Tool-wielding chimps provide a glimpse of early human behavior

Chimpanzees inhabiting a harsh savanna environment and using bark and stick tools to exploit an underground food resource are giving scientists new insights to the behaviors of the earliest hominids who, millions of years ago, left the African forests to range the same kinds of environments and possibly utilize the same foods.

Writing today (Nov. 12) in the Proceedings of the National Academy of Sciences (PNAS), a team of researchers including UW–Madison anthropologist Travis R. Pickering reports evidence of tool use among rare savanna chimps to harvest edible tubers, roots and bulbs.

The finding is important because it chips away at behaviors once seen as uniquely human. It supports the notion that chimpanzees, our closest living evolutionary relatives, can serve as models for understanding some aspects of the lifestyles and behaviors of the earliest members of the human family.

The new study demonstrates that "the understanding and capability to exploit these resources were very likely within the grasp of the first chimp-like hominids," argues Pickering. "It was widely believed that it is a uniquely human adaptation to use tools to dig these things up."

R. Adriana Hernandez-Aguilar of the University of Southern California collected field data as part of her dissertation over nearly two years in the Ugalla Forest Reserve of western Tanzania, an arid woodland savanna that is home to a small population of chimpanzees that have adapted to life beyond the apes’ typical forest habitat. Research at the site has been coordinated by the study’s other co-author, Jim Moore of the University of California at San Diego, since 1989 under the aegis of the Ugalla Primate Project.

"Chimpanzees are not australopithecines, and we can’t conclude that if they do something today, our ancestors must have done it then. But, when integrated with research on the fossil and paleoecological record, modern analogies are useful for investigating our past," Moore explains. "In this case, the Ugalla chimpanzees suggest that underground resources were within reach of our ancestors with similar brain size and hand morphology."

A surprise finding, Moore, Pickering and Hernandez-Aguilar explain, was that the chimps feasted on underground food resources during the food-rich rainy season, and not as a fallback in times of scarcity. That observation, says Moore, "challenges our current hypotheses about the role of such foods in hominid evolution and may help reframe the scientific debate."

Chimps at Ugalla, the authors explain, are facing environmental challenges similar to those of our human ancestors 5 million or so years ago when shifting climate greatly reduced the forests favored by early hominids.

For chimpanzees today, and likely for our human ancestors, open woodlands such as the Ugalla Forest Reserve are marginal habitats where the ability to adapt is challenged. But woodland savannas are also rich in the types of plants that store nutrients in what scientists call underground storage organs — tubers, roots and bulbs of various types. It has been hypothesized that early hominids’ access to such resources through the use of tools was a critical innovation that helped power human adaptation to savanna environments.

"Until now, it has been believed that such food resources, buried beneath the surface of the soil, were beyond the reach of modern chimpanzees," according to Moore.

The underground storage organs of plants in Ugalla can be difficult to get at, as the tubers and roots can be obtained only by cracking a tough layer of earth. To do so, the chimps apparently use sticks, pieces of logs and sturdy bark to penetrate a hard crust of soil to reach looser layers of dirt in which the tubers are found.

While researchers were unable to directly observe the savanna chimps in the act of using tools to dig up the tubers, evidence of such activity by the chimps was found at 11 different sites in Ugalla. Ten of the sites were directly beneath chimpanzee nests. The spoor of chimps — but no other animals — including knuckle prints, feces and chewed wads of the fibrous tubers, provided evidence that the apes were taking advantage of the underground food resource.

Three of the sites, according to the new PNAS report, yielded seven sediment-encrusted digging tools. The worn edges and patterns of adhering sediment of the sticks and bark recovered from the sites implied their use as implements for reaching the tubers.

Some of the underground storage organs excavated by the Ugalla chimps are fibrous and have the appearance of uncooked potatoes, according to Pickering. The chimps chew them like gum to extract nutrients and spit out indigestible wads of fiber. Some of the plants used by the chimps, notes Hernandez-Aguilar, are used as food or medicines by humans.

"Chimpanzees in many parts of Africa are known to consume leaves of several species for medicinal purposes and it will be interesting if it is confirmed in the future that chimps in Ugalla consume underground storage organs for their medicinal properties," says Hernandez-Aguilar.

In Ugalla, chimpanzee density is low, but recent survey data suggest the vast majority of Tanzania’s chimps live in such woodlands. The Ugalla woodland savanna may parallel environments of 5 million years ago when early hominids, the forebearers of the modern human lineage, adapted to the savanna as major climate change dramatically reduced the continent’s forests.

"Savanna chimps, we would contend, are dealing with environmental constraints and problems — evolutionary pressures — that our earliest relatives would have dealt with as well in similar environments," Pickering explains.

And because the tools are organic in nature and subject to rapid decay, evidence of such tool use by early hominids is unlikely to be found in the archaeological record.

In the case of the human lineage, the move to a new environment, scientists believe, triggered the development of anatomical features such as big jaws and powerful grinding teeth to cope with a shift in diet to foods like the underground storage organs of plants.

The new work showing tool-assisted digging by the Ugalla chimps — as well as others’ recent work with another population of savanna chimps who use sticks as small spears to impale bush babies — makes these types of chimps unique and argues for the importance of conserving and studying them further.

"The insights that they may continue to provide concerning human evolution might prove to be quite significant," Pickering argues.

The study was funded by the LSB Leakey Foundation, the National Science Foundation, the Jane Goodall Center at the University of Southern California, the University of California Committee on Research, and the Palaeontology Scientific Trust.