Field of Science

Bipedalism an adaptation to what?

Sean B. Carroll sings the praise of Tim White of the University of California, Berkeley, who has just been named one of the one hundred people who most affected the world in 2010 by Time Magazine. Great news, that an paleoanthropologist and evolutionary biologist can make such headlines.
Since 1981, they have collected thousands of fossils from a valley in Ethiopia whose sediments span 6 million years. Their masterpiece, unveiled last October, is a 4.4 million-year-old skeleton of Ardipithecus ramidus — the oldest and most complete in the human family, predating Lucy by 1.2 million years.

"Ardi," a 4-ft. female, transforms our picture of our early ancestors. Ardi was at home in trees, but she also walked upright. A woodland dweller, she refutes the belief that modern posture was an adaptation to living on the savanna. Gaps in human history remain, but White has filled a big one. [Emphasis added.]

Just because Ardi walked on two feet and lived in trees, we now know that walking upright was not an adaptation to living on the savanna?

I don't think so.

For example. Ardi could have lived in a habitat that was both forest and savanna. It could thus be that it was the selection pressure of walking upright on the savanna that made her (i.e., Ardi's conspecifics) bipedal in the first place.

Alternatively, just because Ardi et al. started walking on two feet while living in the forest, it doesn't follow that walking upright is not an adaptation to life on the ground out of the forest. Bipedalism may not at first have been an adaptation to life on the savanna, but it could well have been a crucial adaptation later on, when they moved into the savanna. So, at first an adaptation to something else while in the trees, and subsequently to life on the savanna. An exaptation, if you like.

It could even be that the first instance of bipedalism was not an adaptation at all, but came about through neutral evolution, which later on made Ardi et al. supremely suited to life on the savanna.

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