Creation by Evolution/The Progression of Life on Earth

by Arthur Smith Woodward

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THE PROGRESSION OF LIFE ON EARTH

By Sir Arthur Smith Woodward

Past-President of the Linnaean and Geological Societies

If we compare the various groups of animals of the present day we shall find that they can be arranged in a series that gradually leads from the simplest to the most complicated—from the lowest to the highest. The lowest forms are minute specks of jelly-like substance, in which feeding does little beyond helping multiplication. Next higher we find animals of more elaborate structure, in which feeding is improved by the presence of small muscles that make grasping easier. Muscles next form a greater proportion of the body and are used for moving about; and in forms still higher we find nerves to control them. Muscles for locomotion work better by being attached to a skeleton, and in the early forms of life this is altogether an outside shell like that of a cockle, a lobster, or a fly. The nerves next gradually become more elaborate and usually tend to be thickest in the head.

New possibilities arise in still higher forms, in which the muscles are fixed to an internal skeleton, around a backbone, and the front end of the nervous system becomes a brain. Next, the blood no longer remains of the same temperature as the surrounding water or air, but is warmed by an improvement in the heart. The brain grows in size and complexity, fostering activity and leading to the development of higher intelligence. Finally, there comes Man, mastering the world by his greatly developed brain.

Beginning by living to eat, the series soon advances toward eating to live. Then comes the reign of flesh, with just enough nerve to make the muscles effective for moving and grasping. Finally, the brain end of the nerve begins to preponderate, so that the animal no longer responds listlessly to its surroundings but improves first in instinct, then in reason, and eventually attains supreme intellectual control.

The question therefore arises whether this regular advance has any meaning. If all animals and Man came into existence at one and the same time in their present forms science might find the meaning of the world of life beyond its ken. The fossilised remains of animals embedded in rocks afford, however, direct evidence that the different kinds appeared on the earth not suddenly, at one time, but in orderly succession, the lower first, the higher later. Existing animals are seen to be merely the scattered and more or less altered survivors of various groups that have had their day one after another during the march of the ages. There seems to have been a slow evolution of life from the lowest to the highest, one group after another flourishing in turn and then dying down, leaving only a few remnants as their posterity. The earth thus records its own history within itself; it writes in imperishable rocks the story of advancing life, and the writing may be as clearly seen and deciphered as the writing on the Rosetta stone, although it is only half a century since Man has systematically attempted to read the story told by the rocks.

The succession of rocks containing fossils was, however, made out in part long before naturalists in general had framed any theories as to the evolution of one group of animals from another, and they therefore were not subject to bias in dealing with the evidence. Indeed, most of the pioneers in geology were firmly convinced that the progenitors of every living thing had been separately and specially created. Fortunately, the inquiry as to the significance of the fossils found began on the western edge of the European continental region, which has in past ages sunk repeatedly beneath the sea and then risen again to become dry land. In thus rising the fossil-bearing beds, which were deposited in successive seas and estuaries, have been somewhat tilted, and their edges have been exposed to view, so that it is easy to examine them and the fossils they contain. More than a century ago William Smith, an English land-surveyor, showed that the order of the rocks that contained these fossils is perfectly clear. Nearly all the chief phases in the succession of life are represented in the old sea beds that now form rocks in the British Isles and the adjacent parts of the European continent. Approximately the same succession has been observed in other parts of the world, and several of the greatest gaps in the geological history of Western Europe have been filled by the discovery of rocks of intervening ages elsewhere.

The order of the formation of the series of fossil-bearing rocks has thus been definitely determined by observing the order in which the layers rest one upon another and by comparing this order in detail in different parts of the world. There is nothing hypothetical in the result of this research. The main difficulty is the imperfection of the record made by the fossils. Generally no part of an animal but its hard skeleton is found; the softer parts have been preserved only in exceptional circumstances. Most of the rocks, at least those of the earlier periods, were formed in seas or estuaries, and so yield remains of land animals only where these have been carried into the water. A fossil is buried by accident and is discovered by accident. We may say that our knowledge of fossils depends on a chapter of accidents. It is not remarkable, therefore, that only a few chapters of the past history of life have been clearly read and that a fact of general significance may be known by not more than a single observation. Nearly every fresh exploration adds something new and shows how much depends on local conditions. So far as it goes, all the evidence points in the same direction — to the slow and regular advance of the world of life in the way already stated. No conflicting evidence has thus far been discovered.

