John Kappelman, a professor of anthropology at the University of Texas, says the new discovery supports that theory.
Thirty years ago, Kappelman said by email, he and a colleague "hypothesized that these belowground plant parts would have offered a rich food resource for early humans who were living in dry seasonal habitats such as bushland and savanna.
"Before that, [yam expert] D. G. Coursey suggested that even the crudest form of a digging tool would have put early humans at a marked evolutionary advantage for exploiting these foods," Kappelman added.
"Well, this new work proves Coursey right, because the tools used by these chimpanzees are as simple as simple can be. And because they consist of thin pieces of wood, [they] are unlikely to be the sort of thing that would be preserved in the fossil record."
Previously, humans were thought to be the only primates to dig up tubers using tools—just as humans were once thought to be the only primates to use tools at all.
Jill Pruetz, an Iowa State University primate researcher not affiliated with the new study, said "it suggests to me that we will continue to redefine humans as we find more out about other species, especially our closest living relatives.
"I wouldn't be surprised to hear about more of these kinds of 'discoveries'—ones that cause us to rethink our definition of what a human is."
(Pruetz made headlines earlier this year with her discovery that chimps make "spears" to hunt other mammals.)
Chimps at Risk
The chimps at Ugalla, the study authors say, face environmental challenges similar to those of our human ancestors about five million years ago, when a changing climate delivered a hit to their forests.
But there are other more immediate factors that could be contributing to the Ugalla chimps' decline.
Ugalla is adjacent to several camps housing people from Congo, Rwanda, and Burundi. These refugees may be responsible for heavy poaching reported in the last five years, study co-author Moore said.
Additionally, a proposed new major road will make travel in the region much easier, and "such roads almost always increase pressure on wildlife," Moore said.
"If the refugees are repatriated to their home countries soon and development along the road is regulated, the area is in decent shape and wildlife will recover," he added.
"If those things don't happen, the outlook isn't good."
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