Scientists have finally proved the generations-old theory that all humans can claim a single set of common ancestors.

Most experts have long argued that early Homo sapiens came out of Africa 55,000 years ago, gradually making their way into Europe, Asia and North America.

The only snag in their theory was Australia's ancient Aborigines, who seemed so different from the rest of us it was hard to prove a positive link with Africa. Until now.

Geneticists yesterday revealed they had found common DNA between peoples of Australasia and Melanesia and nearby New Guinea. Peter Forster, of Cambridge University, said: "For the first time, this evidence gives us a genetic link showing that the Australian Aboriginal and New Guinean populations are descended directly from the same specific group of people who emerged from the African migration."

Dr Forster led a team of geneticists investigating DNA links. Their findings were published yesterday in the journal Proceedings of the National Academy of Sciences.

Crucially, the scientists found no evidence that Homo sapiens from Africa had interbred with indigenous species of humans in Australia, including Home erectus.

Ancient Homo sapiens, probably thousands of years after they left their African homelands, crossed into Australia over a now long-gone land bridge across modern Indonesia and Papua New Guinea, the research shows. They had reached south-east Asia from Africa along a prehistoric "coastal expressway" through the Middle East, India and the Malay peninsula.

Modern humans, similarly, made their way to the Americas across a land bridge, now broken, that once crossed the Bering Straits between Alaska and Siberia.

Homo sapiens on the march from Africa wiped out other human species in their path, such as Homo erectus or, most famously in Europe, the Neanderthals.

Scientists reckon Homo sapiens progressed at a rate of just one kilometre every year. It could have taken 5000 years to get from Africa to Australia.

The land bridge to Australia was flooded 8000 years ago, leaving Aborigines to develop along unique lines until their first encounters with European explorers in the 18th century.

Dr Toomas Kivisild, from the department of biological anthropology at Cambridge University, co-authored yesterday's article.

He said: "There was probably a minor gene flow into Australia while the land bridge from New Guinea was open, but once it was submerged the population was apparently isolated for thousands of years."