Genetic dates are based on a mixture of statistics and best guesses, but the split times calculated by the Danish team are compatible with the more reliable archaeological dates, which record the earliest known human presence in Australia at 44,000 years ago. The Aborigines’ ancestors could have arrived several thousand years before this date.

The Aborigine occupation of Australia presents a series of puzzles, starting with the nature of their stone tools. The early stone tools found in Australia are much simpler than the Upper Paleolithic tools that appear in Europe at the same era. “I don’t understand why they looked so primitive,” said Richard Klein, a paleoanthropologist at Stanford University.

Primitive as the tools may be, the first inhabitants of Australia must have possessed advanced boat-building technology to cross from the nearest point in Asia to Sahul, the ancient continent that included Australia, New Guinea and Tasmania until the rise of sea level that occurred at the end of the last ice age, 10,000 years ago. But there is no archaeological evidence for boats, Dr. Klein said.

Despite the Aborigines’ genetic isolation, there is evidence of some profound cultural exchange that occurred around 6,000 years ago. The stone tools become more sophisticated, and the population increased. The Aborigines did not domesticate plants or animals, but a wild dog, the dingo, first appears in the archaeological record at this time. Researchers led by Peter Savolainen of the Royal Institute of Technology in Stockholm reported this month in the journal Proceedings of the Royal Society B that they had traced the spread of the dingo across the islands of the Pacific by analyzing ancient DNA in the bones of Polynesian dogs.

The dingo originated on the Asian mainland and became part of the Polynesian domestic menagerie along with the pig, the chicken and the rat. This ensemble had reached New Zealand by A.D. 1250. How the dingo arrived in Australia is an “enigma,” Dr. Savolainen writes, because none of the other elements of Polynesian culture are found there.

Even stranger, dogs always travel with their masters, yet there is no sign yet of Polynesian genes in the Aborigine population.