I believe we are the only great apes with long legs and short arms. I am told this is because we walk upright. So why do we walk upright?
Because it is the most energy efficient transport method. We can move over mountains because of it. We still sit on the same muscles as all the other animals
To reach the fruit on the higher branches. It's the same reason we have 5-foot necks, like a giraffe's.
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Doesn't explain why all the other grassland mammals stayed on four limbs. It is most likely tied to tool-use. Upright walking is horrible in too many ways for it to have been a benefit, unless the trade-off was mighty-powerful.
Maybe we always walked upright, there is no evidence that we evolved from not walking upright. Neanderthal walked upright. I think whatever species we came from and evolved from, walked upright. I think we walk upright because we wanted to. Or maybe we just did not evolve the ability to swing from trees and had to.
So... we came from fish that walked upright? And those fish came from single-celled organisms that also walked upright? Trust me, at some point, there was a thing that wasn't walking upright, but an environmental pressure caused a gradual change into something that was walking upright. And it didn't happen back with the fishes, it happened during our co-existence with the other great apes.
"Great Apes" is not a species, so the two are not mutually exclusive. We can be Great Apes, and ALSO be Homo sapiens. It is the distinction between Genera and Species.