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Friday, May 24, 2002

NATURAL SELECTIONS

We dig chimp culture


Most of what we know about ancient cultures comes from what they've left behind. Archaeology tells us, for example, about daily life in England before the Romans came and put an end to bad sanitation, and about intellectual life in Europe before the Dark Ages put an end to learning. We even know that 2,300 years ago, the Yayoi people, like modern Japanese today, ate rice from rice bowls.

We also know how we, modern humans, evolved. Fossils tell us when the first ape stood upright (Australopithecus, in Kenya about 4.2 million years ago), and ancient tools reveal when early hominids first discovered the benefits of technology (2.5 million years ago, in the Ethiopian Rift Valley).

Because archaeologists work with what they can find from excavations, until now they had concentrated on the buried remains left by early human cultures. In other species, it was thought, nothing other than bones were left behind.

A paper published this week in Science describes how archaeological techniques have been used for the first time on a nonhuman species: the chimpanzee. The study promises to illuminate the development of technology in our distant ancestors.

In the remote Tai rain forest in Côte d'Ivoire, West Africa, a team of scientists from different countries (the United States and Germany) and different disciplines (primatology and archaeology) collaborated on a study of chimp nut-cracking behavior.

The Panda oleosa tree produces the archetypal tough nut to crack. Panda nuts are found throughout tropical Africa, but only a few populations of chimps have worked out how to crack them. Those chimps, particularly in Co^te d'Ivoire, transport stones from hundreds of meters away to specific food-processing sites. There they sit and whack the nuts with stone hammers against the tree roots.

Here's where archaeology comes in. As they work on their primitive kitchen tables, the chimps scatter nut shells and flakes of rock onto the forest floor, creating what archaeologists call an "activity area."

At one site, around a tree where chimps had been seen processing nuts for many years, the researchers excavated almost 40 kg of nutshell and 479 stone pieces, weighing a total of 4 kg. The stone flakes were produced unintentionally by the chimps as they worked, and some were recovered from depths of 21 cm (they had been naturally buried). As in human archaeology, it is repeated occupation of the same spot and the accumulation of large amounts of refuse that creates a site providing the most information. In this case, the scientists estimate that the site is more than 100 years old, indicating that tool use has been practiced in the region for many generations.

Christophe Boesch, a professor at the Max Planck Institute for Evolutionary Anthropology in Germany, and one of the authors of the paper, has been researching the chimps of the Tai rain forest since 1979. His work has shown that mothers share nuts with their offspring while the juveniles are learning the behavior. They take several years to become proficient.

"Some of the stone byproducts of chimpanzee nut-cracking are similar to what we see left behind by some of our early ancestors in East Africa during a period called the 'Oldowan' [more than 150,000 years ago]," said Julio Mercader, a rain forest archaeologist at George Washington University, Washington, D.C., and the lead author of the paper.

This means that protohumans may have been using tools earlier than we now think.

"We know that flaked stone tools were used 2.5 million years ago, but stone tools may have been used by hominids as long ago as 5 million years," said Melissa Panger, a primate-tool specialist at GWU and another co-author of the paper. "If we look for assemblages of stone pieces like those we have found left behind by the chimpanzees, we can infer that those assemblages may relate to tool use, even if we don't have the tools."

The paper heralds a new area of study for archaeology, primatology and palaeoanthropology, one that will help us understand past primate activity, both of chimps and their closest relatives, the humans.

For example, a major question in human evolution is "How did our brains grow so big?" The first hominid in our genus, Homo habilis, 2 million years ago, had a 700-ml brain; we have a 1,450-ml brain. One hypothesis for the huge increase is that tool-use in early hominids provided the evolutionary drive for larger brains -- which in turn enabled our ancestors to use more complex tools . . . and so on. But if archaeology shows that chimps have been using tools for a long time, too, without a concurrent increase in brain size, then another selection pressure might have been more important in human brain development.

"The data highlight how much more we still need to learn about our closest living relative, the chimpanzee, in order to understand humanity's uniqueness," said Boesch. And we shouldn't waste any more time. "Everyday chimpanzees are being killed in the wild and their forest habitat is being destroyed. All of us have a responsibility to care for other species."

E-mail Rowan Hooper at rowan@japantimes.co.jp


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