Creation by Evolution/The Evolution of the Horse and the Elephant

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4609264Creation by Evolution — The Evolution of the Horse and the Elephant1928Frederic Brewster Loomis

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THE EVOLUTION OF THE HORSE AND THE ELEPHANT

By Frederic Brewster Loomis

Professor of Geology, Amherst College

The horse and the elephant are so well known and their characteristic features are so striking that a study of the changes which have taken place in their ancestors to bring them to their present forms should be of general interest. The horse and his associates, unlike other animals, has on each foot only a single toe—the hoof—and the elephant is unique in possessing that wonderful organ, the trunk, which is adapted to so many uses. In our study of the evolution of these animals we shall have to turn to the geologist for the evidence, which consists of bones entombed in beds of sand and clay, most of them now hardened to rock, laid down in different parts of the world during what is called the Tertiary period (see the accompanying geologic time table) and part of the succeeding Quaternary period, in which we are now living. The order of succession of the animals whose forms are thus revealed must be determined by the order of the deposition of the beds in which the bones are found. In a series of such beds the one at the bottom was laid down first and the overlying beds were laid down in the order in which they appear, one above another. The bones found in these beds belonged to animals that lived and died about the time the beds were formed. Each bone found occupied a certain known position in the skeleton of the animal, had certain distinctive features and performed certain definite, well-known functions.

The Horse

The work of tracing the development of the horse is relatively easy, especially the forms of the horse that lived in America, for we have a very extensive series of fossil remains of American horses, taken from beds that are piled in succession one upon another (Fig. 1). More than 200 different kinds of American horses have been discriminated, and in addition to these about 30 kinds have been found in Europe and nearly as many more in South America, Asia, and Africa. Some beds have yielded thousands of teeth and jaws, some have yielded other parts of the bony frame, and most of the types of horses are represented by complete skeletons.

Geological Table Showing Evolution of the Horse

Period		Epoch	Length of epoch in millions of years	Millions of years ago		Kinds of horses
Quaternary	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	Recent		1		Equus
		Pleistocene
Tertiary	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	Pliocene	8	9	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	Plesihippus, Hipparion
						Pliohippus, Hippidium
						Protohippus
		Miocene	13	22	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	Merychippus
						Parahippus
						Hypohippus
						Anchitherium
		Oligocene	13	35	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	Miohippus
						Mesohippus
		Eocene	20	55	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	Epihippus
						Orohippus
						Eohippus

The genus that includes the modern horse (Equus) is represented today by the domestic horse (of which there are

Fig. 1.—Evolution of the horse. Stages passed through from the earliest four-toed Eohippus (upper left) to the three-toed Mesohippus (upper middle) to a form with the side toes reduced (Merychippus, upper right) to the small, clumsy, one-toed Equus scotti (lower left) to the modern horse (lower right).

Reproduced from photographs by W. E. Corbin of fossils found in successive layers of rocks and now preserved in the Museum at Amherst College.

many breeds), the wild horses of Mongolia, the half asses (the kiang and the onager), the asses, and the zebras. The genus includes eight to twenty species of living animals, the number discriminated depending upon the judgment or the fancy of the naturalist making the classification. Most of the later fossil horses are so nearly like the living horses that they have all been placed in the genus Equus. Any one who sees restorations of these animals at once calls them horses, zebras, asses, though some are much smaller than the living animals of these kinds.

North America was an early home of the horse, whose remains have been found in deposits in Wyoming that were laid down in Eocene time. (See the geological table.) At that time the climate of North America was warmer than it is now and Alaska was linked to Asia by land over which horses migrated. The Eocene lignite beds and gypsum deposits of France contain abundant bones of horses, and bones are found also in England, which was then connected with the Continent.

The earliest horses whose remains are found in America are the Eocene forms known as Eohippus (the “dawn horse”), some of which stood only about a foot high at the shoulder. The fore foot had four toes, the hind foot three toes, but each showed a vestige of an additional toe. The teeth were simple and short. Three kinds of Eocene horses have been distinguished, called Eohippus, Orohippus, and Epihippus. These horsed appear to have lived in Western North America and in England at nearly the same time. Some of them appear to have inhabited either park-like openings in forests or the forests themselves.

