Imagine being able to walk from Papua to Australia without taking a boat or plane. It might sound impossible, but tens of thousands of years ago, the two regions were connected by land.
Back then, Papua and Australia were part of a vast landmass called Sahul. It included the island of New Guinea, now divided between the Indonesian region of Papua and Papua New Guinea, as well as Australia and the Aru Islands.
So, what would it take to connect them again? We would not need to move the islands around. We would simply need to lower sea levels until the land now submerged beneath the water emerged.
How Could Papua and Australia Be Connected Again?
The key lies in the Torres Strait, the stretch of water separating Papua New Guinea and Australia. Parts of the seabed connecting the two regions lie only about 12 meters below today's sea level.
If sea levels dropped far enough, this shallow seabed could emerge as dry land. Something similar happened during the last Ice Age, around 26,000–19,000 years ago. At the time, global sea levels are estimated to have fallen by around 120 meters, with some studies suggesting a drop of 134–140 meters.
Why did sea levels fall? During an Ice Age, large amounts of water become trapped in thick ice sheets on land. This leaves less water in the oceans, causing sea levels to drop. When the Earth warms and the ice melts, the water flows back into the oceans, causing sea levels to rise again.
These changes repeatedly connected Australia and New Guinea by land. Over the past 125,000 years, at least one land route between them is estimated to have been available for more than 90 percent of that period.
When a Gulf Became a Lake
When sea levels fell, the changes extended beyond the land connecting Papua and Australia. The Gulf of Carpentaria, now part of the sea between the two regions, also became a vast lake.
Lake Carpentaria is estimated to have stretched about 600 kilometers from east to west and 300 kilometers from north to south. Despite its enormous size, it was only around 15 meters deep. Grasslands surrounded the lake, creating a landscape very different from the gulf we see today.
Further south, similar changes opened a land route to Tasmania. The connecting landmass, known as the Bassian Plain, is estimated to have existed between around 43,000 and 14,000 years ago.
Why Did Papua and Australia Separate Again?
After the Ice Age ended, the Earth began to warm. Ice sheets melted, and the water flowed back into the oceans. Sea levels gradually rose, covering the low-lying land that had once connected different regions.
Around 8,000 years ago, the land bridge between Australia and New Guinea was finally submerged. The route that people could once cross on foot became a stretch of water, while higher areas remained above the surface as islands in the Torres Strait.
However, this change happened after humans had already settled in Sahul. Archaeological evidence suggests that people were living in northern Australia around 65,000 years ago, long before rising seas separated Australia and New Guinea as they are today.
How Did the First Humans Reach Sahul?
One piece of evidence of early human life was found at Madjedbebe, a rock shelter in northern Australia. Research published in Nature estimates that humans lived there around 65,000 years ago.
This discovery raises an interesting question: if humans reached Sahul tens of thousands of years ago, how did they get there?
Although Australia and New Guinea were once connected, people traveling from Asia still had to cross the sea to reach Sahul. They could not walk the entire way because stretches of water separated the islands along the route.
Research published in Scientific Reports suggests that humans probably did not reach Sahul simply by being carried away by ocean currents by chance. They may already have been able to choose when to set out and steer their boats toward their destination.
In other words, early humans may have been capable of planning open-sea journeys lasting several days at least 50,000 years ago.
The story of Sahul shows how changes in nature can reshape the land and influence human journeys. When sea levels fall, regions that were once separated can become connected.
When sea levels rise, those routes disappear beneath the water, leaving islands behind. The map we know today is the result of changes that have unfolded over thousands of years.
