It seems to have arisen between 21 and 33 million years ago, when primates were becoming more human-like.

The gene emerged about the time the path that led to humans, chimps, orangutans and gorillas was splitting off from that of old and new world monkeys.

The gene could have duplicated itself, creating many new ones specific to humans, according to researchers at Harvard University in Massachusetts.

Genetic clues

Science has long sought to explain why we are different from our closest animal cousins - the primates.

Our findings have potential implications for understanding genetic differences between humans and other primates Dr Daniel Haber

Knowledge of the human DNA sequence gained by the Human Genome Project allows the question to be explored by comparing stretches of DNA.

The newly-discovered gene, known as Tre2, is found in very few mammals apart from humans and their closest relatives.

It is absent from more primitive primates such as the lemur, but is found in higher primates such as gorillas, chimps and orangutans.

Ascent of humans

The gene seems to have emerged when two other genes fused together during the evolution of higher primates.

Half of it is similar to an ancient gene found in many animals, while the rest has much in common with a gene confined to human-like primates.

Its sudden appearance relatively late in the history of the animal kingdom could have been the trigger for the evolution of humankind, although so far this is only a theory.

The products of the gene are found mainly in the testes, so the researchers think that it may be linked to human reproduction.

"Our findings have potential implications for understanding genetic differences between humans and other primates," says team leader Dr Daniel Haber.

The research is published in the Proceedings of the National Academy of Sciences.