… What is most striking is just how much can be read from a few ancient skulls if you know how. Bipedalism is a case in point: no scientist has seen our putative ape-like ancestors in the flesh, yet they deduce from a host of clues that these ancestors were walking on two legs some 4m years ago. For example, if the hole through which the spinal cord enters is on the underside of the skull, this suggests an upright posture â€“ in quadrupeds, including great apes such as chimpanzees, this hole tends to be more at the back of the skull. (You can work out why this is by going down on all fours: you will find your head naturally positioned so as to be staring at the ground rather than looking forwards â€“ which is not conducive to avoiding sabre-toothed tigers.)
Countless other clues, from toes to hips, help give an idea of how a specimen might have looked when alive. But they leave open the question of why. Coming down from the trees to wander about upright would, as Tattersall points out, have been hugely risky. It meant leaving a way of life that had been successful for millions of years to develop new foraging strategies in the face of the ferocious predators that prowled the grasslands. There are many theories purporting to explain why our ancestors became bipedal, from minimising exposure to the baking African sun to increasing long-distance trekking efficiency. But none seems entirely adequate to explain such a revolution. This is, as Tattersall puts it, â€œhugely unfortunateâ€, as everything that has come since â€“ up to and including you reading this article â€“ follows from those first two-legged steps.
Tattersall does an excellent job of showing how we can sketch the story of our origins from the few precious fossil remains, while at the same time not glossing over our ignorance of such crucial details. But the psychologist David P Barash is even bolder in embracing the unknown: his latest book, Homo Mysterious: Evolutionary Puzzles of Human Nature, is a catalogue of the many unresolved riddles of our history.
A surprising number of these mysteries concern female sexuality. The male orgasm, for example, serves a rather obvious seed-sowing function â€“ but what is the point of its female equivalent? The popular hypothesis that such ecstasy enhances the likelihood of a subsequent pregnancy is, Barash informs us, entirely without evidence. The idea that it might motivate reluctant ladies also seems flawed: other animals donâ€™t require an eruption of bliss in order to continue the family line. Perhaps it is simply a â€œnon-adaptive by-productâ€ â€“ an incidental development to which evolution is indifferent?
Barash doesnâ€™t think so, preferring to believe that it is more important than that. But in what way remains a riddle â€“ and only one of many posed by the female body. Scientists cannot explain, for example, why women have prominent breasts even when they are not suckling children. Other mammals donâ€™t. Yes, of course, men are drawn to these protruding sacks of fat â€“ but why? No one knows, though theories abound. …