Human Evolution: Why Did Our Ancestors First Walk Upright?

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That may make sense, even to a chimp, but it's not all that easy for a chimp to walk upright. Unlike humans, for example, a chimp can't stand on one leg and let its leg bones carry the weight. A chimp has to use muscles for that, because its legs are structured differently, and that can be tiring.

But the gradual evolutionary changes, which had to take millions of years, brought a mixed bag. It made it possible for early humans to roam over vast areas, picking low hanging fruit, and carrying supplies, tools, and kids. It also made them appear larger and more intimidating.

But we still have a backbone left over from the years when our more distant ancestors were chiefly horizontal, both in the water and on the land. It wasn't designed to work in a vertical position, which is why modern humans suffer from sore backs, slipped disks, arthritis, and so forth.

So when did all this begin? According to the Smithsonian Institution's Human Origins Program, it probably started at least six million years ago with changes in the leg bones of one of the earliest hominoids, Sahelanthropus. Next came the knee, a couple of million years later.

By around three million years ago, according to many experts, our ancestors were pretty much like us, at least structurally, and probably moved around mostly on two feet, which had lost the ability to cling to a branch. Last year in the journal Science, researchers reported the discovery of a fossilized foot in Ethiopia that was clearly made for walking, not climbing trees.

That foot was stiff enough to push off from the ground when walking, and flexible enough to absorb the shock of touching down, so it was a monumental change. No more swinging in the trees.

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