Chimpanzees may have been using stone "hammers" as long as 4,300 years ago.
An international research team, led by archeologist Julio Mercader of the University of Calgary in Canada, said Monday that it had uncovered the hammers, dated to that time, in the West African country of Ivory Coast. It would be the earliest known use of tools by chimpanzees.
The hammers were used to crack nuts, a behavior still seen in chimps in that area, the researchers said in a paper in the online edition of Proceedings of the National Academy of Sciences.
The finding may indicate that a "chimpanzee stone age" began in ancient times, the researchers say.
The earliest reports of stone tool use by chimpanzees in this area date to the writings of Portuguese explorers in the 1600s.
The stones were about the size of cantaloupes with patterns of wear indicating they were used to crack nuts.
Mercader said there weren't any farmers living in this region 4,300 years ago, so it is unlikely chimpanzees picked it up by imitating villagers.
Others aren't so sure. The tools may predate farming in the area but not necessarily human contact, said anthropologist Stanley H. Ambrose of the University of Illinois at Urbana-Champaign.