Also, several types of starch grains were found on the stones, which the researchers say is residue from cracking local nuts.
Some experts had believed that the chimps at Mercader's study site learned to crack nuts by watching people break apart the seeds of African palm oil trees and other tropical species.
Agustín Fuentes, an anthropologist at Indiana's University of Notre Dame, said he's not surprised by the new research, but he's happy about it.
"It puts the nail in the coffin on those who say chimp tool use is atypical," he said.
"Most people have already bought into that. But now you can say, Look, you've got a 4,000-year-old tradition."
The most primitive human stone-tool sites are in Olduvai Gorge in East Africa.
Tools there date back to 2.6 million years ago, when people were deliberately modifying stone tools by flaking rocks to create razorlike edges. Chimps today don't change the shapes of the stones they use as nutcrackers.
But the chimps' stones may be similar to stone tools used by humans before our ancestors began to chisel rocks for specific purposes.
Today in Côte d'Ivoire's Taï rain forest, mother chimpanzees still teach their infants the art of nut cracking.
It takes young chimps about seven years to master the technique. To split a nut without pulverizing it, the chimps must apply 2,200 pounds (1,000 kilograms) of force.
Fuentes said that Mercader and colleagues' work emphasizes that the difference between chimps and humans is not the ability to use tools, but the ability to modify the tools and share that information.
For Fuentes, the research "knocks humans off the pedestal of tool use," but it affirms our unique ability to communicate.
A chimp mother might teach her offspring to crack nuts, but chimps are not really communicating about how to use tools and where to get them, he said.
"Nothing," he said, "goes to the level of information-sharing and technology that [humans are capable of]."
Editor's note: Julio Mercader's investigation of the stone tool use of present and past chimpanzees in West Africa has been funded in part by the National Geographic Society's Committee for Research and Exploration.
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