4. Two Legs - Some Theories Considered
This seems to be the silly season for speculative theories about the cause of bipedalism, judging from the frequency with which they have been appearing recently. They all have to fit the scene which Yves Coppens describes so well - the drying of the Western side of the Great Rift Valley in Southern Africa: and the fact that chimpanzee remains are concentrated to the west of the Rift; and hominid remains to the east.14 Apart from that, we have informed guess work.
An hypothesis favoured by many has been Man the Mighty Hunter - the first hominid who stayed in the plains when the forests receded, and went upright to run faster, and to see prey or dangers far off. But can we seriously believe that some newly jumped-up biped had any significant speed advantage over the predators of the savannah? Walking on two legs is mechanically difficult and requires major changes in the configuration of the bones and muscular actions used for locomotion. The vertebrae have to become load bearing. According to Lovejoy, 'Lucy' must have come at the end of a long evolution towards two leggedness.15 It seems improbable in the first stages of the changeover that a new biped would have been faster than a quadruped.16 Human babies strive to stand upright, but revert to crawling when they are in a hurry. Scavenging, a less flattering image of our earlier selves, is open to similar objections. Hyenas and dogs are good scavengers without abandoning four leggedness. Scavenging is at best a partial explanation.
If it was not speed, then what was the advantage of the new posture? Dean Falk seems to suggest that bipedalism was produced by the need to keep the brain cool.17 She describes physical adaptations such as cooling holes in the cranium and more exposed pathways for blood leaving the brain, and suggests that the cooling was enhanced by going upright. Better cooling would be needed by a creature which used its brain more, especially when that brain grew larger. By the square-cube law, the surface to volume ratio becomes less favourable for cooling as the brain grows bigger. While one can see that a larger brain needed new cooling mechanisms, it is begging the question to argue that this was a cause of bipedalism. Surely the question which needs to be asked is what new activity had led to greater brain activity and the need for cooling? Other large bipeds, like ostriches and kangaroos, are certainly not noted for their brain-power - so what were the proto-hominids up to?
Another recent idea, from Nina Jablonski and George Chaplin,18 is that Savannah dwelling apes stood upright as a display of aggression. Surely this again is a very thin cause for such a difficult bodily transformation. Chimpanzees run social lives, stylised aggression and all, without undergoing the discomforts of becoming bipedal.
The idea of bipedalism being an adaptation to the dry Savannah is challenged by a watery hypothesis - the Aquatic Ape who may have taken to the water, wading and losing his hair in the process. Elaine Morgan suggests that this could have taken place in the Rift Valley when it was an inland sea.19 The wader's buoyancy in water would doubtless make uprightness easier. This hypothesis can also account for the high position of the breasts of the human female, and would explain a human nose which impedes the entry of water. Her argument that the detailed development of the human embryo suggests a return to the water is persuasive. Unlike any other primate, it even has a coating of wax (vernix caseosa) at birth.20 Like other mammals which have returned to the sea, humans can control their breathing - a prerequisite of speech. The 'aquatic ape' could certainly have enjoyed a rich protein diet from shell fish and the long-chain molecules in fish oils would favour the growth of nervous tissue.
The Morgan hypothesis has the essential quality which others lack; it suggests a unique and major change of behaviour as the impulse towards bipedalism. Yet though the aquatic ape is an attractive hypothesis, it does not account for the use of tools or language. Unless the salt deposits of the Rift Valley yield up real evidence, Morgan's marine ape must remain no more than an attractive idea.