The beginnings of life will probably never be known, for there is reason to believe that the earliest animals were softbodied, without skeletons. They probably originated in the open sea and acquired hard parts only when they settled down within reach of the surf. By the time that any of them had gained enough skeleton to be regularly fossilized, toward the dawn of the Cambrian period, members of most of their early predecessors had disappeared, so that their earliest history is unknown. Swarms of other soft-bodied animals were living at that time, for more or less vague impressions of them occur in a peculiar bed of greasy shale of the Cambrian period in the Rocky Mountains of Canada.

It is clear, however, that before backboned animals appeared or before animals acquired skeletons, the backboneless groups flourished widely and were at some times and places represented by larger animals than any of their kind of later date. Great armoured cuttlefishes, for example, and gigantic lobster-shaped animals were the rulers of the seas before the earliest backboned animals — the fishes — began to flourish. Soon after the appearance of fishes the lower groups just mentioned lost their leading place, and most of them died out. A new era had begun, in which fishes increased both in numbers and in size. The Old Red Sandstone, both of Europe and of North America, laid down as sand millions of years ago, tells of ages when some of the fishes were stranded in pools that at times dried up. Under these circumstances some of them passed from gill-breathing to lung-breathing animals and acquired paddle-like legs suitable for scrambling about on land.

Thus arose the first backboned animals that spent part of their life on land and part in water—the amphibians, which are now represented by the newts, salamanders, frogs, and toads. Then, through more tribulation of drought and desert in the Permian epoch, there came the equally cold-blooded reptiles—lizards, crocodiles, alligators—animals capable of living all their life on land. They found conditions so easy that they literally swarmed over all lands and even invaded the air as flyers and the sea as swimmers. They increased immensely in bodily bulk until some of their latest representatives in the Cretaceous period (the period of the Chalk) were the biggest masses of flesh that ever lived on land.

A few of the more progressive of these reptiles rather early began to show signs of becoming something better, and by the time the giants of the group were worn out, the progressives had become warm-blooded animals, with an improving brain and very active legs. These were the mammals, quadrupeds which soon began to suckle their young and care for them in their youth. At the beginning of the Tertiary period they took the place of the giant reptiles, which had disappeared. Birds also took possession of the air.

During the Tertiary period the mammals occupied every sphere of life on the land, and as they became more completely adapted to their surroundings their brains grew relatively larger and more useful. As the flesh-eaters advanced in power of jaw and in cunning, and as the vegetable-feeders gradually acquired teeth for grinding hard grasses and nimble feet for running rapidly on plains, their brains kept pace with their needs. Some of the mixed feeders, which lived in the forest and underwent only slight bodily changes to adapt them for swinging about in trees and to feeding on fruits and small animals, became even better equipped with brain. These were the monkeys and the apes. In the apes the brain was especially complicated, and there is reason to believe that in a few that eventually took to life on the ground the brain gradually became very large. Thus arose the distant ancestors of Man, who is shown by fossils to have existed only in a very late geological period. Man himself, indeed, did not appear until the latest geological period—until many of the other mammals were ready for his use for food and domestication.

Fossils do more than prove this general progression of life on earth. They show that there are definite changes—some of them progressive—in each group as it is traced through successive geological periods. They also show that these changes are more or less gradual, not sudden. Fishes may be considered a good example. The oldest fairly well-known fish-like animals, those of the Silurian period, have no hard parts beyond scales and plates in the skin. We can infer from certain markings in the fossils that they had, inside, the beginnings of a backbone and also of a skull, which contained a brain like that of a fish, but neither backbone nor skull was hard enough for fossilisation. Some of these earliest fishes took to life on the bottom of shallow waters, and their skin-armour thickened into bony plates for protection.

In the next period (the Devonian or Old Red Sandstone) the swimmers as well as the bottom-dwellers gradually acquired an elaborate skin armour that was covered with shining enamel, and hence they are described as “‘ganoid,” from the Greek work for resplendent. Their arm fins and leg fins were stiff paddles, used more for crawling on the bottom than for propulsion or balancing in swimming. Some that had powerful jaws, such as Dinchthys (“terrible fish’’), became gigantic, having heads three or four feet across and armour in places three or four inches thick. Like our early steam battleships, they specialised in weight of armour, and like these battleships they were soon superseded by rivals which depended for success on swift movement rather than on stolid defence. In a few of the Devonian ganoids the two pairs of paddles were replaced by ordinary flexible fish fins, which were strengthened by fin rays, and in some the paired fins were halfway between these two patterns.

During the next period (the Carboniferous) ganoids with flexible paired fins predominated, but they were still handicapped as swimmers by the low degree of hardening of the internal bones, by the incompleteness of the tail as a swimming apparatus, and by the unfinished mechanism of the fins along the middle of the body above and below. The tail was formed by the tapering end of the body, turned upward to make an upper lobe; the real fin was below this, as in the sharks and sturgeons of the present day.

