes managed to crawl out of the muddy waters at times when either the supply of oxygen in the water or the supply of living food there was insufficient; that at such times these creatures wriggled along on their bellies much as some eel-like fishes do now under similar circumstances, except that they used the stout fore and hind paddles to increase their hold on the mud and to assist in pushing the body forward. From that time on, the stout fan-shaped fore-paddles began to be bent at the future elbows and wrists, while the hind paddles were bent in the opposite directions at the future knees and ankles; meanwhile the fan-shaped bony rods of the paddles broke into segments and gave rise to the bones of the fingers and toes.
In many of the earliest land-living vertebrates there were five principal rods or digits on each hand and foot, and possibly a small nodule or reduced ray in front of the thumb or the great toe, and another behind the little finger or the little toe; but these extra digits in most lines of animals were early reduced to vestiges, so that the five-rayed hand and foot became the standard form. Thus man in common with other vertebrates has inherited the basic pattern of his five-fingered hand and five-toed foot from the earliest land-living vertebrates, perhaps of the days of the Devonian coal forests. Man also owes to these Devonian fishes his very ability to breathe.
Man has, moreover, inherited from these amazingly ancient animals each and every one of his twenty-eight skull bones, as well as every other bone of his entire skeleton. But the earlier types possessed many more bones in the skull than man does. The late Professor Williston, of the University of Chicago, showed that as we pass from earlier to later vertebrates reduction in the number of the skull elements is the rule, although there is here and there an exception due to the fragmentation of one of the remaining elements.
Fishes, in common with all other vertebrates, have, on each side of the head, the three semicircular canals which act like spirit levels and assist the animal in maintaining its equilibrium. Below the semicircular canals is a sack supplied with nerves. A part of this sack corresponds to the true organ of hearing in man. The cavity of the human ear (behind the drum membrane) is represented in fishes by the cavity of the gill chamber. Fishes have no drum membrane, which first appears in the amphibians, but its place is occupied in the fishes by the bony shell or operculum covering the outer side of the gill chamber. When the air-breathing fishes were changed into amphibians the opercular series disappeared, leaving only a notch at the outer upper corner of the skull, upon which was stretched the drum membrane of the ear.
Thus, by the time amphibians originated from some air-breathing lobe-finned fishes, all the fundamental problems in the production of the ground plan of the human organization had been solved, and it is literally true that the oldest amphibians were on the whole much nearer to man than they were to the oldest known ostracoderms. Nevertheless, a host of improvements in the entire mechanism were still to be worked out, and to these striking advances we now turn as we pass from the oldest amphibians to the oldest reptiles; thence to the theromorph series of reptiles, which became more and more mammal-like; thence to the oldest mammals, onward to the most primitive placental mammals of the closing age of the dinosaurs; thence to the tree-shrews, lemuroids, monkeys and apes that grow steadily more like man; finally to man himself in the last few million years of the billion-year history of life.
Origin of the Reptiles
In their individual development or embryology the amphibians, like the fishes, went through a water-living stage of development, in which they had functional gills; but the earliest reptiles succeeded in laying their eggs and rearing their young wholly upon land and in this way were able not only to invade the drier uplands and many parts of the earth where water was scarce, but to avoid the intensive competitive warfare for living food that must have raged in the swamps of the coal forests.
The earliest reptiles were still so much like their contemporary relatives, the amphibians, in most of their skeletal characters that some of them were on or relatively near the borderland between the two classes. Such a form is Seymouria, from the Permian of Texas, a fine skeleton of which is mounted in the University of Chicago. The pattern of its skull bones, as seen from above and from the side, conforms closely to the primitive amphibian type and is an almost ideal archetype of every later skull, including that of mammals and of man himself. The same is true of the underside of the skull, including the arrangement of the numerous elements of the upper jaws, palate, and base of the cranium. It may be said of Seymouria, as of many other generalized forms, that, if we had not discovered them, we could have predicted their existence, so closely do they conform to the ideal conditions inferred to be the starting point for later advances.
