Reconstructed setting· The columns of time
The long walk
75 ka → 45 ka
This backdrop shows the exit of Homo sapiens from Africa, then its arrival in Australia: a thin line of walkers, mangroves (coastal forests that grow in salt water), a watercraft at the water's edge and kangaroos.
Backdrop representing the walk of Homo sapiens from Africa to Australia: a thin line of walkers for the small number of those who left, mangroves for the hypothesis of a coastal route, a watercraft for the sea crossings and kangaroos for the fauna found on arrival.
What the evidence shows
The line of walkers is thin because those who left Africa were few. McEvoy and colleagues (2011) analysed the DNA of people from seventeen populations around the world. Their analysis traces, over time, effective population size, that is, the number of individuals who pass on their genes. Among the ancestors of those who live outside Africa today, it fell sharply at the time of the exit. This fall is the mark of a founder effect: a small group that separates from its population of origin carries only part of that population's genetic diversity. According to Hallast and colleagues (2020), people living outside Africa today descend almost entirely from those who left it in a single exit, about 70,000 to 50,000 years ago. The mangroves illustrate the hypothesis of a coastal route: these forests bordered much of the shores of the Indian Ocean, and the walkers would have followed those shores. According to Erlandson and Braje (2015), these forests offered resources that may have attracted coastal groups. The watercraft figures because access to Sahul, the continent then joining Australia and New Guinea, required open-sea crossings; the oldest known occupation there is dated to about 65,000 years, an age contested by other work that brings it to 50,000. The kangaroos represent the fauna that the first arrivals found there: Sahul was home to large mammals, birds and reptiles, among them kangaroos exceeding 200 kilograms. These giant kangaroos are extinct. Prideaux and colleagues (2010) studied the fossils of a cave in south-western Australia: the first traces of humans in the region date from 49,000 years ago and, in less than ten thousand years, almost all the large mammals had disappeared from it. They conclude that the arrival of humans was probably decisive, and recall that by about 40,000 years ago Australia had lost more than 90 per cent of its large animal species. Hocknull and colleagues (2020) conclude otherwise for the north-east of Australia, where very large animals still lived at that date and disappeared only later. There, the extinctions coincide with a lasting deterioration of climate and environment, and these authors do not accept the hypothesis of a rapid extinction caused by humans across the whole of Sahul.
Bounds and conventions
A single landscape, a mangrove shore, stands for thirty millennia, from 75,000 to 45,000 years ago, and for regions with different climates. It depicts the coastal route, which remains a hypothesis: other routes, through the interior of Asia, are also considered, and the number of routes taken remains debated. The kangaroos are drawn from living species: the sources of this page do not describe what the extinct giant kangaroos looked like.