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Fossil evidence of mysterious 'southern Denisovans' yet to be found
When modern humans arrived in the islands of southeast Asia, they may have encountered a range of ancient human species.
But it turns out that only one of these ancient human species left their genetic legacy in the people who live there today.
The islands of southeast Asia cover a vast area, with countless islands scattered across the seas from Taiwan in the northwest to Australia and New Guinea in the southeast. Spread across all these islands are number of diverse nations and cultures, including those in Indonesia, the Philippines, Malaysia and Brunei, collectively known as Island Southeast Asia.
Over the past few hundred thousand years, these islands have been home to a number of ancient human species who would have walked and island-hopped right across the archipelago.
The DNA of more than 400 people currently living in Island Southeast Asia has revealed the relationship between modern humans who arrived in the region 50,000-60,000 years ago and the ancient humans who were already there.
This DNA shows signs of ancestry from the Denisovans, who are currently only known from fossils in Asia, but there is no genetic evidence of the ancient humans whose bones have been found in the area.
The results of a recent study led by the University of Adelaide have been published in Nature Ecology and Evolution.
The front of the replica Sangiran 17 cranium of Homo erectus from Java, Indonesia.
A menagerie of human species
Island Southeast Asia contains one of the world's richest fossil records documenting human evolution. Some of these fossils date to at least 1.3 million years ago.
These tropical islands were once inhabited by three distinct ancient humans who have been recognised in the fossil record.
Homo erectus is the first human ancestor thought to have spread throughout Eurasia, having dispersed from Africa to reach as far east as the island of Java (in Indonesia), where the first fossils of this species were found. It is thought that H. erectus survived in southeast Asia until at least 108,000 years ago.
Island Southeast Asia was also once inhabited by two species of miniature (or 'hobbit') humans. The first of these to be discovered was Homo floresiensis, from the Indonesian island of Flores. Just two years ago, a second species of tiny human was unearthed in the Philippines and called Homo luzonensis. These two species are thought to have survived until around 50,000-60,000 years ago.
But hidden within the DNA of many modern humans living in this region today are the telltale signs of a fourth ancient human species that has usually been assigned to the Denisovans, a sister lineage of the Neanderthals, currently known from fossils and DNA from Siberia and China. However, no Denisovan fossil remains have yet been found in Island southeast Asia, although it is strongly believed that the so-called 'southern Denisovans' must have lived there.
The recent study has found that while it is possible that the ancient humans known from fossil remains overlapped with the arrival of modern humans in the region, there is no genetic evidence that they interbred with modern humans or the Denisovans.
Prof Chris Stringer, a researcher of human evolution at the Museum, was involved in the study.
'The known fossils of H. erectus, H. floresiensis and H. luzonensis might seem to be in the right place and time to represent the mysterious "southern Denisovans",' says Chris. 'But their ancestors were likely to have been in place in Island Southeast Asia at least 700,000 years ago, so their lineages are too ancient to represent the Denisovans, who are more closely related to the Neanderthals and us.'
It is currently thought that the Denisovans and Neanderthals only split from our lineage around 600,000 years ago.
By searching caves on these tropical islands, such as the Liang Bua Cave in which H. floresiensis was found, researchers hope they may find some remains of Denisovans ©Rosino/Wikimedia Commons
Why haven't we found Denisovan bones in Island Southeast Asia?
The Denisovans have so far left us little physical evidence of their existence. To date, their fossil legacy includes a finger bone, teeth and skull fragment from a Siberian cave, and a lower jawbone from a Chinese cave on the Tibetan plateau.
But just like the Neanderthals, the Denisovans have left their genetic legacy in modern humans through ancient interbreeding. As a result, billions of people in Asia and Australasia have Denisovan DNA.
Dr João Teixeira, from the University of Adelaide, is the lead author of this latest study. He explains, 'We know from our own genetic records that the Denisovans mixed with modern humans who came out of Africa 50,000-60,000 years ago both in Asia, and as the modern humans moved through Island Southeast Asia on their way to Australia.
'The levels of Denisovan DNA in contemporary populations indicates that significant interbreeding happened in Island Southeast Asia.
'The mystery then remains: why haven't we found their fossils alongside the other ancient humans in the region? Do we need to re-examine the existing fossil record to consider other possibilities?'
Exciting discoveries ahead
The next major discovery could be fossil evidence of the Denisovan people in the islands of Indonesia and perhaps even Papua New Guinea and Australia.
Fossils preserve particularly well in caves, so caves on the islands of Sumatra, Borneo and Sulawesi may be the most promising places to start looking for physical evidence of the enigmatic southern Denisovans.