The Mammal-Like Reptiles
We come next to the theromorphs, mammal-like reptiles of Permian and Triassic time, nearly all of which were fierce carnivorous animals. The most primitive of these were lizard-like forms from the Permian rocks of Texas and Russia; but some of them, called pelycosaurs, were specialized side lines. The most noteworthy advance by the earlier members of this group was the development of an opening, or temporal fossa, in the bony mask covering the back part of the skull on each side behind the eyes. The most primitive amphibians and reptiles had this temporal region covered with a continuous shell of bone, but in the mammal-like reptiles the tissue surrounding the jaw muscles (which lay just beneath this bony shell) seems to have acquired the power first of fastening itself to the surface of the bone and then of sinking into the middle of the bony area. Meanwhile the bone itself, while giving way in the middle of the area, strengthened itself around the margins of the area to which the jaw muscle was attached. In this way bony arches grew up around the margins, and an opening appeared toward the middle. Such openings have been developed in different parts of the temporal region in different groups of reptiles; some reptiles had on each side two temporal openings or fossae, one above the other, and other reptiles had none. The mammal-like reptiles had but one, which was surrounded in the later members of the series by the postorbital, parietal, squamosal, and jugal bones. This temporal fossa may be traced through the ascending series of mammal-like reptiles into the early mammals, thence into the tree-shrews, lemurs, monkeys, apes, to man himself. The bony bar below this opening, variously called the jugal or malar or zygomatic bone, may similarly be traced upward to man.
We likewise owe to these lower predatory reptiles the reduction in number of the wedge-like pieces that finally grew together to make a single vertebra or functional unit of the backbone, there being four pairs in the early amphibians but only two fully developed pairs (neurocentra and pleurocentra) in the higher mammal-like reptiles and mammals. These aggressive and progressive animals also took many other steps toward the mammalian type of skeleton, including the human type, most of them correlated with their improved running powers; for whereas the earlier forms had crawled almost on their bellies with sprawling arms and legs, the cynodonts, or later mammal-like reptiles, carried their bodies well off the ground, almost like the more primitive mammals, as we know from the detailed form of their limb bones.
Among the improvements introduced by the cynodonts was the reduction of the phalangeal formula (that is, the number of bony segments or phalanges in the different fingers and toes of each forefoot) to the mammalian number, as indicated in the following table:
| Digit. . . . . . . . . . . . . . . . . . . . . . . . . . . . | Ⅰ | Ⅱ | Ⅲ | Ⅳ | Ⅴ |
| In earlier reptiles. . . . . . . . . . . . . . . . . . | 2 | 3 | 4 | 5 | 3 |
| In cynodonts. . . . . . . . . . . . . . . . . . . . . . | 2 | 3 | 3 | 3 | 3 |
| In primitive mammals. . . . . . . . . . . . . | 2 | 3 | 3 | 3 | 3 |
| In primiates (including man). . . . . . . . | 2 | 3 | 3 | 3 | 3 |
Similarly in the hind foot the primitive reptilian phalangeal formula was:ⅠⅡⅢⅣⅤ23454; but the formula in the higher mammal-like reptiles and mammals was: ⅠⅡⅢⅣⅤ23333 Certain of the lower mammal-like reptiles show transitional stages, for in them one phalanx of the third digit and two phalanges of the fourth digit were much reduced in length and appear to be almost on the point of disappearing (Broom). Very possibly the reduction in the number of phalanges in cynodonts was associated with the newer method of walking more on the ends of the fingers and toes.
Thus it may be seen that man has inherited not only the number of his fingers and toes but even the number of the small bones on each finger and toe from the mammal-like reptiles of the Triassic period.
Man also owes to the higher mammal-like reptiles a whole series of structural improvements in his skull, teeth, and jaws, which the limits of space here compel us to deal with in a summary fashion, for the teeth of these reptiles are already reduced to two sets, corresponding to the milk teeth and the permanent teeth of man, and are differentiated into incisor, canine, premolar, and molar teeth, as in the mammals, including man. Moreover, the cynodonts clearly foreshadow the mammals in the progressive predominance of the dentary or tooth-bearing bone of the lower jaw, one on each side, which in the mammals is the only surviving one of the numerous separate pieces found in the lower jaw of reptiles.
Origin of the Egg-laying Mammals
The higher mammal-like reptiles take us almost to the threshhold of the mammals, and they almost exactly divide the structural differences between mammals and primitive reptiles. Meanwhile the typical or modernized reptiles, including the turtles, lizards, crocodiles, and dinosaurs, acquired divergently specialized characters that carried them far away from the mammal-like reptiles, which, as we have seen, were progressing toward the mammals. Most of the mammal-like reptiles certainly became extinct, but we must suppose that certain others went on and gave rise to the mammals through such intermediate links as Dromotherium and Microconodon, tiny animals from the Triassic of North Carolina, each of which is as yet known only from one-half of its lower jaw. At any rate, the mammal-like reptiles as a group realize all the predicted characters for the ancestors of the mammals, and those ancestors, when they are more fully known, should be intermediate between some of the mammal-like reptiles on the one hand and the mammals on the other.
