The first advance from arboreal to terrestrial life is marked by the power of walking more or less erect on the hind limbs and thus releasing the arms; this power is developed to a greater or less degree in all the anthropoid apes; with practice they become expert walkers. The additional freedom which the erect attitude gives to the arms and to the movements of the hands and the separate movements of the fingers is especially noticeable in the gibbon. The cultivation of the powers of the hand reacts upon the further growth and specialization of the brain; thus the brain and the erect attitude react upon each other. In the gibbon there is a marked increase in the size of those portions of the brain which supply the centres of touch, vision, and hearing.
Fig. 23. The evolution of the brain. Outlines (side view) of typical human and prehuman brains, showing the early development of the posterior portions of the brain and the relatively late development of the anterior portions, the seat of the higher mental faculties.
Discussion as to how the ancestors of man were fashioned has chiefly dealt with the rival claims of four lines of structural evolution: first, the assumption of the erect attitude; second, the development of the opposable thumb; third, the growth of the brain; and fourth, the acquisition of the power of speech. The argument for the erect attitude suggested by Lamarck, and ably put by Munro[(4)] in 1893, indicates that the cultivation of skill with the hands and fingers lies at the root of man's mental supremacy. Elliot Smith's argument that the steady growth and specialization of the brain itself has been the chief factor in leading the ancestors of man step by step upward indicates that such an advance as the erect attitude was brought about because the brain had made possible the skilled movements of the hands.
Fig. 24. The evolution of the brain. Outlines (top view) of typical human and prehuman brains, showing the narrow forebrain of the primitive type and the successive expansion of the seat of the higher mental faculties in the successive races.
The true conception of prehuman evolution, which occurred during Miocene and Pliocene times, is rather that of the coincident development of these four distinctively human powers. It appears from the limb proportions in the Neanderthal race that the partly erect attitude and walking gait were assumed much earlier in geologic time than we formerly imagined. The intimate relation between the use of the opposable thumb and the development of the higher mental faculties of man is sustained to-day by the discovery that one of the best methods of developing the mind of the child is to insist upon the constant use of the hands, for the action and reaction between hand and brain is found to develop the mind. A similar action and reaction between foot and brain developed the erect gait which released the hand from its locomotive and limb-grasping function, and by the resultant perfecting of the motion of thumbs and fingers turned the hand into an organ ready for the increasing specialization demanded by the manufacture of flint implements.
This is the stage reached, we believe, in late Pliocene times in which the human ancestor emerges from the age of mammals and enters the age of man, the period when the prehistory of man properly begins. The attitude is erect, the hand has a well-developed opposable thumb, the centres of the brain relating to the higher senses and to the control of all the motions of the limbs, hands, and fingers are well developed. The power of speech may still be rudimentary. The anterior centres of the brain for the storing of experience and the development of ideas are certainly very rudimentary.
Change of Environment in Europe
Considering that the origin and development of any creature are best furthered by a struggle for existence sufficiently severe to demand the full and frequent exercise of its powers of mind and body, it is interesting to trace the sequence of natural events which prepared western Europe for the entrance of the earliest branches of the human race. The forests and plants portray even more vividly than the animals the changing conditions of the environment and temperature which marked the approach and various vicissitudes of the great Ice Age.