By the end of Eocene time or a little later all the European horses seem to have disappeared, for the deposits laid down in Europe about that time contain no bones of horses. In America, however, they flourished, and they continued to multiply and to grow larger during the following epoch, the Oligocene.

The Oligocene horses were larger than their predecessors, and each of their feet bore three toes. This three-toed horse is called Mesohippus. It appears to have been confined to the American continent, a fact suggesting the temporary severance of land connection between North America and Asia. The later Oligocene horses (Miohippus) were larger and had longer teeth and smaller side toes than the earlier forms.

During the Miocene epoch, which followed the Oligocene, Western North America was inhabited by many kinds of horses, among them one that has been called the “forest horse.” The Miocene horses were also three toed and had low-crowned teeth. The forest horses spread from America to Asia and Europe, where their remains are found; but they appear to have died out in America in mid-Miocene time and later in Europe. The main line of the horses, however, continued to exist and underwent great changes, all originating in America. These changes appear to have been determined by environment. The teeth became longer and harder to adapt them better to grazing; the feet, which in the earlier horses were first five-toed, then four-toed, and then three-toed, advanced toward a single-toed form, the side toes becoming useless (Fig. 2). These changes indicate growing adaptation to life on grassy plains. The grass of these plains is harsher than that in or near forests, containing more silica, and horses that feed on it must have hard teeth. A hard, small hoof is also peculiar to plains horses, as well as long legs, for the horse must be able to escape from enemies, such as wolves and other carnivorous animals.

In their adaptation to life on plains in Miocene time the horses differentiated into three groups, the types of which are known as Pliohippus, Protohippus, and Hipparion, each probably representing adaptation to life on a certain type of plain—the grassy plain, the brushy plain, and the desert plain.

Fig. 2.—Principal stages in the evolution of the teeth and the fore foot of the horse. Showing the increase in the complexity of the grinding teeth and the gradual loss of toes on the front foot.

1. Four-toed horse (Eohippus). Eocene epoch.

2. Early three-toed horse (Mesohippus). Oligocene epoch.

3. Later three-toed horse (Merychippus). Miocene epoch.

4. One-toed horse (Equus) . Pleistocene and Recent epochs.

At the beginning of the next geological epoch, the Pliocene, the three types just named were living in America. During this epoch America was again united to Asia by a stretch of land that extended across Bering Sea. Over this land horses migrated from America to Asia and from Asia to Europe, where they became abundant and were differentiated into several species. Horses also found their way from North America to South America across the Isthmus of Panama, then recently emerged.

The Pliocene horses were all plains horses of the three groups named. They were all three toed, but the side toes in most species were so small that they did not touch the ground. Pliohippus, which lived only in North America, had the smallest side toes; Hipparion, which lived in North America and in Europe, had the largest side toes and had teeth of a distinctive pattern.

In the Pleistocene epoch, which followed the Pliocene and which included the Ice Age, horses were abundant in both America and Europe. These horses, which were presumably the descendants of Pliohippus, are so nearly like the horses now living that they have all been placed in the genus Equus. Pleistocene deposits found in all parts of the United States, from California through Texas to Florida and northward to Nebraska and Pennsylvania, have yielded remains of horses, some smaller than even the smallest living pony, some as large as any living horse, and one species (Equus giganteus) the largest horse known.

The Pleistocene epoch is the period of maximum development of the horses in number, size, and variety. During this epoch horses made their way from America by way of Alaska and an isthmus across Bering Sea to Asia and Europe. Remains of Pleistocene horses are found in Alaska. The ice sheet of the glacial age, although it covered northeastern America, did not extend west of the Rocky Mountains, so that Alaska was then temperate enough to permit horses to live there. During this epoch horses made their way also to South America over the Isthmus of Panama and spread as far south as Argentina.

