Wild Chimps Rocked On: Apes left unique record of stone tools

Archaeologists, by definition, uncover the remnants of past human activity. With the first excavation of chimpanzee stone tools at an African site, however, the scope of their work has entered virgin terrain.

STRIKING FINDS. Archaeologists have identified hammering stones at a chimpanzee nut-cracking site in western Africa. Image by C. Boesch, Montage by Steve Smart

Chimps transported suitable pieces of stone to the undated site and used them to crack open nuts placed on thick tree roots, according to Julio Mercader of George Washington University in Washington, D.C.

“At least some wild chimpanzees have produced stone [artifacts] and left behind an archaeological record of their nut-cracking behavior,” says Mercader, who directed the excavation. He described the recent discoveries at the annual meeting of the Paleoanthropology Society, held last week in Denver.

Researchers previously had reported that chimps living in western Africa’s Ta forest avidly stockpile stones at places with broad tree roots or stumps that serve as anvils for cracking nuts. This activity may represent a learned behavior peculiar to the local animals, since chimps living in other parts of Africa don’t use stone implements (SN: 6/19/99, p. 388).

Mercader and his coworkers excavated a Ta forest site called Panda 100. Trees bearing so-called Panda nuts grew in this region until 1996, when they died out. The chimp artifacts haven’t been dated yet.

The researchers chose their dig site after noticing four large tree roots that displayed pounding marks made by stones. Excavation of trenches at the site yielded two more tree roots with similar markings. Fragments of nutshells were recovered around all six roots.

Moreover, Mercader’s group unearthed 479 stone artifacts, often in close proximity to the shell fragments. These artifacts included the remains of hammering stones, thin flakes that had been pounded off those stones, and pieces of shattered rock.

The earliest known stone tools, made by human ancestors in eastern Africa around 2.6 million years ago, consisted of sharpened chopping implements and larger rocks used as anvils. Chimps’ hammering stones recovered at Panda 100 are about the same size as those ancient choppers, Mercader says. However, implements used by human ancestors show more evidence of having been intentionally modified than do those attributed to chimps, he notes.

The Ta forest discoveries suggest that archaeologists may be able to investigate links between nut-cracking tools employed by chimps and human ancestors, says wild-chimp researcher William McGrew of Miami University in Oxford, Ohio. Homo species cracked nuts with stone implements at least 780,000 years ago (SN: 2/23/02, p. 117: Available to subscribers at Almond Joy, Stone Age Style: Our ancestors had a bash eating wild nuts .).

“There seems to be a signature of chimpanzee archaeology at Panda 100, which is pretty cool,” remarks Nicholas Toth of Indiana University in Bloomington, who studies ancient stone tools. Still, he adds, “the Ta forest material that I’ve seen looks fairly crude.”

In contrast, human ancestors’ earliest known tools exhibit remarkably sophisticated workmanship, Toth says. In a study presented at the Paleoanthropology Society meeting, he and his coworkers discerned that 2.6-million-year-old stone tools and present-day stone chopping implements fashioned by experienced tool makers required similar skills.

Toth also notes that two captive bonobos, or pygmy chimpanzees, have learned to make chopping tools out of rocks with considerable proficiency, though not up to human skills. Chimps’ tool-making disadvantage largely derives from having large hands that can’t manipulate objects as well or generate as much striking force as human hands can, Toth says.

Bruce Bower has written about the behavioral sciences for Science News since 1984. He writes about psychology, anthropology, archaeology and mental health issues.

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