The mammals apparently originated during late Permian or early Triassic time, in a semi-arid region that was subject to extreme changes of temperature. In South Africa, one of the homes of the mammal-like reptiles, the polished surfaces of the older Permian rocks clearly indicate the presence of great continental glaciers, which are generally accompanied by periods of extreme cold and aridity, followed by warm interglacial periods.
The chief difference beteen a typical mammal and a typical reptile is that the mammal has far more perfect devices for regulating its own body temperature and thus compensating for changes of temperature in the environment. As mammals have an active diaphragm and similar improvements, they can generate more heat in proportion to their weight than reptiles, and in their hair they have a superior substance for retaining the body’s heat; also, by means of sweat glands, they can lower their own temperature through evaporation.
Typical mammals have also become able to hatch their eggs within the body, eliminating the egg-shell and bringing forth their young alive, but most reptiles conserve the old habit of egg-laying, which is also retained to-day by the most archaic living mammals, the duckbill (Platypus) and the spiny anteater, both of Australia. These two ancient forms connect the mammals with the extinct mammal-like reptiles in several respects. The opossum, kangaroo, and other marsupials, taken as a whole, represent an early stage in the higher method of reproduction, for their young are born in a very immature state and are subsequently developed in the mother’s pouch.
Origin of the Placental Mammals
In all the higher mammals, such as the dog, cat, horse, cow, elephant, monkey, ape, and man, the internal development is carried further than in the marsupials, and a more extensive connection between the growing embryo and the womb is established by means of the after-birth or placenta. Here is another fundamental structure which man obviously owes to his mammalian predecessors.
The same is true of the breasts of the female. From the presence of rudimentary breasts in male mammals (including man) people sometimes infer that the remote ancestral forms must have been hermaphrodites, but all available evidence indicates rather that in the ancestral mammals the sexes were just as distinct as they are to-day.
The presence of breasts in the female of the human species and their ability to secrete milk for the nourishment of the young were among the facts which led the great Swedish naturalist Linnaeus, in 1758, to list mankind within the class first called by him Mammalia, and the presence of only a single pair of breasts, together with other considerations, led Linnaeus to group man as a member of his order of Primates, which included also the apes, monkeys, lemurs, and bats.
To return to the fossil mammals: Our knowledge of the very long period after the mammals first appeared, during the ages when the dinosaurs and other reptiles dominated the scene, is extremely meagre. From a study of recent mammals Huxley predicted that the remote common ancestors of all the highly diversified placental mammals would be found to be small insectivorous forms, not unlike some of the recent insectivores, such as the Madagascar tenrec (Centetes) in general appearance and habits. From a study of the teeth of later mammals Professor Henry Fairfield Osborn likewise predicted that the ancestral placentals, living perhaps in the Lower Cretaceous period, would have teeth of the generalized insectivorous type. Quite recently Dr. Roy C. Andrews and his colleagues have contributed important evidence in favor of this view by discovering in a Lower Cretaceous formation in the Gobi desert in Mongolia the fossil skulls of several kinds of small mammals. Some of these combine features of the later insectivores and primitive carnivores and thus appear to afford a generalized pattern for the divergent evolution of the insectivores, carnivores, herbivores, tree-shrews, and primates, all of which are first definitely known to have lived in Palaeocene time in North America, at the beginning of the Tertiary period, or so-called Age of Mammals.
Palaeontologists are confident that these already diversified mammals were not suddenly created in Palaeocene time, holding that they were derived by evolutionary changes from the more primitive mammals of the Cretaceous and Jurassic periods, from some of which they inherited certain striking characters of their dentition.
Once we have passed out of the obscurity of the very long period during which the reptilian hosts dominate the fossil record and the mammals remain far too inconspicuous, we come to the much fuller record supplied by the fossils of the Age of Mammals, estimated by Barrell to be about sixty million years in duration, with its six great epochs—Palaeocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene.
Origin and Evolution of the Primates, Including Man
Throughout this enormously long period, which was short, however, compared to some of its predecessors, the fossil records are relatively abundant for some great orders of mammals, such as the hoofed mammals, and extremely meagre for the primates. In western North America at the beginning of the Palaeocene epoch, some sixty million years ago, there lived relatives of the existing tree shrews, and in the next higher beds (Lower Eocene) we find the ancient relatives of the lemurs and tarsioids, which are found also in the Eocene of Europe. In the Lower Oligocene beds of Egypt have been found two lower jaws of extraordinary interest, one (Parapithecus) combining the characters of the tarsioids and the anthropoids, the other representing a primitive pro-anthropoid ancestral to the gibbons and perhaps to the branch leading to the higher apes and man. In the Miocene and Pliocene beds of India and Europe we find the broken jaws of possibly a dozen kinds of anthropoid apes, some of which (Dryopithecus) appear from the details of their teeth to be closely related both to the existing anthropoids and to man. In the Upper Pliocene beds we find possible traces of early man in the shape of crude flint implements; in the Pleistocene beds have been found the remains of many individuals of the Neanderthal race in Europe; also the famous skull of Pithecanthropus, in Java. In the closing stages of the Pleistocene epoch the modernized Homo sapiens appears.
