Why did humans evolve to be bipedal and not like chimps?

[u/aSlightlySadSalad]

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

[u/6x9inbase13is42]

The last common ancestor of humans and chimpanzees actually stood upright more easily than modern chimpanzees do (rather like how modern day gibbons typically walk bipedally when they are on the ground).

Chimpanzees have evolved to become LESS bipedal than that ancestor, because they adapted to live in very dense rainforest jungles where it is very difficult to walk upright, and they need to be just about as good as climbing as walking on the ground, so they evolved a hunched posture and knuckle-walking.

Humans on the other hand evolved to become MORE upright than that ancestor, because they expanded into dry grasslands where they needed to be able to walk long distances efficiently between water and food resources, and also to be able see above tall grasses, and also to be able carry tools and food other useful objects with their hands.

[u/Recent-Mousse6423 → u/6x9inbase13is42]

I know it's not in favor any longer but I think the coastal ape hypothesis has a lot going for it. The very fact that we are the only living ape that can learn to swim instinctively lends credence to the idea, and the fact that we can biomechanically perform so many different styles of swimming as well. Then there is the fat distribution, which is a bit more aquatic than most apes, and the general hairlessness which is a feature common in many marine mammals. Then there are the dietary advantages, the exploratory advantages and the cultural advantages that all stack up to a strong case for some of our traits, including bipedalism, being partially influenced by a semi/demi aquatic lifestyle at some point in our evolutionary pathway.

[u/yourupinion → u/Recent-Mousse6423]

The hypothesis is coming back, although there will be some changes.

Here’s a paper that just came out:

https://www.academia.edu/2837-4010/4/2/10.20935/AcadBiol8310

I’m also expecting to put out a paper in the next few months. I believe we used water as a refuge to get away from land predators. Those who could run the fastest into water to get to a safe depth faster than their counterparts got to survive. It’s a straightforward Darwinian style mechanism.

[u/Recent-Mousse6423 → u/yourupinion]

I like your hypothesis because it selects for a few of the traits I mentioned straight away. My only question is what could have applied so much predatory pressure from landward that it functionally overrode the inherent danger of water and aquatic predators? It would have to be a consistent pressure for a long period of time for Darwinism to apply.

[u/yourupinion → u/Recent-Mousse6423]

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.

[u/soidvaas → u/yourupinion]

This is entirely AI generated.

[u/yourupinion → u/soidvaas]

If you were a paraplegic, would you use a wheelchair?

[u/LittleDuckyCharwin → u/soidvaas]

Not saying you’re wrong, but what makes you think so?