Physically he belongs to the great branch or phylum of vertebrate animals having a backbone—sometimes only a rod of cartilage—an internal locomotive skeleton, giving the possibility of great strength and swiftness, and of large size. Large size, with its greater heat-producing mass relative to its radiating surface, implies the possibility of warm blood, or constant high temperature, resulting in greater activity of all the organs, especially of the glands and the nervous system. Large size, as a rule, is accompanied by long life—giving opportunities for continuous and wide experience, and hence for intelligence. Yet most vertebrates have remained cold-blooded, and only a “saving remnant” even of men is really intelligent. Man belongs to the highest class of vertebrates, the Mammals, which produce living young and suckle them. Among the highest mammals, the Primates, or apes, the length of the periods of gestation, of suckling the young, and of childhood, with its dependence upon the mother, have become so long that she absolutely requires some sort of help and protection from the male parent. From this necessity have sprung various grades and forms of what we may venture to call family life, with all its advantages. How many mammals have attained genuine family life and how many men have realized its possibilities?[1]
The upward march of our ancestors was neither easy nor rapid. They were anything but precocious. They were always ready to balk at progress, stiff-necked creatures who had to be driven and sternly held in the line of progress by stronger competitors. The ancestors of vertebrates maintained the swimming habit, which resulted in the development of the internal skeleton and finally of a backbone, not because it was easiest or most desirable, but because any who went to the rich feeding-grounds of the sea-bottom were eaten up by the mollusks and crabs. Our earliest air-breathing ancestors were crowded toward, and finally to the land, and into air-breathing by the pressure of stronger marine forms like sharks, or by climatic changes.[2] Reptiles, not mammals, dominated the earth throughout the Mesozoic era, and harried our ancestors into agility and wariness; at a later period the apes remained in the school of arboreal life mainly because the ground was forbidden and policed by the Carnivora. They and their forebears were compelled to forego some present ease and comfort, but always kept open the door to the future.
In spite of all this vigorous policing, malingerers and deserters turned aside from the upward line of march at every unguarded point or fork in the road, escaped from the struggle, and settled down in ease and stagnation or degeneration, like our very distant cousins, the monkeys and lower apes. Long-continued progress is a marked exception, not the rule, in the animal world, and is maintained only by the “saving remnant.” And these continue to progress mainly because Nature is “always a-chivying of them and a-telling them to move on,” as Poor Joe said of Detective Bucket, and her guiding wand is the spur of necessity.
The Primates, or apes, are, as we have seen, the highest order of the great class of mammals. Most of them, like other comparatively defenseless vertebrates, are gregarious or even social.[3] They have a feeling of kind, if not of kindness, toward one another. This sociability, together with the family as a unit of social structure, has contributed incalculably to human intellectual and moral development. Man is a Primate, a distant cousin of the highest apes, though no one of these represents our “furry arboreal ancestor with pointed ears.” Arboreal life was an excellent preparatory training toward human development. Our primate ancestor was probably of fair size. In climbing he set his feet on one branch and grasped with his hands the branch above his head. Foot and leg were used to support the body, hand and arm for pulling. Thus the hand became a true hand and the foot a genuine foot, opening up the possibility of the erect posture on the ground and the adaptation of the hand to higher uses. Meanwhile the climbing and leaping from branch to branch, the measuring with the eye of distances and strength of branches, the power of grasping the right point at the right instant, and all the complicated series of movements combined in this form of locomotion furnished a marvellous set of exercises not only for the muscles but for the higher centres in the cortex of the brain. Very probably gregarious life and rude play, so common among apes, was an extension course along somewhat similar lines.
Our ancestors became at home in and well adapted to arboreal life, but the adaptation was never extreme. It was rather what Jones[4] has called a “successful minimal adaptation.” They used arboreal life without abusing it by over-adaptation, which would have enslaved them, and made life on the ground an impossibility when the time came for their promotion to this new and more advanced stage.
At the close of his arboreal life the ape had inherited or acquired the following assets: His vertebrate and mammalian structure had given him a large, vigorous, compact, athletic, adaptable body. The mammalian care of the young had insured their survival, but only at the expense of great strain and risk of the mother. Something at least approaching family life was already attained. Arboreal life with its gymnastic training had moulded the body, differentiated hand and foot, given the possibility of erect posture, emancipating the hand from the work of locomotion and setting it free to become a tool-fashioning and tool-using organ. The ape has keen sense-organs, an eye for distances, and other conditions; and the use of these powers has given him a brain far superior to that of any of his humbler fellows. These are full of great possibilities and opportunities, if he will only use them.
But why did our ancestor descend from his place of safety in the trees and live on the ground, exposed to the attacks of fierce, swift, and well-armed enemies? Very few of the Primates, except the rock and cliff-inhabiting baboons, ever made this great venture. There must have been some quite compelling argument to induce him to take so great a risk. The change took place probably at some time during the latter half of the Cenozoic or Tertiary period, the last great division of geological time, the Age of mammals.[5] The earliest Tertiary Epoch, the Eocene, was a time of warm and equable climate, when apes lived far north in Europe, and doubtless in Asia also. Some of these apes were of fair or large size, showing that conditions were favorable and food abundant. The next epoch, the Oligocene, was similar but somewhat cooler. The third, the Miocene, was cooler still and dryer. Palms now forsook northern Europe, being gradually driven farther and farther south. Life became more difficult, food scarcer. Apes could not longer survive in northern Europe, but had to seek a warmer, more favorable, environment farther south, for many of the fruit and food trees had been crowded out and famine threatened.[6] But insects and other small and toothsome animals remained on the ground, and were abundant along the shores of rivers and lakes. There, too, were fruits and berries, roots and tubers. There the food supply was still more than sufficient.
Thus far we have glanced at Europe only. But the same changes are taking place in Asia, the cradle and home of most placental mammals, the main area of a huge zoological province of which Europe was but a westward projection, and with which America had direct connection from time to time in the region of Behring’s Straits. Here, during late Miocene and early Pliocene times, in the latter part of the Cenozoic era, a dryer and somewhat harsher climate had been accompanied by the appearance of wide plains fitted for grazing animals, as well as stretches of forest, with all varieties of landscape favoring great diversity as well as abundance of mammalian life. It was, perhaps, the golden age for most mammals, when food was plenty, climate not too severe, and every prospect pleased. This slow and gradual, but fairly steady, lowering of temperature was to culminate in the Great Ice Age of the Pleistocene Epoch, so destructive to mammalian life in the northern hemisphere.
A second climatic change, perhaps even more important than the lowering temperature, was the increase of aridity. Even during the Oligocene Epoch “the flora indicates a lessening humidity and a clearer differentiation of the seasons,”[7] The great trough of the inland sea which had stretched from the Mediterranean to the Indian Ocean began to rise, the first uplift taking place along the Pyrenees and western Alps. The Miocene was marked by a series of great movements. The old inland sea was displaced, subsidence gave place to uplift, and the greatest mountain system of the globe, including the Alps and the Himalayas, began to grow through vast repeated uplifts in the crust.[8] The continents were elevated and widened. The forest-dwelling types became restricted and largely exterminated, and animals of the plains, in the form of horses, rhinoceroses, and the cloven-hoofed ruminants, expanded in numbers and in species. This profound faunal change implies dryer climate. There was now a lesser area of tropic seas to give moisture to the atmosphere. The mountains were now effective barriers, shutting off the moisture-bearing winds from the interior of the continents.