Wild gorillas have been seen using "weapons" for the first time, giving a new insight into how early man learned to use sticks and stones for fighting and hunting millions of years ago.
Researchers observed gorillas in the Cross River area of Cameroon throwing sticks, clumps of earth and stones at human "invaders".
It is the first time that the largest of the great apes has been seen to use tools in an aggressive way.
Experts believe that our ancestors may have learned to use sticks and stones in a similar way to frighten away predators.
The scientists noticed the unusual behaviour during a three-year study. They believe the animals might have learned to throw objects from humans who were seen throwing stones at the gorillas.
Jacqueline Sunderland Groves, from the University of Sussex in Brighton, a member of the Wildlife Conservation Society team, said: "The area is largely isolated from other gorilla groups, but there are herdsmen on the mountain.
"In one encounter a group of gorillas threw clumps of grass and soil at the researchers while acting aggressively. Another gorilla threw a branch. A third encounter saw the gorillas throwing soil at a local man who was throwing stones at the apes."
A gorilla was seen to use tools once before in the Congo, using sticks to test the depth of water and to cross swampy areas.
The findings suggest that the use of tools may predate the evolutionary split between apes and humans six million years ago.