Standing and walking is, for us, so easy, so natural, that we take it for granted. Many mammals can stand upright for a short time, and even walk. But it takes effort, and they soon slump back to all fours, the typical mammalian state. Hominins—species in the human family—are different. Walking upright is their default. Locomotion on all fours, using hands and feet to walk, is, in contrast, unnatural and difficult. The adoption of bipedality, by a lineage of apes living in Africa 7 million years ago, was one of the more remarkable, unlikely, and puzzling events in the entire history of life. It required a complete re-engineering of the entire body, from head to toe.
When the backbone evolved half a billion years ago, it was a structure held horizontally, in tension. In hominins, it moved through ninety degrees, to be held vertically, in compression. No more radical alteration in the engineering requirements of the backbone has happened since it first evolved, and it can only be regarded as maladaptive—witness that back problems constitute one of the most costly and frequent causes of illness in humans today.
The question remains—why? The easy answer is that bipedality is just one of many peculiar modes of locomotion that apes have tried over millions of years, including swinging using elongated arms, as in gibbons; clambering using all four limbs as hands, as orangs do; and the unique quadrupedal knuckle-walking of chimps and gorillas. But why hominins tried bipedal walking, rather than any other mode of getting from one place to another, remains an open question. Certainly, life in open country does not require it. Many large monkeys, such as macaques and baboons, live in open country and remain with all four feet firmly planted on the hard, dry ground.
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