5. Two Legs - An Alternative Suggestion
These different hypotheses all depend on a two stage process to explain, first, why some apes became biped and then, second, that their hands were free for tool using to develop. But are two hypotheses necessary in the first place?
Let us consider a simpler possibility. Suppose instead that bipedalism arose because the first hominids were using their hands, which became too useful to waste on knuckle walking. Suppose that they were holding useful things. The proposition 'upright because of tools' is simpler than 'some reason for upright; and then some reason for tools.' After all, our close relatives the chimpanzees are capable of using
a range of tools, like stones to crack nuts, straws to winkle out termites, and sticks to reach things. There is no fatal improbability in the idea that the first upright steps and frequent hand-use began when the new hominid line started about 5 m.y.a. I have pointed out the difficulty of working flint, and that it seems most likely that only a creature which is already using things will have the motor skills to work stone.
Deliberately made stone tools, then, belong to a much later time. Many simpler things, however, can be used to great effect. Plain pebbles can crack nuts and sea-shells, or can be thrown as missiles. A dead animal offered, apart from its meat: shoulder blades to dig with; a bladder to carry water; hard, pointed horns; gut; a jawbone studded with teeth... and so on. Above all, it is necessary to imagine what power there was in a stick. It can scrape and dig, revealing new food sources like roots and bulbs; for primates have nails not claws, so they are poorly fitted for digging. The stick can extend the reach and knock down fruit and nuts. As a club it is an energy storing weapon - other animals can only store energy by charging at high speed. Furthermore, like throwing stones or using a hammer, a club requires accurate prediction of its curved path. There is no time for feed-back corrections, and Calvin considers that this required more advanced neural capacities.21 And lastly, it might have made walking easier for a learner-biped just as it does for an ageing one.
We are not suggesting a craftsman tool-maker, an earlier claimant to be homo faber, but a rough opportunist; homo bricoleur or thing user.22 He did not even need an opposable thumb. The opposable thumb is important for the precision grip in delicate operations, but the power grip is enough for grasping sticks and stones. In any case, how would an opposable thumb have come about unless it had been favoured with success? An essential feature to grasp about evolution is that a new feature does not appear before it is used. The feature only improves because it is already being used successfully. This truth is encapsulated in the saying that 'Birds do not fly because they have wings. Birds have wings because they fly.'
Primate origins left other important legacies besides the useful hand. Our brachiating branch of the primate line had acquired a particularly useful arm socket. Apes, unlike monkeys, swing beneath branches and have a much wider arm movement -handy for a prospective tool user. You can scratch behind your other ear; monkeys can't. Life in the trees also called for good binocular vision, our highly developed judgement of distance is based on a number of mental processes, not just stereoscopy; it is affected by the familiar size of things, overlapping, even colour.23 Primates in trees needed this three dimensional mental power to recognise things every which way up. In fact, our brains are very good at rotating things mentally to compare them.24 Even more important, the further up the primate line one looks, the more we find a tendency to learning, imitation and inventiveness.