A tiny piece of plant resin shows that humans lived on an island in eastern Indonesia at least 55,000 years ago – and thus provides insight into the likely route that modern humans took on their migration to Australia.

We know that modern humans reached Australia from mainland Asia by heading southeast, crossing what is now Indonesia and many other islands in Southeast Asia. The exact timing is disputed, says Dylan Gaffney of Oxford University. Modern genetic evidence suggests humans arrived less than 50,000 years ago, but archaeological evidence suggests an earlier arrival, “perhaps 65,000 or even 80,000 years ago,” he says.

In addition, the exact route they took is also disputed because the geography of the region was different back then. The Earth was in a cold ice age, so more water was trapped in ice sheets and sea levels were lower. This means that some land masses that are now islands were connected to continents. In the western part of this region, Borneo, Sumatra and Java were all part of the Asian mainland – while in the eastern part, New Guinea was connected to Australia.

This means that there were two possible routes that people could have taken to reach Australia. The northern route is from Borneo directly east to Sulawesi and on to New Guinea and then south to Australia. The southern route is via Java, Bali and Timor to northern Australia.

To find out how people made this journey, Gaffney and his colleagues excavated Mololo Cave on Waigeo Island, one of the Raja Ampat Islands west of New Guinea and along the northern route. In the sediments on the cave floor, the team found evidence of human habitation, such as charcoal and some stone fragments.

Crucially, Gaffney’s team found a piece of resin just 1.4 centimeters in diameter. It has a square shape, suggesting it was cut from a tree rather than accumulated naturally. Radiocarbon dating shows it is at least 55,000 years old.

The resin was probably used as fuel for fires, says Gaffney. “It’s very flammable and a good source of light in caves.” But there are other possibilities, too, such as as a fragrance or glue. Whatever the purpose, it shows that people were on Waigeo at least 55,000 years ago. “We show that people used the northern route,” says Gaffney.

The discovery supports the idea that humans first reached Australia via the northern route, says Kasih Norman of Griffith University in Queensland, Australia. Geographical models have always pointed to the northern route because crossing by sea is easier. “On the northern route, you have to make more water crossings between islands, but the crossings themselves are shorter,” she says. Plus, “you can always see the next island.”

But most archaeological excavations have focused on the southern route, Norman says. Only in recent years have researchers like Gaffney begun to investigate the northern option.

One of the most important discoveries, published in July, is a 50,000-year-old cave painting of a pig found on Sulawesi along the northern route. Similarly, a study published in May found that there was no evidence of humans on Timor 44,000 years ago. Timor lies on the southern route, suggesting that this route was used later.

A final intriguing mystery in all this concerns the absence of Denisovans – an extinct group of people known to have lived on the Asian mainland – in Australia’s fossil record. Many populations in Southeast Asia carry Denisovan DNA: These include people from Papua New Guinea, who have DNA from two different Denisovan groups. This suggested, but did not prove, that Denisovans lived on New Guinea. Yet there is no sign of Denisovans in Australia. “As far as we know,” says Norman, “there has never been anyone (but homo sapiens) Here.”