The most notable evolutionary changes in the horses consist of an increase in size, changes in the size and structure of the teeth, and, most conspicuous of all, changes in the form of the foot. The stock from which the horse was derived was probably five-toed, the foot conforming in its general pattern to the common mammalian foot, but the stress for speed appears to have centered on the third toe, leading to the elimination of four of the toes, resulting in a one-toed, swift-running animal, as the geological record shows. Relics of two of the last toes to disappear are seen in the splint bones of the modern horse.

The geological record reveals to us, of course, only the bony parts of the numerous horses whose remains have been preserved in the rocks and discovered. The differences in these parts have enabled us to discriminate many species, but if we knew the differences in mane and in tail and in colour we might increase greatly the number of species. We do know enough to assure us that a continuous series of horse-like forms inhabited the earth for ages and that Western North America was the principal scene of their remarkable development. All the American horses, however, finally became extinct from some cause or causes not yet discovered, perhaps a parasitic or a contagious disease. Thus the horse, which lived and developed for more than forty million years in America, died out on its native soil; but it survived in the Old World, though in smaller numbers and in forms less varied than it had in America in the Ice Age.

When America was discovered and explored by Europeans there were no horses in the country. Although some horses escaped from the Spanish. conquerors and became wild, both in North America and South America, our domestic horse is a descendant of European breeds, which are numerous and extremely diverse in size as well as in other features.

In early historical time the domesticated horse was used to draw chariots and as a riding animal. From the earliest stages of its domestication it was highly prized. The horses of one country were traded for those of another, or were captured, and great care was taken in their breeding. Some of the various breeds differ more widely than some wild species, and the changes made under domestication have been far more rapid than those which occur among horses in a natural state. Among wild horses the changes that occur in a thousand years, or even in a million years, may be slight, but by successive changes from age to age we proceed from the tiny four-toed horse of Eocene time to the large single-toed horse of to-day. Every year has revealed more and more intermediate forms, until the fact of the gradual evolution of the horse is now recognized by all biologists.

The Elephant

The elephant is not only the largest living land animal but the most peculiarly built. Its wonderful prehensile trunk and its long, heavy tusks are its most striking features. (See Fig. 3.) It has relatively short and straight legs, a short body, and a very short neck, so that its head is carried high above the ground; its lower jaw is short, it has no front teeth, and only one grinding or cheek tooth in each jaw. Each grinding tooth is composed of 17 to 25 plates and weighs 15 to 20 pounds. Elephants’ teeth are so large and hard that they form fossils which are easily recognized. Fossil teeth and bones of elephants and of their relatives, the mastodons, have been found in North America, South America, and Europe, countries in which elephants no longer live, as well as in Asia and Africa. The elephants appear to have always lived in dense forests and to have fed on the vegetation they afforded.

Only two species of elephants are living to-day, the African and the Indian elephant, but in the Ice Age, which occurred in the Pleistocene epoch, there were more than twenty species. In the Pliocene and Miocene epochs there were few true elephants but many mastodons, and they inhabited all

Copyright by Martin Johnson, author of “Safari.”
Courtesy of G. P. Putnam’s Sons.

Fig. 3.—A midnight visitor. An African elephant surprised by the flashlight at the waterhole just back of the Johnson Camp at Lake Paradise, June 25, 1927.

By a process of evolution “his nose has turned into a hand, his ears into fans, and his teeth into ivory spears.” Parts of an animal develop in response to needs for them and vanish with the passing of such need.

the continents except South America and Australia. In the Oligocene and Eocene epochs there were no true elephants, and the mastodons appear to have lived only in Africa.

The earliest known member of the elephant family is a tiny form, just over two feet high, whose remains have been found near the Fayum Oasis, in Egypt. It is of upper Eocene age, and is a mastodon known as Moeritherium. This primitive mastodon may have given rise to the true elephants, although its neck is short and heavy, its feet are short and compact, its head is of normal length, and it has nearly a full set of teeth. The second incisor in the upper jaw, however, is large, and the third incisor and the canine tooth are small and appear to be on their way to being lost. It also has incisors in the lower jaw, the second one large, but no third incisor or canine tooth. In each jaw there are six low-crowned grinding teeth. This form seems to have lived near rivers and ponds and to have subsisted on soft vegetation, the large upper and lower incisors suggesting that they were used to dig up bulbs and roots. Remains of mastodons in this stage of development are found only in Egyptian beds.

