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Primate teeth.

The top set of gigantic teeth belong to australopith, our ape ancestor. At bottom left are chimpanzee teeth; human teeth are at right.

Roots of human civilization may have been just that

By Deane Morrison

From M, fall 2005

Approximately 5 million to 7 million years ago, our ape ancestors called australopiths left the rain forest for the grassy, tree-poor savannah, forsaking the year-round supply of fruit--the mainstay of modern chimpanzee diets--and the abundant leaves they used as fallback food when fruit was scarce. No one knows why they left, but the move to the savannah is thought to have spurred several key developments in human evolution, including bipedalism, stone tool making, increased reliance on hunting, and, later, the evolution of large brains. The savannah apes may have adopted game as their primary food, but they also needed edible fallback plants that were available in the dry season. Anthropologists Greg Laden, from the University of Minnesota, and his Harvard colleague Richard Wrangham postulate that the fallback food that allowed them to survive was fleshy underground storage organs (USOs) like roots and tubers. "Once roots were 'discovered,' chimp-like creatures would not only be able to survive on the savannah, but may well have been compelled to extend their range into more and more open habitats," says Laden. Evidence for this scenario includes the facts that savannahs are richer in USO foods than rain forests, and hominid apes that lived in savannahs millions of years ago had gigantic teeth and jaws, suggesting they chewed hard foods like roots. The teeth were adapted to grinding rather than shearing, which is consistent with a fallback diet of hard rootlike food rather than leaves.

That the size of teeth and jaws reflect the apes' secondary, not primary, food sources should not be surprising, says Laden. Primary foods for apes are either fruit or meat, and neither requires heavy-duty chewing. According to previous work by Laden and Wrangham, the masticatory apparatus became less formidable in later human ancestors as they learned to soften their fallback food by cooking it. The work is published in the Journal of Human Evolution. If the researchers are right, human history owes a huge debt to the likes of turnips and rutabagas.