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Big mystery: What made us stand tall?

/ Source: Special to MSNBC

Why did humans begin to stand erect? There are a lot of theories out there, going all the way back to Darwin who suggested that humans became bipedal to free their hands in order to make and use stone tools like arrowheads and spears. But we now know from the fossil record that humans began to walk upright about 4 million years ago — long before they started making such sophisticated tools.

About 4 million years ago is when we observed a change in orientation of the tibia — a bone between the knee and the ankle — so that it is held upright rather than angled to the outside as it is in apes. But there is no evidence that hominids started systematically crafting tools before 2.6 million years ago, shedding doubt on Darwin’s hypothesis.

Another theory has to do with the earliest of humans moving out onto the savannah. This theory suggests that early hominids needed to stand upright to look over tall grasses to spot predators and potential prey. We don’t think that’s how things happened based on what we see in the fossil record.

In fact, there is new evidence that shows that the earliest human ancestors were not out on the savannah, but still in the woods.

An idea that has recently gotten a lot of press is the notion that hominids started to stand upright for improved thermal regulation when living in open country. By standing upright, they had more air circulating around the body and less direct sunlight. Again, there is a problem in assuming that early hominids were living in open country before they had these adaptations.

A need to carry?
The most likely explanation for hominids becoming bipedal involves the need for freeing up the hands for something — carrying simple tools (sticks, stones and other found objects), food or water.

You have to realize that by becoming bipedal they were giving up some real advantages to running on all fours. First, you become a very slow runner. You can’t outrun a chicken, much less a lion. And you give up grasping feet, so you can’t seek refuge in trees as easily.

So, since hominids lost so many adaptations for running fast on the ground and climbing trees, we know that the change had to be critical for survival and reproduction.

Was travel a factor?
It may be that our ancestors needed to carry food and simple weapons from place to place to take advantage of ecological resources that were distributed sparsely across the landscape. At this time in Africa, between 5 and 2 million years ago, the climate was cooling and drying, which may have influenced resource density and distribution. They may have needed to travel long distances with rocks and sticks for extracting food or defending themselves from predators.

More evidence that becoming erect had to do with the need to pick things up and carry them comes from what we see in animals in existence today.

Chimps, for example, stand erect when they are carrying food. They also move on two feet when they’re carrying a branch or throwing sticks and stones.

All in all, it’s really an interesting puzzle and one that’s not easily solved. The theories are, for the most part, based on logical arguments and everyone, of course, thinks their argument is the most logical.

Carol Ward is an associate professor in the department of anthropology and the department of pathology and anatomical sciences at the University of Missouri at Columbia.