Chimpanzees from West Africa were cracking nuts open using stone tools in prehistoric times, according to a study released on Monday that suggests some chimp populations may have been using this kind of tool technology for thousands of years.
Researchers have speculated that the tool-using behavior seen in some chimp populations might stretch back to ancient times and this study provides the first solid proof to support that theory.
The evidence comes from the world's only known prehistoric chimpanzee settlement in the Tai rainforest of Ivory Coast. archeologists who were excavating the site last year discovered stone "hammers" that date back 4,300 years.
The researchers were able to match microscopic starch residues found on the implements with several nuts that are known to be staples of the chimpanzee diet but not the human diet.
Hammers
Further study showed that the hammers -- essentially irregularly shaped rocks about the size of cantaloupes -- could not have been the result of natural erosion and were too large to have been used by humans.
The discovery suggests that this nut-cracking behavior has been passed down through more than 200 generations of chimps in the Tai forest and that "chimpanzee material culture has a long prehistory whose deep roots are only beginning to be uncovered," the authors of the study said.
The earliest observations of tool use among modern wild ape populations dates from the 19th century, although it has only been in the past few decades that the phenomenon has been the focus of serious study, with primatologists documenting very different patterns of tool-use among African chimp populations.
This study not only shows that some chimps had developed this skill thousands of years before these observations were made, it also raises anew the question of how exactly these primates developed skills such as crushing nuts with rocks and sticks or using sticks to "fish" termites out of mounds.
imitating humans
Some scientists believe that chimps developed these skills by imitating humans, but 4,300 years ago there was no farming in this part of the Tai rainforest, so the animals could not have acquired the skill from local villagers, the authors said.
That fact implies that either the chimps evolved the behavior independently of man, or that both humans and great apes inherited the skill from a common ancestor millions of years ago.
"The first non-human archeological site of considerable antiquity raises interesting ideas about our common heritage with chimpanzees," said Alison Brooks, a professor of anthropology at George Washington University and a research associate at the Smithsonian Institution in the US capital.
"Why and how did this group of chimpanzees maintain nut-cracking behavior while other chimpanzee groups living in locations with the same nuts available did not?"
The study, which appears in the Proceedings of the National Academy of Sciences, was written by Julio Mercader, an archeologist at the University of Calgary in Alberta, Canada with help from a team of collaborators from Germany, the UK, the US and Canada.
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