Aping the Stone Age

Chimp chasers join artifact extractors to probe the roots of stone tools

For chimpanzees living in a forest surrounding the village of Bossou in Guinea, cracking nuts is a serious task with important steps. They are: First, lug large rocks to a spot near a nut-bearing tree, such as an oil palm. Next, gather the nuts and place them on the rocks. Then, obtain a smaller, graspable rock. Finally, smash the armored treats and let the shells fly. As clutches of apes pound away with devastating precision, these nut bashers create an unholy din akin to a human rock band.

APING THE STONE AGE Chimpanzees in Tai National Park in Ivory Coast use stones for cracking nuts, and older chimps pass these skills on. Studies of the way modern chimps use tools may provide clues for piecing together aspects of human prehistory. Clive Bromhall/OSF/Animals Animals
HINTS FROM CHIMP CULTURE After a juvenile Bossou chimp grooms a dominant male, the adult gives the juvenile access to cracking tools and a stash of Coula nuts. Learned behaviors, such as this exchange, often spread through chimp communities that live close together. S. Carvalho/Univ. of Cambridge and Primate Research Institute at Kyoto Univ.
STONE AGE ART Many cave stencils are of left hands, so stencilers probably used their right hands to paint. Shown are finds from Borneo (top) and Papua New Guinea. From top: Bradshaw Foundation; Nancy Sullivan & Assoc.

In fact, these West African chimps rock out in a surprising way. In this corner of the jungle, chimps appear to think more carefully about implements and how to assemble them than many scientists had assumed. A team led by anthropologist Susana Carvalho set up a nut-cracking lab in the forest near Bossou by placing seven piles of nuts and several dozen stones of various sizes, shapes and types inside a clearing. Over five field seasons, 14 of 17 chimps that regularly visited the clearing consistently reused the same pairs of stones, the scientists report in a special October issue of Animal Cognition. Most chimps, Carvalho says, used the stones together as one tool, a nutcracker.

Carvalho suspects that watching the Bossou chimps at work will provide clues to the origins of the Stone Age, the 2.6 million years during which members of the human evolutionary family are known to have used and made stone tools of increasing complexity. She is one researcher participating in a scientific movement to merge strains of archaeology, anthropology, primatology and psychology into a hybrid field dubbed primate archaeology: the study of current and past material culture among apes and perhaps other nonhuman animals.

“Very few archaeologists and paleoanthropologists have seen a wild ape, and very few primatologists have done any excavation or analysis of artifacts and fossils,” says Cambridge primatologist William McGrew, who has studied wild chimps for more than 40 years. “We have much to offer one another.”

If, for example, the modern chimps’ pounding leaves signature damage on the stones, then sites holding remains from ancient African hominids — now-extinct members of the human evolutionary family — could be probed for similarly marked stones. Such markings could reveal tools that may have preceded the earliest known stone tools, known collectively as the Oldowan industry, named for Olduvai Gorge in Tanzania. These single-edged cutting implements date from around 2.6 million years ago. Researchers could also search the modern chimps’ nut-cracking site for sharp stone fragments produced during pounding. Such accidentally sharpened fragments may have stimulated the invention of Oldowan tools.

Early hominids possessed chimp-sized brains and some other chimplike traits, making living chimps a reasonable group to compare with hominids, Carvalho contends. (That view is controversial; SN: 10/24/09, p. 9).

Carvalho, a graduate student at the University of Cambridge in England, and 17 other scientists set out the case for primate archaeology in the July 16 Nature. New investigations inspired by this perspective are set to appear in an upcoming Journal of Human Evolution.

For now, primate archaeology focuses on chimps, though gorillas and orangutans, capuchins (SN: 2/14/09, p. 12), crows (SN: 8/29/09, p. 5), dolphins (SN: 1/3/09, p. 13) and other animals also make and use tools. One line of research explores how cultural traditions in tool use and other behaviors spread among African chimp communities, with possible implications for understanding cultural links among ancient hominid groups. Another project combines chimp, hominid and modern human data to explore the mystery of why most people are right-handed. And new finds in Africa reveal the possibility of a chimp stone age.

Chips off the old block

Primatologist Andrew Whiten of the University of St. Andrews in Scotland also suspects that primate archaeology will provide a glimpse into the way toolmaking evolved and spread among both chimps and hominids.

