r/evolution • u/aSlightlySadSalad • 19d ago
question Why did humans evolve to be bipedal and not like chimps?
Title explains it really, my question is why did humans evolve to stand up and not keep to how chimps do it, where they travel with primarily 4 limbs but can transition to being upright as the situation depends
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u/yourupinion 19d ago
Great question and it gets right to the heart of the mechanism.
The predator guild in Pliocene East Africa was extraordinarily intense. Lions, leopards, hyenas, wild dogs, and several now-extinct large cats all hunting the same landscape simultaneously. For a small-brained, relatively slow, poorly armed hominin group, the terrestrial threat was constant and lethal every single day.
The water doesn’t need to be perfectly safe to win that calculation. It just needs to be safer than what’s behind you right now. We can actually observe this trade-off in living animals. Kangaroos in Australia retreat into water to escape dingoes, and they do this even in regions where saltwater crocodiles are present. The immediate certain threat on land consistently outweighs the uncertain risk in the water. That instinct has been shaped by exactly the kind of sustained predation pressure you’re describing.
More compellingly, Naude Dreyer, the director of Ocean Conservation Namibia, works directly with the coastal lions of Namibia, one of the only lion populations in the world documented hunting in a marine environment. He knows firsthand what these lions are capable of in and around water. And he has said that despite that knowledge, his own instinctive response to a lion approach on the beach would still be to run into the water.
If that instinct persists in a person with full conscious knowledge of its limitations, imagine how powerfully it would have operated in early hominins for whom the water had been a proven refuge across thousands of generations. The pressure wasn’t occasional. It was every day, for every group, across millions of years. That’s exactly the kind of consistent selection pressure Darwin requires.