In the Oligocene beds, which lie just above the Eocene, the remains of another elephant-like form are found. This form which is called Palaeomastodon, has the second incisor of both the upper and lower jaws considerably enlarged. Of the other teeth, the first and third incisors and the canine of the upper jaw have disappeared, as has also the first incisor of the lower jaw; all that is left of the front teeth in either jaw is the second incisor. These incisors of the lower jaws are flattened and the jaw is elongated, so that in spite of the fact that the neck has shortened the mouth still reaches the ground. The two incisors of the upper jaws have greatly enlarged and spread to either side. Between these upper tusks lay the upper lip, prolonged enough to reach to the end of the lower jaw and to manipulate the food dug up with the lower incisors. Palaeomastodon was about four and a half feet high and seems to have improved or specialized in the habit of digging roots and fleshy vegetables. In these same Oligocene beds of Egypt we find still another and more progressive form, Phiomia, which may well be called the “long-jawed mastodon,” for the lower jaw is still longer and the neck is still shorter.

At the beginning of Miocene time there was in Europe a group of long-jawed mastodons closely related to Phiomia. They migrated from Africa to Europe and increased in size until they were about eight feet high. The lower jaw was as much as six feet long. After reaching Europe these long-jawed Mastodons spread over the continent and migrated to Asia and finally to North America across land that then connected the continents. In Pliocene time this form culminated in Trilophodon giganteus, which was almost as large as the later mammoths. In most animals the neck elongates as they increase in size, so that the mouth can be brought to the ground for feeding or drinking, but in Trilophodon the neck steadily shortened, and the necessity of reaching the ground has been met by elongating the jaws. The two large shovel-like teeth of the lower jaws indicate that these large forms were still digging roots and fleshy bulbs for food. The upper jaw is not so long, but the two upper tusks are long enough nearly to touch the ground and probably aided the lower tusks in digging and pushing aside the earth. The upper lip must have been correspondingly long.

This is a critical time in the history of the elephants. The dinotheres, mastodons, mammoths, and elephants of later time all seem to have gone through this long-jawed stage. In the Miocene epoch some of the long-jawed mastodons changed from the habit of digging to that of browsing on leaves and twigs. The shovel-teeth were no longer needed. The long lower jaws were no more an advantage. So we come to a series of elephants in which the lower jaw is shortening. The necessity for reaching the ground, at least for water, still remains, and for this purpose the long upper lip is used, but being no longer supported it becomes pendant, a proboscis. These new forms with a shortened lower jaw are the mastodons. There are many intermediate stages, such as Mastodon longirostris of the lower Pliocene of Germany, in which the lower jaw is still of considerable length. Mastodon atticus from Pikerni, in Greece, has a long chin, and while young has incisors in the lower jaw. Throughout the Pliocene there are several species of mastodons in Europe and Asia. By Pleistocene time they had reached North America, where they flourished throughout the Ice-Age and for a short time afterward. Among even the American mastodons there is occasionally found one which has vestiges of tusks in the lower jaw, like the one at Amherst College, which has tusks nine inches long.

The upper tusks, after they were no longer used for digging, did not disappear, as would be expected, but instead turned upward and forward, increasing in size, so that those of a well-grown adult are usually seven to eight feet long, and a single tusk weighs over a hundred pounds. Projecting so far in front of the animal they may be of some use in pushing or lifting, perhaps in fighting; but these uses can hardly compensate for the inconvenience of such projections, or for the effort involved in carrying so much weight out in front of the head. Like the antlers of the elk or moose, they seem to be overdeveloped structures, and were probably among the features that caused the extermination of these great beasts. Though the mastodons flourished throughout the Pleistocene epoch and for a short time thereafter, they all died out 15,000 to 20,000 years ago. They had attained a height of about ten feet, which is about the size of most of the larger living elephants. They were, however, shorter legged and more massive in build than the living elephant.