Researchers disagree about whether chimps display cultural conventions, such as styles of making stone tools, that resemble the long-standing behavioral traditions — from pottery patterns to religious rituals — that demarcate different human cultures (SN: 6/19/99, p. 388). But converging lines of evidence indicate that wild chimps indeed invent distinctive types of tools within communities, and these tools get passed from one generation to the next as a kind of cultural legacy, Whiten says. He and his colleagues Kathy Schick and Nicholas Toth, both archaeologists at Indiana University in Bloomington, agree that modern chimps’ stone tools offer a framework for identifying precursors of Oldowan tools at ancient hominid sites and the beginnings of the spread of tool-based cultural traditions among hominids.

“The nature of chimp culture has become one of our best indicators of the likely forms of hominid culture during at least the earliest phase of the Oldowan,” Whiten says. The
Oldowan period of toolmaking lasted for 1 million years, until double-edged hand axes appeared in Africa around 1.6 million years ago.

A new analysis of artifacts previously unearthed at 20 Oldowan sites in East Africa reveals considerable evidence of stone-on-stone pounding that was probably a legacy of earlier hominids, the researchers say. Stones pounded to crack nuts contain smooth pits, Schick and Toth note. Stones pounded to break off sharp-edged Oldowan flakes, which are also used as tools, display rough pits. Further research will attempt to distinguish between these two tool varieties at Oldowan sites.

For roughly 50,000 generations, Oldowan toolmaking techniques got passed from hominid experts to novices. In recent experiments, Whiten has found that captive chimps display a similar capacity for learning how to use tools by observing more experienced comrades.

A vivid demonstration was reported in 2008. Whiten and a colleague studied 12 wild-born, juvenile chimps living in an African sanctuary. The experimenters supplied stones, and one 5-year-old chimp named Mawa immediately started using the stones to crack palm nuts, a talent he had apparently learned in his native community. The other chimps  showed no sign of knowing anything about nut cracking, reflecting an absence of this activity in their home groups.

Nine of those chimps — all at least 3 years old — cracked nuts with stones after watching Mawa demonstrate his skills for a few days. The same animals had previously been clueless when given nuts and stones but no access to Mawa.
Schick and Toth have observed a comparable learning process in a family of captive pygmy chimps that has never lived in the wild. In 1990, Toth began teaching one of the male chimps, Kanzi, to strike one stone against another to produce a sharp-edged implement. Kanzi soon learned to make and use an adequately sharpened stone to cut a cord and gain entry to a box containing food.

Over the past several years, Kanzi’s younger sister Panbanisha has developed her own stone-sharpening proficiency by watching Kanzi and humans make stone tools. Panbanisha’s two children, Nyota and Nathan, have now begun to experiment at pounding stones together.

Hints from chimp culture

What happened in Kanzi’s family apparently gets magnified in the wild, where many chimp families live together, says biological anthropologist Stephen Lycett of the University of Kent in Canterbury, England. A new investigation led by Lycett suggests that chimps, like people, have evolved a strong preference to copy behaviors exhibited by a majority of those in their home groups.

Lycett’s team used data on the number of shared traditions — including types of tool use and social grooming rituals — in different African chimp communities to construct a tree of branching social relationships among these groups.

The team was surprised to find that the chimps’ cultural ties stretched to distances similar to those documented among human hunter-gatherer groups.

Chimp groups in close proximity display more behaviors in common than do chimp groups living far from each other. Consider that the use of stone hammers and anvils for nut cracking extends through only a limited number of neighboring chimp communities in West Africa. Groups in East and West Africa share virtually no behaviors in common, supporting the chimps’ assignment to separate species. Overall, the findings suggest that cultural traditions regularly get passed from one generation to the next in chimp communities, but that these traditions typically get passed from one community to only a few neighboring groups, Lycett says. This group-to-group sharing usually happens via migration of females when they reach sexual maturity.

Whiten estimates that chimp groups share cultural traits across a distance of no more than 700 kilometers.

As it turns out, seven Oldowan sites in East Africa lie within 700 kilometers of a major Oldowan location called Koobi Fora. If the chimp model of cultural spread applies to ancient hominids, then a common tradition for making stone tools may have spread from Koobi Fora to some or all of its surrounding sites, Whiten suggests. Scientists have yet to test his hypothesis.

“Stone-flaking technology is one of the best candidates for examining the possibility of shared cultural traits in the prehistoric record,” Whiten says.

Evolution’s right hand

Stone tools, both chimp and human, can also be used to pry into the enduring mystery of why most people are right-handed, says archaeologist Natalie Uomini of the University of Liverpool in England.

By at least 120,000 years ago, right-handedness frequently occurred among Neandertals, Uomini asserts. Neandertals’ cutting tools tended to get resharpened on one side in a way that was most likely done using the right hand. Neandertal fossils have larger attachment areas for muscles on the right arm than on the left arm, indicating a preference for using the right arm.