Although the fossil record of the evolution of the Primates is meagre it tends to show that the various groups appeared in the following succession: (1) tree shrews, (2) lemuroids and tarsioids, (3) monkeys, (4) pro-anthropoids, (5) diversified anthropoids, (6) primitive man, (7) modernized man. But this is also precisely what one would predict from a comprehensive comparative study of the surviving families of Primates, with special reference to the structure of their brains, skull, teeth, hands and feet, and other parts of the body. The main branches of the order are all represented to-day by surviving members. By making comparisons first within each group and then between groups, including both the fossil and the recent forms, it has been possible to decipher the main record of evolution of the brain, teeth, and various parts of the skeleton.
Taking the series as a whole, it shows a remarkable gradation of forms and structures, culminating in various side branches and also in man. It will remain for future palaeontologists to correct errors and to amplify the details of the process of evolution, but the general sequence of events has been worked out independently by a number of investigators, whose results yield a remarkably concordant, consistent story. Thus Keith has shown that when we pass from the monkeys to the gibbons, which stand near the base of the anthropoid-man radiation, we find that the gibbon has already effected profound readjustments of the viscera and skeleton to its habit of sitting upright and of brachiating, or extending the arms upward and leaping from branch to branch. Keith finds that on the whole the gibbon is nearer to man in this internal readjustment to the upright position than it is to the lower primates. Elliott Smith and his students in England and Tilney in America have worked out the sequence in brain structure from tree-shrew to lemur to monkey, gibbon, orang, chimpanzee, gorilla, man, and they find progressive changes culminating in the enormous expansion of the brain and intelligence in man. The present author has published a series of works on the recent and fossil Primates, dealing especially with classification and phylogeny as founded on studies of skull structure, dentition, and various parts of the skeleton, in which the same general sequence as that derived from a study of the brain has been worked out. With the collaboration of Dr. Milo Hellman, the writer finds that, as formerly suggested by Dubois, the Miocene and Pliocene group of anthropoids called Dryopithecus clearly foreshadow both the modern anthropoids and man, and that certain species of this broadly inclusive “genus” appear to be at least not distantly related to the actual common ancestor of man and chimpanzee. Remane, after the most profound studies, has shown that the canine teeth and anterior premolars of man retain many tell-tale evidences of derivation from a stage in which these teeth were like those of certain female chimpanzees; and the great anthropologist Schwalbe left as his legacy to the world a masterly analysis in which he endorsed the conclusion that man and the chimpanzee are the offshoots of an ancient common stock. Professor Henry Fairfield Osborn, although formerly doubting the derivation of man from an arboreal stock, now finally accepts the remote arboreal origin of man and the derivation of both man and anthropoids from a common anthropoidean stock. He therefore differs from the present writer chiefly in inferring that even as far back as Lower Oligocene time the cleavage between man and apes had already begun. But whether this cleavage took place in Lower Oligocene time or somewhat later, the conclusion remains, based upon a vast accumulation of evidence, that the higher anthropoids, especially the chimpanzee and the gorilla, are man’s nearest surviving relatives, and that the remote “common ancestor” of perhaps ten million years ago was a tailless, partly tree-living, pro-anthropoid, in many respects far more like a young female chimpanzee than like a modern white man.
Conclusion
The natural egotism of man made him easily credulous of the story that the first man, although made from the dust of the ground, was also created perfect in the image of God. The knowledge that man has struggled upward to his present estate from less intelligent animals is still practically denied to the majority of mankind.
The gospel of evolution as outlined above is not the writer’s invention; it has not been built up, like early systems of religion, in an endeavor to propitiate the gods without; it is simply a very condensed outline of what Nature is gradually revealing to those who carefully examine her records. When man fully realizes what he has come from and the long, slow steps by which he has reached his present condition, he will be better able to apply intelligent measures toward correcting his infirmities and toward guiding his evolution along profitable paths in the future. One can do no better than quote the noble words of Charles Darwin:
We must, however, acknowledge, as it seems to me, that man, with all his noble qualities, with sympathy which feels for the most debased; with benevolence which extends not only to other men but to the humblest living creature; with his god-like intellect, which has penetrated into the movements and constitution of the solar system—with all these exalted powers—man still bears in his bodily frame the indelible stamp of his lowly origin.
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