Of all the continents, the human settlement of Oceania remains the most mysterious. We know that ancient humans walked across the now sunken continent of Sahul to reach Australia, and now some ancient trees reveal when they did it.
While it's easy to understand how humans got from the species's birthplace in Africa to Eurasia - after all, they're connected - it's not as immediately obvious how humans reached the Americas or Oceania. The land bridge that once connected Alaska and Siberia explains how humans reached the Americas, but what about Australia, which is separated by thousands of miles from the nearest major landmass?
The answer is that, until just a few thousand years ago, Australia wasn't an island continent. As recently as 18,000 years ago, the islands of New Guinea, Australia, and Tasmania were highland regions linked together by dry lowland areas. Around that time, the final ice age ended and the glaciers receded, causing water levels to rise and the lowland areas to submerge, becoming what we now know as various small seas and straits.
So how do humans fit into this picture? There's evidence for humans on Australia dating back some 40,000 years, but we still don't have a great grasp on the timeframe in which they crossed from Asia to Sahul and down into modern Australia. That's now beginning to change as archaeologists have discovered clear evidence of humans in the New Guinea highlands some 50,000 years ago. This habitation, in what is now the country of Papua New Guinea, is the earliest known presence of humans on the former Sahul landmass.
The settlement reveals the people there chopping down trees and cooking food from throughout the highland regions, as well as reaching out into lower altitudes to gather starchy yams. The trees were likely cut down with stone axes to let more sunlight shine down on the plants that the settlers wanted to eat and use.
Archaeologists speculate that this was one of many temporary settlements throughout New Guinea, allowing the settlers to move quickly and easily throughout the highlands in search of the best food. This mobile, nomadic lifestyle would have then set the stage for the final migration to Australia a few thousand years later.
[Original paper at Science; additional perspective here]