The archaeological record from ancient Homo sapiens that lived during the same time as Neandertals shows similar signs of a right-handed skew, Uomini notes. In addition, European cave art from more than 20,000 years ago contains a preponderance of left-hand stencils made by individuals who apparently liked to hold paint-blowing tubes in their right hands.

Indiana University’s Toth has reported that most Oldowan toolmakers from nearly 2 million years ago were probably right-handed, based on the direction in which they sharpened stones. But that evidence remains preliminary, Uomini holds.

Primatologists have established that wild chimp communities display a variety of hand preferences. Groups can be right-handed, left-handed, evenly divided between right- and left-handed or can contain ambidextrous individuals.

But a trend of relatively stronger right- and left-handedness does appear in chimp groups that regularly use tools, such as nut-cracking stones or sticks for poking into termite mounds to remove the edible insects. Making and using tools requires a division of labor between the hands, Uomini says. Pressure to learn increasingly complex techniques pushed hominids toward a right-sided bias, she hypothesizes.
Many researchers suspect that specific genes contribute to human hand preferences. Uomini hypothesizes that people and chimps share a genetic propensity to use one hand more than another on tasks that demand dexterity. Genes for right-handedness, though, have evolved in humans alone, she proposes.

Uomini favors a hypothesis advanced by French researchers to explain the retention of left-handedness in about 15 percent of the human population. In this scenario, minority left-handers have enjoyed a long-standing advantage over majority right-handers in face-offs and combat because lefties are more used to facing righties than vice versa. The result: Preservation of a genetic tilt to the left in some people.

“The question of left-handedness is still open, although we know that there was a small minority of Neandertal left-handers,” Uomini says.

Preliminary evidence from visitors to a Dutch archaeological center supports the idea that increasingly complex tool operations elicit a right-handed preference. A nut-cracking station included three stone anvils, 21 hammer stones and an assortment of nuts. Another station had puzzles: four to nine flint pieces that could be fit together into a single unit.

Among 14 people who tried to crack nuts at least six times, 12 exclusively held the hammer with their right hands, just as they had signed up for the game with their right hands. Another two right-handed signers used the hammer ambidextrously. In contrast, 34 of 38 people who gave the puzzle at least six tries manipulated flint pieces ambidextrously.

Uomini’s finding of greater right-handedness on the more complex nut-cracking task recalls a 1995 study led by anthropologist Linda Marchant of Miami University in Oxford, Ohio. Marchant’s team found that members of three modern-day hunter-gatherer groups preferred to use their right hands when holding tools, such as cutting implements, that require a precision grip. Ambidexterity characterized other hand actions, such as waving at insects and poking at objects.

Marchant, who regards predominant right-handedness as an evolved trait in hominids, is intrigued by Uomini’s ideas about the impact of complex tool use on hand preferences. Other than Marchant’s 1995 paper, little is known about handedness in hunter-gatherer populations. “A full-scale project really needs to be done,” Marchant says.

Stone Age for chimps

Even greater uncertainty surrounds a category of stone artifacts that no one knew about until a few years ago. In 2007, researchers working in a West African forest were the first to report the discovery of chimp archaeological sites (SN: 2/17/07, p. 99).

Three excavations in Taï National Park in Ivory Coast yielded more than 200 stone pounding implements that chimps used to crack nuts at least 4,300 years ago. A team led by archaeologist Julio Mercader of the University of Calgary in Canada and primatologist Christophe Boesch of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, noted that the prehistoric stones looked like the nut-cracking rocks today’s chimps employ in the same forest.

This discovery raised questions about how far back in time chimps began to use stones as pounding tools and whether there is any way to distinguish prehistoric chimp implements from those of ancient hominids.

Further insights should come from a new chimp archaeological site that Cambridge University’s Carvalho discovered in 2006.

She came upon an abandoned nut-cracking site while surveying an area inhabited by chimps in Guinea. Initial excavations revealed hammer and anvil stones, along with pieces of nut shells, buried in soil layers.

Next year Carvalho will conduct studies of microscopic damage, wear marks and organic residue on the Guinea stones. Burned bits of wood from the site will provide radiocarbon age estimates for the finds. Then the results will be submitted for publication.

“We hope for some exciting news and plan to compare the new stone assemblage with Mercader’s,” says Carvalho.

Like Bossou chimps today, their long-gone predecessors, it seems, can still make a rock-solid racket.

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