Though the jaw was shortened, the mastodons never developed complex grinding teeth, but had five rather simple grinders in each jaw. Apparently they remained browsers to the end of their days.

While the mastodons were developing their upper tusks and losing their lower ones, there arose a group of elephants which lost the upper tusks and retained the lower ones, though the lower jaw had shortened so much that it could no longer reach the ground. These animals were the dinotheres, which arose in early Miocene time from long-jawed mastodons and increased to a maximum size of but little less than that of the mastodons themselves. By late Miocene time they reached their height of development, only to die out in early Pliocene time. In the dinotheres the lower tusks were not simply retained but were enlarged and recurved. It is hard to guess the use to which such tusks could be put. The back teeth of the dinothere resemble those of the mastodon, and probably the animal had a proboscis, similar to that of the elephant, else it would have had no means of reaching the ground to drink.

In late Pliocene time, while the typical mastodons were browsing on leaves and twigs, some of the group began to feed on grass. As already suggested, grass carries considerable silica in its stems and leaves, so that animals which feed on it must have hard teeth. The first change taken to harden the teeth was to increase the number of cross ridges from three or four to six or eight, and then to increase the height of each ridge. Forms that attained this stage of development are known as stegodons. They were short-jawed and lived in Asia in Pliocene time. They were abundant both in numbers and in species. Some of them had enormous tusks, one tusk in the British Museum being nine feet and nine inches long.

The cross ridges of the teeth continued to develop in number and in height, and to supplement them, the cement, which usually is found only around the roots, worked its way up around the outside and into the valleys between the ridges, until the high ridges were welded together. Such teeth may show from ten to twenty-seven ridges and may reach a height of eight to twelve inches. From the time the valleys are filled with cement these forms are known as true elephants, mammoths if extinct, elephants if living. This increase in the size of the tooth, coming at the same time as the shortening of the jaw, has caused a curious manner of succession in the teeth of elephants. The first (rather small) grinding tooth comes into place soon after birth. It is used and worn for two or three years. Then the second grinder comes up behind it, crowds it out toward the front and takes its place. In a similar manner, one after another, the rest of the grinding teeth come in, crowding out the predecessor, until in about the fifteenth year the last molar, the largest one, comes into position, and this one functions for the rest of the elephant’s life, 200 years or so. These true elephants are grazing forms. They flourished in Pleistocene time all over the world except in Australia. In North America, along the ice front, roamed the woolly mammoth, Elephas primigenius, mostly about nine feet high, the same mammoth that roamed in northern Europe and Asia and that has been preserved for us, frozen in the ice, in Siberia and Alaska. In southeastern North America there were the Columbian and Jeffersonian mammoths, ranging from eleven to twelve feet in height. In the Southwest lived the imperial mammoth, the greatest of all the elephants, measuring in height thirteen and a half feet. Just after the Ice Age all these animals died out, just why it is hard to say; perhaps by some contagious disease.

Of the several Pleistocene mammoths in Asia, only one has survived, the Indian elephant. The Pleistocene elephants of Africa are less well known, but two species are still living on that continent, the African elephant, some forms eleven feet high, and a dwarf form from the Congo, which is but four to five feet high. With the changes which took place in Europe after the disappearance of the ice sheet, some land areas became separated from the mainland, and the elephants on these new islands were restricted in their wandering and breeding. On Malta, for instance, there were developed two dwarf forms, related to the elephants of the mainland but consistently small—Elephas melitensis, some five feet high, and the tiniest of all elephants, Elephas falconi, but three feet high. Sicily, Cyprus, and Crete also had dwarf varieties.

