The extinct human lineage nicknamed "the hobbit" may not be a distant relative of modern humans as previously thought. Instead, hobbits may be members of the mysterious close relatives of modern humans known as Denisovans, and may have interbred with ancestors of modern humans on the islands of Southeast Asia, researchers say.
Although modern humans, Homo sapiens, are now the only surviving human lineage, other human species once roamed across Earth. For instance, previous research suggested Homo erectus, the most likely ancestor of modern humans, made its way out of Africa by at least 1.8 million years ago. In contrast, modern humans may have only begun migrating out of Africa about 200,000 years ago.
In the past 20 years, researchers have discovered many new branches of the human family tree on the islands of maritime Southeast Asia, which includes Brunei, Indonesia, Malaysia, the Philippines, Singapore and East Timor. These human ancestors include the extinct species Homo floresiensis, often known as "the hobbit" for its miniature body, as well as the even smaller Homo luzonensis. Both species survived until about 50,000 to 60,000 years ago, meaning they may have lived in the region at the same time as modern humans.
Recently, scientists have detected signs that extinct groups of humans not only overlapped timewise but also had sex with the modern humans of maritime Southeast Asia. For example, fossil DNA suggests the ancestors of modern Papuans and South Asians interbred with a southern branch of the mysterious Denisovans, who were close relatives of Neanderthals.
But even though modern people in these regions have relatively high levels of Denisovan DNA, suggesting significant interbreeding, no Denisovan fossils have been found in the region — the only traces of this enigmatic group found so far were a finger bone and jawbone unearthed in Siberia and Tibet.
Now, researchers suggest that either the hobbit H. floresiensis or its smaller cousin H. luzonensis or both may actually be southern Denisovans. They detailed their findings online March 22 in the journal Nature Ecology and Evolution (opens in new tab).
To shed light on the prehistory of maritime Southeast Asia, the study researchers analyzed more than 400 modern human genomes from across the world, including more than 200 from the islands of Southeast Asia and New Guinea. Scientists hunted specifically for genetic sequences that were significantly different from those usually detected in modern humans, because such DNA may have come from extinct human lineages such as H. floresiensis or H. luzonensis.
The new study confirmed prior work that found relatively high levels of Denisovan ancestry in people of maritime Southeast Asia, New Guinea and Australia — up to 3% to 6% of their DNA comes from Denisovans. It did not show evidence of interbreeding between modern humans and older lineages, such as Homo erectus.
The researchers also found traces of highly divergent genetic sequences in Denisovan DNA — extracted from specimens found in Siberia — that may have come from very distant relations of modern humans, which might suggest Denisovans could have interbred with an archaic human lineage such as H. erectus about 1 million years ago, before Denisovans split into southern and East Asian branches.
So what might these new findings suggest? One possibility is that H. floresiensis and H. luzonensis are very distant relatives of modern humans as currently thought, evolving from H. erectus or a similarly ancient lineage, and that Denisovans are a completely separate lineage. In this scenario, neither of these smaller-sized Homo species would have interbred with either Denisovans or modern humans.
Another more extraordinary possibility is that H. floresiensis and H. luzonensis may differ significantly from modern humans in terms of anatomy, but either or both might be closer relatives of modern humans than often suggested. In this scenario, these human species might not have differed from modern humans as much genetically as previously thought, explained study author João Teixeira, a population geneticist at the University of Adelaide in Australia. If so, either or both of these lineages might be examples of southern Denisovans, in which case, they would have interbred with the ancestors of the modern humans of maritime Southeast Asia, potentially explaining the high levels of Denisovan ancestry found in modern people there, he noted.
"Maybe H. floresiensis and H. luzonensis are not very divergent super-archaic groups as we currently assume," Teixeira told Live Science.
However, not everyone who was part of the study agreed with that conclusion. Study co-author Chris Stringer, a paleoanthropologist at the Natural History Museum in London, noted archaeological evidence suggested H. floresiensis and H. luzonensis were living in maritimeSoutheast Asia since at least 700,000 to 1 million years ago, long before the Denisovan lineage first evolved. Given that, he argued the hobbit and its cousin may be too ancient to be the southern Denisovans.
However, the oldest supposed fossils associated with H. floresiensis and H. luzonensis in the region may not actually have belonged to these species, Texeira noted.
Instead, those fossils may be traces of an earlier group. So it might still be possible that either H. floresiensis or H. luzonensis — or both — arrived later to their respective isles and could still potentially be Denisovans.
This suggested connection between hobbits and Denisovans remains uncertain because scientists have yet to successfully analyze DNA from any fossils of H. floresiensis or H. luzonensis, Teixeira cautioned.
"It's hard for DNA to preserve in the tropics," he said. "At the moment, this idea is only speculation. But H. floresiensis and H. luzonensis are definitely at the right place at the right time to be southern Denisovans."
To help fill in the missing branches of the human family tree in the islands of Southeast Asia, researchers should not only continue searching for DNA in human fossils from this region, but also look for fossils in other areas such as Australia, Teixeira said.
All in all, Teixeira predicted, "the next big find in human evolution is due to occur in island Southeast Asia."
Originally published on Live Science.