During the Permian and Triassic periods, which followed, the tail in the more progressive fishes lost its upper body lobe by shrinkage and became a most efficient tail fin, and the middle fins were gradually brought up to the most efficient form. The internal skeleton was also gradually hardened.

Early in the next period (the Jurassic) the backbone in some fishes was completed. Each joint or vertebra was deeply hollowed at each end to admit soft, elastic substance and so to give the great flexibility that is needed for rapid swimming. The bony skull was also completed. At the same time the scales became thinner and deeply overlapping and contained very little bone substance and enamel.

In the following Cretaceous seas there were a few fishes that had the bony support of the tail as well formed as that of most existing bony fishes; indeed, the reign of the modern thin-scaled bony fishes, completely adapted for rapid movement in water, had begun, and the only subsequent changes were those which have given almost endless variety to this thoroughly efficient race. It is also interesting to note that the fishes which achieved these latest developments include nearly all those that are used as food by man today.

To summarise briefly: The first fishes were encumbered with outside armour and their fins were not very well formed for swimming. Next, the paired fins became thoroughly adapted for balancing, and then the tail fin was improved until it became a perfect propeller. After this the inside skeleton became bony and gradually grew more efficient and more complicated, and the scales of many forms became thin. Thus, by progressive stages, in a definite order, fishes were continually improved for locomotion and for feeding in water, and there arose the possibility of the infinite variety found among the existing bony fishes.

A student of fossils recognises that when any kind of animal shows a tendency to change in some particular part, the degree of this change increases in successive generations, especially if the change at first gives it some advantage. Among the later ganoid fishes of the Jurassic period there are some that tend to assume the form of a swordfish, which has a powerful tail that fits it for darting as well as for swimming. The snout begins to thrust itself forward at the front of the upper jaw. Toward the later part of the Jurassic period the snout even forms a pointed weapon. In the middle part of the Cretaceous period, which followed, the pointed snout in some of these fishes became longer. Toward the end of the Cretaceous period (represented by the Chalk), the snout is much elongated and occasionally forms even a sharp blade, as deadly as that of some existing sword fishes. The increasing power of the snout was thus acquired by gradual growth, which can be followed in the fossils stage by stage.

Among land mammals, or quadrupeds, the deer are very interesting for the same reason. Fossils show that the earliest deer had no horns, or antlers. The next deer had small antlers, but none of them were forked more than once. A little later, in the Tertiary period, some of the deer had antlers with from two to four prongs. In the later part of the Pliocene epoch, in the Tertiary period, some of the deer, when full-grown, had antlers even larger and more complex than any deer existing at the present day. Indeed, it is probable that these deer were handicapped by their over-grown antlers and so died out.

Overgrowth of a part that has begun to show progressive enlargement is often observed among fossils. The gigantic tusks of the elephants that lived in late Pliocene and Pleistocene times are further examples. Also the great canine teeth of the sabre-toothed tigers, which lived with them. In both these animals the enlargement was doubtless a hindrance and eventually helped to put an end to them. Excessive enlargement of this kind must have been usually a hindrance, but there is one great enlargement, already mentioned, which proved to be an advantage — that of the brain in mammals. The great growth of the brain which led to the appearance of man, with his superior mental equipment, was the natural result of the progressive development of the brain in the higher mammals during the Tertiary epoch. Though the fossil apes were very different from modern apes, they must be regarded as the ancestors of both the modern apes and man. Not all the stages between the ape and man have yet been found, because the higher the brain power the more wary the animals would become in avoiding accidents by which their remains could be buried in the earth; but the few fragments that are known show that the links certainly existed. The teeth and jaws of fossil apes suggest that they belonged to animals which may have been ancestral to man as well as to modern apes, and the oldest known fossil human skulls and jaws exhibit more ape characters than any human skull and jaw of the present day.

The oldest jaws of apes thus far discovered are from the early Tertiary (Oligocene) deposits of Egypt and belong to animals smaller even than the existing gibbons, the smallest living apes. They have a short, bony chin and small canine teeth. By a very slight reduction of the canine teeth and equally slight changes in the molar teeth the heads of these apes would approach in form the modern human head. By an enlargement of the canine teeth and a lengthening of the bony chin they would acquire the jaw of an existing ape.

The jaws of the next higher apes, from the middle Tertiary (Upper Miocene and Lower Pliocene) of Europe, represent larger animals, equalling in size a modern chimpanzee. The so-called "forest-ape" (Dryopithecus) now has powerful canine teeth, and so is approaching the modern apes rather than man; but its molar teeth are remarkably human in appearance, and the short, bony chin was less prominent than that of the chimpanzee and gorilla.

