The tools are 4,300 years old, say researchers

The discovery represents the oldest evidence of tool use by our closest evolutionary relative.

The skill could have been inherited from a common ancestor of chimps and humans, the authors say, or learnt from humans by imitation.

Alternatively, humans and chimps may have developed tool use independently, the Proceedings of the National Academy of Sciences journal reports.

Chimpanzees were first observed using stone tools in the 19th century.

Julio Mercader and colleagues found stone tools at the Noulo site in Ivory Coast, the only known prehistoric chimpanzee settlement.

Nut crunch

The excavated stones showed the hallmarks of use as tools for smashing nuts when compared with ancient human or modern chimpanzee stone tools.

The use of stone tools may have a deep evolutionary origin

"Chimpanzee material culture has a long prehistory whose deep roots are only beginning to be uncovered," write the researchers in Proceedings.

The tools were found to be 4,300 years old, which, in human terms, corresponds to the later Stone Age, before the advent of agriculture in the area.

The age of the tools was determined by subjecting charcoal from the same ground layers to the technique of radiocarbon dating.