The successive types that have been thus briefly described form a regular series, illustrated in Fig. 4. On looking at this figure we find it impossible to resist the conclusion that we have here the stages in the evolution of existing elephants—that these animals have come into existence by a series of gradual changes. Little swamp-dwellers with numerous simple teeth capable of crunching succulent aquatic vegetation become adapted, step by step, to life in a forest. The limbs were converted into stout pillars, to support the increasing bulk of the body and to stamp down small plants, and the toes were fused together into an insensitive mass, practically unpierceable by spines or thorns. The snout was drawn out into a muscular, flexible proboscis, capable, on the one hand, of gathering up ground vegetation and, on the other, of taking toll from the foliage of trees.

Fig. 4.—Evolution of head and molar teeth of mastodons and elephants. A, A′, Elephas, Pleistocene; B, Stegodon, Pliocene; C, C′, Mastodon, Pleistocene; D, D′, Trilophodon, Miocene; E, E′, Palaeomastodon, Oligocene; F, F′, Moeritherium, Eocene. (After Lull.)

The front teeth were reduced in number, those which remained becoming tusks, found in the lower as well as the upper jaw and first used for digging. Ultimately the lower tusks were suppressed, the lower jaw was shortened, and the upper tusks became enormously developed, to serve as weapons and to present a firm surface across which the trunk could break torn-off branches into pieces. The grinding teeth also became fewer and at the same time larger and more complex, to constitute an effective milling apparatus for crushing tough vegetable food. In an adult Indian or African elephant only four of these enormous teeth are in place at the same time, and each of them consists of three substances of different degrees of hardness, which wear unequally, so that the crown is always kept rough, for if smooth it would not be an efficient grinding surface. The densest of the three substances, the enamel, projects in a series of transverse ridges, narrower and more numerous in the Indian species than in its African cousin.

The head has necessarily become large and heavy, having to support the massive trunk and huge teeth (tusks and grinders) and to give attachment to the complicated muscles of the trunk and the large muscles by which the lower jaw is moved up and down in chewing. A long neck is obviously incompatible with so large a head, though the weight of this is less than might have been expected, for the unusually thick wall of the skull is not solid but is traversed by complicated air spaces.

The story of the Elephant, thus briefly outlined, is sufficient in itself to prove the evolution of that particular order of mammals, and a similar story could be told of other orders of that class and of many other groups, both high and low.

Since the time of Darwin and Wallace, the doctrine of evolution has permeated and revolutionized every department of human thought, and as a scheme of creation is immeasurably more reasonable than its crude predecessors. So far from belittling our conceptions of a Supreme Intelligence, it adds immensely to the dignity and wonder of the universe.

REFERENCES

Horses

Matthew, W. D. and Chubb, C. H. Evolution and Domestication of the Horse. Guide Leaflet of the American Museum, No. 36, 1924.
Loomis, F. B. The Evolution of the Horse. 1926. Marshall Jones Co.
Osborn, H. F. Equidae of the Oligocene, Miocene and Pliocene of North America. Mem. Amer. Museum Nat. Hist., new series. Vol. II, 1918.
Antonius, O. Stammesgeschichte der Haustiere. 1922.

Elephants

Osborn, H. F. The Elephants and Mastodons Arrive in America. Jour. Am. Mus. Natural History, Vol. XXV, No. 1, pp. 3–23, 1925.
Osborn, H. F. Phylogeny and Classification of Elephants. Amer. Phil. Soc. Proc, Vol. LXIV, pp. 17–35, 1925.
Guide to the Elephants in the British Museum, 1922.

The remains and the impressions of ancient animals and plants found in the rocks—the fossils discovered by our geologists—show a gradual multiplication of species from age to age, and a gradual increase in the complexity of forms and functions. The story told by the record of the rocks is a story of progress from the simple to the complex, from the lower to the higher. The earliest true vertebrates were the simplest fishes; next came the higher, more complex fishes; then the amphibians, able to live both in water and on land. The reptiles, such as lizards and crocodiles, came next; then birds, which, as the record shows, were derived from the reptiles; and the mammals, some of them, such as the apes and man, of very recent origin. The geologic record reveals strikingly the great story of evolution, and discovery after discovery is making the details of that record more clear and more convincing.—Editor.