Teeth and fragments of jaws of several other apes from rocks of the same age in India show that in this region there must have been more variety among apes than is seen anywhere at the present day. There is, in fact, good reason for supposing that these animals may have included some of the actual ape-like ancestors of man. None of them were larger than a chimpanzee or small gorilla.

The fossil we call Pithecanthropus (“ape-man’’), from a late Tertiary deposit at Trinil, in Java, is distinctly larger—as large as an average man. It is known by the top of a skull, some molar teeth, and a long, straight thigh-bone. The skull is shaped like that of a gibbon, having immense bony brow ridges, but it is nearly large enough to have contained a human brain, and an impression of the brain cavity shows that it had a few human characteristics. Some authorities, indeed, regard Pithecanthropus as an overgrown gibbon; others believe that it belongs to the same family as man. Better specimens are needed to determine exactly its relationships.

The earliest undoubted men are known only by remains of skeletons from Europe, which show some peculiarities of apes. Eoanthropus (the “dawn-man”) is represented by parts of a skull and lower jaw from a river deposit at Piltdown, Sussex, and is especially interesting as approaching an ape in the shape of its lower jaw and front teeth. It has as good a forehead as any modern man, and the size of the brain case is well above that of the lowest existing savages; but the skull lacks the beautiful dome-shape of the ordinary modern human skull, and the neck must have been unusually thick. The shape of the bony chin is unlike that of man and is almost identical with that of a young chimpanzee. Indeed, the whole of the bone of the lower jaw is remarkably ape-like, and it is shown to be human only by two of the molar teeth, which remain in their sockets. The canine teeth are much larger than those of modern man, and the canine of the lower jaw interlocks with its opposing tooth, as in the apes. The only other known human skull that apparently makes some approach to the same form is a fossil skull of an Australian native found in a river deposit at Talgai, in Queensland.

Another fossil human jaw, found in a sand pit at Mauer, near Heidelberg, Germany, is ape-like in the downward and backward slope of the bony chin. In other respects, however, it is typically human, though it is unusually thick and heavy. It is probably almost or quite as old as the Piltdown jaw, just mentioned. At the beginning of the Pleistocene epoch, therefore, there existed in Europe more than one race of men that resembled apes in the peculiarities of their jaws.

Later deposits in Germany, Belgium, and France—even some so far away as Palestine—have yielded remains of men with a large brain case and typically human jaws, but with the bony forehead inflated into great brow ridges like those of a chimpanzee. These early men are known by almost complete skeletons, because they had learned to bury their dead, and several of their burial places in caves and rock-shelters have been discovered. They represent the Neanderthal or Mousterian man, so called because the first skeleton to attract attention was found in a cave in the Neanderthal near Düsseldorf and the stone tools which this kind of man made were first studied in the cave of Le Moustier, in the Dordogne. Neanderthal man walked with a shuffling gait, not quite upright, as proved by his gorilla-like neck and thigh bone. Indeed, he combined in one body more ape characters than are seen in any other low kind of Man.

The cave-floor deposits and others later than those containing Neanderthal man yield no remains of any but typical modern man, Homo sapiens. Some of these remains suggest that the human races of the northern hemisphere were at first less distinctly separated than they are at the present day; but the skeletons found are still too few to warrant definite conclusions. Fossil skulls from Wadjak, in Java, and from northern Rhodesia, in South Africa, seem to show that the lowest modern race that survives in the Australian region was formerly more widely spread in the southern hemisphere. One skull that appears to have belonged to a member of this race, found in a cave at the Broken Hill mine, in northern Rhodesia, is remarkable as having the largest bony brow ridges ever seen in a human skull. It is probably an example of “reversion” to a form common in some ancestral race.

The succession of fragments of apes and men already found among fossils therefore justifies the expectation that further discoveries will reveal a multitude of links between the lower (or animal) and the higher (or human) group. The chain of life is undoubtedly complete to its uppermost limit.

REFERENCES

• Boule, M. Les Hommes Fossiles. Paris, 1923. English Translation by Ritchie. Edinburgh, 1924.
• Keith, A. The Antiquity of Man. London, 1925.
• Knipe, H. R. Evolution in the Past. London, 1912.
• Lucas, F. A. Animals of the Past. Amer. Mus. Nat. Hist., New York, 1922.
• Lull, R. S. Organic Evolution. New York, 1922.
• Lull, R. S. and others. The Evolution of Man. Yale Univ. Press, 1922.
• Osborn, H. F. Men of the Old Stone Age. New York, 1915.
• Pirsson, L. V. and Schuchert, C. A Text-Book of Geology. New York, 1915.
• Scott, W. B. A History of Land Mammals in the Western Hemisphere. New York, 1924.