Walking on your two back legs isn’t that unusual in nature. Birds have evolved to do it, most likely from dinosaur ancestors that walked on their back legs. Kangaroos get about very efficiently on two legs too. But it is more common for mammals to stay on four legs, as it takes less balance and generally means you can run faster, either in the direction of prey, or away from predators. Humans are the only apes that habitually walk on two legs, although other apes, and monkeys, aren’t totally quadripedal as they do sometimes stand up and walk.
So what makes an ape start to walk on two legs? It can’t be because it would make them good hunters in the future. It can only be because it confers an advantage at the time. That’s the way natural selection works. And that’s the problem I have with the idea that we began to walk and run on two legs in order to hunt more efficiently. Sure, our large bum muscles do make us good walkers and runners now, but in the earliest days of transition from walking on all fours to walking on our back legs, we didn’t have those developed muscle groups. We were probably as slow and awkward as the baboon in the picture above. So why did we do it?
The baboon is wading through flood waters in the Okavango delta in Botswana, an inland river delta that doesn’t reach the sea. The delta is a large geological depression in the ground and the water, which comes from highlands in Angola, is evaporated in the baking heat of the desert and transpired by the plants in the delta. Once a year this desert area receives a flood of around 11 cubic kilometres of water. And it isn’t unique in Africa. There are similar inland deltas today in Sudan and Mali.
Suppose a group of apes, some of the common ancestors of humans and chimps, were living in or near an area that was affected by geological land movement. This movement effectively cut them off from other apes of the same species when it became permanently flooded. The stranded apes would have to wade from small island to small island. Good waders would be more likely to meet mates who were also good waders. Mums that stood up and waded wouldn’t risk drowning babies that were hanging onto their belly fur. Natural selection at work. And we’d be very likely to eat quite a bit of fish and other aquatic food sources rich in Omega 3 fatty acids. Brain food.
It’s not that fanciful, or unlikely. It is at least worthy of consideration as a possible root of bipedalism.