3 Human Prehistory

In a famous passage from The Descent of Man, Darwin writes as follows:

As soon as some ancient member in the great series of the Primates came, owing to a change in its manner of procuring subsistence, or to a change in the conditions of its native country, to live somewhat less on trees and more on the ground, its manner of progression would have been modified; and in this case it would have had to become either more strictly quadrupedal or bipedal. Baboons frequent hilly and rocky districts, and only from necessity climb up high trees; and they have acquired almost the gait of a dog. Man alone has become a biped; and we can, I think, partly see how he has come to assume his erect attitude, which forms one of the most conspicuous differences between him and his nearest allies. Man could not have attained his present dominant position in the world without the use of his hands which are so admirably adapted to act in obedience to his will. As Sir C. Bell insists “the hand supplies all instruments, and by its correspondence with the intellect gives him universal dominion.” But the hands and arms could hardly have become perfect enough to have manufactured weapons, or to have hurled stones and spears with a true aim, as long as they were habitually used for locomotion and for supporting the whole weight of the body, or as long as they were especially well adapted, as previously remarked, for climbing trees. Such rough treatment would also have blunted the sense of touch, on which their delicate use largely depends. From these causes alone it would have been an advantage to man to have become a biped; but for many actions it is almost necessary that both arms and the whole upper part of the body should be free; and he must for this end stand firmly on his feet. To gain this great advantage, the feet have been rendered flat, and the great toe peculiarly modified, though this has entailed the loss of the power of prehension. It accords with the principle of the division of physiological labour, which prevails throughout the animal kingdom, that as the hands became perfected for prehension, the feet should have become perfected for support and locomotion. With some savages, however, the foot has not altogether lost its prehensile power, as shewn by their manner of climbing trees and of using them in other ways. (84)

The basic outline of human prehistory, outlined in this pregnant passage, has not much altered since Darwin’s day, though he was unaware of the many hominid species that have subsequently appeared in the fossil record. I can summarize current thinking roughly as follows. Our primate ancestors were at one point primarily tree dwellers. At this stage there was probably little in the way of language or social cooperation, and the body would be adapted for tree climbing, branch suspension, and brachiation (swinging from branch to branch). Food would likely consist of fruits, nuts, and insects. All limbs would be prehensile, with little to differentiate the feet and hands. Tool use would be virtually nonexistent.¹ Trees would afford shelter from sun and rain, as well as safety from ground-dwelling predators. This lifestyle would have been in place for millions of years, stable and satisfactory—just as it still is for many primates. Modern humans are descended from this ancient stock of arboreal ape like creatures, subject to the principles of ancestral preservation and incremental adaptation. Simplifying, we can say that “we” were once tree dwellers, though at this stage our ancestors were not yet real Homo sapiens.²

But then, for some reason, we came down from the trees (our ancestors did): maybe climate change affected the food supply in the trees, maybe the babies became too big and heavy to carry in trees, maybe there was competition from other species, or maybe it was a combination of things. In any case, we (they) descended to the ground, possibly over many thousands of years, maybe more quickly. Our habitat thus changed dramatically. At first we had only the old adaptations to arboreal life, which now had to serve us in terrestrial living. This initiated a long process of natural selection, probably fairly brutal, in which mutations that favored terrestrial living were powerfully selected for. Darwin sketches the general form that these took: the bipedal posture, straight-legged striding, differentiation of hands and feet, development of the hand as manipulating organ, different modes of feeding, child care, and so on. What we have here is a major change of habitat interacting with an old species design, which produced intense selective pressure. Our bodies (and brains) are terrestrial modifications of a preexisting arboreal form.

After many thousands of years (estimates vary), man became a dedicated toolmaker, a language user, and a sophisticated social being—and then later an artist, a scientist, a philosopher, and do on. Above all, he developed a large and complex brain (encephalization). This is what I am calling the Transition. The theory suggested by Darwin is that the hand played a critical role in making the Transition possible—it was the bridge, the fulcrum, the mediating mechanism. The hand was once an organ for locomotion, branch gripping, food gathering, and probably fighting, but it grew into another kind of organ altogether. These later adaptations of the hand (of which more later) were built upon the preadaptations present in its arboreal incarnation—gripping fingers, basically. The hand became liberated from locomotion by upright bipedal posture, and it came into its own to serve other adaptive functions.³ This led to a process of coevolution between hand and brain, as the brain increased in capacity to serve the hand and the hand developed new powers because of the expanding brain. The disadvantages of the bipedal gait—problems of balance, spine fragility, slow running—were eventually compensated for by the adaptiveness of the new arrangement. Free hands were far more vital to survival on the ground than the disadvantage of easily tripping over on just two feet (and our brains had to become better at keeping us thus balanced, which led to further enhancements).

Thus, in sum, we became what we are today because we were driven down from the trees and forced to adapt to a new and hostile world. If life in the trees had remained hospitable, we would never have made the Transition, as our primate relatives have not. Our hands would not have been transformed into the prehensile prodigies they are today, had we retained our ancient habitat. What looked like a catastrophe turned out to result, in the fullness of time, in an unprecedented species success, largely because of what the hands became under the new selective pressure. We snatched victory from the jaws of defeat. What looked like impending extinction eventuated in unparalleled species proliferation. And the hands played a pivotal role in this reversal of fortune. They allowed us to claw our way back after a shocking and perilous deracination.⁴

That is the general picture I will be working with. It will be spelled out more fully as we proceed, and its implications explored, but the basic message should be clear: man is an ex-brachiator, an expelled tree dweller, a hand specialist, a recent (and unlikely) biological success, a touch-and-go proposition for much of his early existence, and a creature still very much in the making.⁵ The question is how to understand man’s later evolution given his inauspicious beginnings, both as to form and fortune. How, in particular, did the hand contribute to the development of language?