These changes would have been noticeable in Europe north of the Alps, but were far more so in central Asia along the northern face of the great plateau of Thibet, with its eastern and western buttresses, and its towering rampart of the Himalayas on the south, cutting off the warm moisture of the Indian Ocean. Northward of this vast plateau and westward over the far less elevated Iranian plateau and Afghanistan, forest was fast being replaced by parklands of mingled groves and glades, or by grassy plains, or even by dry steppes. Dessication, aridity of climate, was fast compelling forest and arboreal mammals to migrate or radically change their habits of life.[9]
Almost all the apes found their old environment and continued their arboreal life by migrating far southward through India or into Africa. But at the rear of the retreating host were forms from the cooler northern regions. They were hardy and vigorous, and probably larger than most of their fellows. Possibly some of them were caught in isolated decreasing areas of forest surrounded by steppe or plain. Some of them, at least, began to descend from the trees, to seek the new food supplies of riversides, glades, and thickets, and thus gradually to become accustomed to life on the ground. It was a very hazardous experiment; only the most hardy and wary and the quickest in perception, wit, and movement survived. Among these were our ancestors, driven like all their forebears by the spur of necessity into a new mode of life under trying conditions.
They were still only apes, with long arms and short legs, and probably scrambled mostly on all fours. They had heavy brows, retreating foreheads, projecting jaws, and a brutal physiognomy. Of the mental life of the man who was to be descended from them there were few signs. They were bundles of very slight possibilities.
But let us not “despise the day of small things.” They were still far from the invisible line between apedom and manhood. Physically they resembled man quite closely. They had hand and foot, and a fair-sized brain, though they had scarcely begun to realize the possibilities of these structures.
Arboreal life could teach them little more; continuance in that school would have meant a very comfortable stagnation. They were now promoted to a new school of vastly more difficult problems, greater risks and dangers, and more severe and trying discipline. They had had an excellent course of manual and sensory training; now they must continue this and add to it the use of whatever wits they had, under peril of death. Nature was still compelling them to “move on.”
This descent to the ground probably was accomplished either in India or on the Iranian plateau, or somewhat farther to the northeast, somewhere in the great horseshoe of parkland which curved around the western buttress of the great central Asiatic plateau of Thibet. Can we locate it somewhat more definitely?[10]
At this time, during the Pliocene Epoch, there were being deposited in India the so-called Siwalik strata—vast, ancient flood-plains, stretching for a distance of 1,500 miles along the southern foot-hills of the Himalayas. They are composed of materials washed down from the mountains by a system of rivers, persisting with little change into the present. Says Osborn of the mammals found here: “It is altogether the grandest assemblage of mammals the world has ever seen, distributed through southern and eastern Asia, and probably, if our vision could be extended, ranging westward toward Persia and Arabia into northern Africa. It is the most truly cosmopolitan aggregation because in its Upper Pliocene stage it represents a congress of mammals from four great continents.... The only continents which do not contribute to this assemblage are South America and Australia.”[11] The older, Miocene, portions of this fauna are chiefly browsing forest forms, emphasized by the absence of both horses and Hipparion, as well as of grazing types of cattle and antelopes. Grazing forms, showing the decline of the forest and the spread of open parkland and grassy areas, become abundant during the Pliocene Epoch. “Among the Primates we find the Orang, an ape now confined to Borneo and Sumatra; also the Chimpanzee, another ape, now confined to Africa, the Siwalik species displaying a more human type of dentition than that of the existing African form.”
In the older, Miocene, portion we find Sivapithecus, an ape which Pilgrim considers as more nearly resembling man than any other genus of anthropoids, while Gregory speaks of it as belonging to the anthropoid line.[12] Somewhat later, in late Pliocene or early Pleistocene, there was living not far away, in Java, a far more renowned form, Pithecanthropus erectus, Du Bois, which seems to stand almost exactly midway between higher apes and man. The remains consisted of two molar teeth, a thigh-bone, and the top of a skull. The cranium is low, the forehead exceedingly retreating, giving but very small space for the frontal lobes of the brain. But the brain-cast, made from the cranial cavity, shows, according to Du Bois, that the speech area is about twice as large as in certain apes, though only one-half as large as in man. In size the brain stands somewhat above midway between the highest recent apes and the lowest existing men. The thigh-bone shows that Pithecanthropus could have stood and walked erect quite comfortably. There has been and still is much difference of opinion regarding the position of this most interesting being. Opinion was long divided nearly equally between those who considered it as the highest ape and others who held it to be the very lowest man.
It is worthy of notice that, when Pithecanthropus was alive, “Java was a part of the Asiatic continent; and similar herds of great mammals roamed freely over the plains from the foot-hills of the Himalaya Mountains to the borders of the ancient Trinil River, while similar apes inhabited the forests. At the same time the Orang may have entered the forests of Borneo, which are at present its home.”[13] Where man’s distant cousins, the anthropoid apes, and his still nearer relation, Pithecanthropus, were all living and some, at least, apparently progressing, could hardly have been far from his original home. But the climatic conditions of that time lead us to seek his original cradle somewhat farther northward than India, or even Beluchistan, and nearer to, if not in, the great steppe zone of central Asia. We lose sight of our ape-man as he is advancing toward the threshold of manhood, not far away. Whether we think that Pithecanthropus was approaching or had already passed it depends much upon where we draw the line between ape and man, a line largely artificial and as difficult to fix as the day and hour when the youth becomes of age, and what human characteristics we select to mark it. In his erect posture and some other physical traits he seems already to have attained manhood; mentally he was probably far inferior to even the lowest savage races of to-day. We are not sure whether he was our ancestor or merely a cousin of our ancestor, once or twice removed; we still lack foundations for any hypotheses as to exactly when, where, or how the erect ancestral ape-man emerged into real manhood.
Millennia passed between the days of Pithecanthropus and the first human migrations, and we may imagine primitive man as having become fairly well accustomed to life on the ground, and as having mastered his first lessons in meeting its dangers and difficulties. He had probably taken possession of a much wider area than the home of the ape-man, perhaps of the whole of the parkland zone curving around the western buttresses of the plateau of Thibet. From this region routes of migration radiated in all directions, all the more open because of the elevation of land which lasted through Upper Pliocene and early Pleistocene times.[14] Sumatra and Java then formed an extension of the Malay Peninsula, reaching more than 1,000 miles into the Indian Ocean; while the Orang seems to have been able to reach Borneo somewhat earlier. The way was equally clear westward into Europe, the Dardanelles being then replaced by a land bridge, while a second bridge spanned the Mediterranean over Sicily into Italy, and a third existed at Gibraltar.[15] These routes were evidently followed by herds of great herbivora, and probably by the earliest human emigrants into Europe.