Among the hundreds of living primate species, only humans are naked. Two kinds of habitat are known to give rise to naked mammals—a subterranean one or a wet one. There is a naked Somalian mole rat which never ventures above ground. All other non-human mammals which have lost all or most of their fur are either swimmers like whales and dolphins and walruses and manatees, or wallowers like hippopotamuses and pigs and tapirs. The rhinoceros and the elephant, though found on land since Africa became drier, bear traces of a more watery past and seize every opportunity of wallowing in mud or water.

[...]

One hypothesis used to be that they first developed big brains and began to make tools, and finally walked on their hind legs to free their hands for carrying weapons. But we now know that it was bipedalism that came first, before the big brain and tool-making.

However, if their habitat had become flooded, they would have been forced to walk on their hind legs whenever they came down to the ground in order to keep their heads above water. The only animal which has ever evolved a pelvis like ours, suitable for bipedalism, was the long-extinct Oreopithecus, known as the swamp ape.

Today, two primates when on the ground stand and walk erect somewhat more readily than most other species. One, the proboscis monkey, lives in the mangrove swamps of Borneo. The other is the bonobo or pygmy chimpanzee; its habitat includes a large tract of seasonally flooded forest, which would have covered an even more extensive area before the African climate became drier.

A snake is found by a child, who screams and points. The women and children gather round and beat it with sticks.

The larger hunters return, and make their bipedal displays, causing the women hand over the fruit. The hunters tuck in, magnanimously sharing the feast with their mates, reinforcing the pair bonding.