EVOLUTION: Humans didn't descend from trees

Humans are closer to apes than has been imagined, according to a controversial theory that suggests chimpanzees once walked upright but then returned to the trees. An Australian scientist, Simon Easteal, says humans and chimpanzees diverged from a common origin, between 3.6 and 4 million years ago. This is much later than the generally accepted belief of anthropologists that the evolutionary change occurred 5 million years ago or earlier.

Mr Easteal, of the Australian National University in Canberra, arrived at the conclusion after recalculating the frequency at which natural genetic mutations occur. If he is correct, the earliest fossils that appear to be human - dated about 4.4 million years ago - cannot be human after all.

Mutations of a given gene in a group of related species are thought to occur at a fairly constant rate as they evolve. Working from fossil evidence indicating when evolutionary changes occurred, scientists have worked out an average genetic mutation rate for human development.

Mr Easteal told New Scientist magazine: "We didn't come down from the trees - they went up into the trees."