Current DNA evidence suggests that humans evolved in East Africa about 130,000 years ago and did not migrate from our home continent until 80,000 years ago; at that time, expansion of the Sahara would have discouraged migration to the north and it is thought that they followed the coast of the Red Sea (or crossed a shallow portion of that channel) and then the southern coast of Asia, reaching Indonesia about 70,000 years ago.
By that time, the Wisconsin Glaciation was underway and sea levels were much lower than they are today. In fact, the islands of Indonesia were joined to form a broad peninsula and New Guinea was continuous with the Australian Continent. While the oldest human fossils found in Australia date from 40,000 years ago, artifact evidence suggests that humans first reached that Continent between 60,000 and 55,000 years ago. Since there is no evidence of human seafaring before the Phoenician and Polynesian cultures developed, some 4000-3500 years ago, it is presumed that the first human Australians, having crossed at least 50 miles of open sea, arrived accidentally, perhaps swept southward by a tropical storm. Of course, others may have set out on rafts to find them and met the same fate.
Until additional evidence is discovered, the timing and circumstance of man's initial colonization of Australia will remain uncertain. It is known (via DNA evidence) that native populations of New Guinea and Australia share a common ancestry (though the islands separated 8000 years ago as sea levels rose) and that humans occupied all regions of Australia, including Tasmania, by 30,000 years ago. Of course, we also know that the earliest European settlement was established in 1788, at least 53,000 years after the first humans set foot on the Continent.
