When did humans start walking upright? Are we supposed to walk on all fours? And what would our world look like if we still walked on all fours? There are a lot of theories that attempt to explain why humans started to walk on two legs. By the way, this ability is called bipedalism and it's one of humanity’s defining physical characteristics.

Anyway, about 4 million years ago, ape-like ancestors left the safety of their trees and took the first step toward humanity. They started carrying food over long distances and using tools to prepare it. But what if they hadn’t?

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