Prehistoric species of men apparently weren't too finicky in their mating habits. A study done in India to detect the migration of hominins in Asia through genetics was small, but had startling results: the discovery of a new human ancestor, at least for some southeast Asians.

This mysterious hominin coexisted with the Neanderthals and Denosivans, but was a different species. No remains of it have been found yet – just distant echoes of its genes in Andaman islanders existing today.

The traces of the mysterious humanoid's DNA were found in 10 islanders belonging to the Jarawa and the Onge tribes, who inhabit the Andaman Islands, deep in the Bay of Bengal. The sequences were compared with genetic sequences of 60 people from mainland Indian populations, with different ethnic histories, and with the publicly available data from other populations, the scientists explain.

The study's results are definitive evidence that the human ancestor Homo heidelbergensis gave rise to more lineages than just Neanderthals and Denisovans, author Partha Majumder of the National Institute of Biomedical Genomics in Kalyani, West Bengal told The Hindi. The known species - today's humans, Neanderthals and Denisovans (and presumably Red Deer Cave Man too) stemmed from a common ancestor about 600,000 years ago, Majumder added.

Open gallery view Google Map of Andaman Islands, in the Bay of Bengal. Credit: Google Maps, elaboration by Haaretz

The study also supports the theory that all Asian and Pacific peoples existing today share a single origin, say its authors led by Mayukh Mondal. They all stem from a single wave of prehistoric expansion out of Africa, contradicting an earlier theory of two independent waves of migration, say the scientists.

New branch on the human tree

The research produces a new branch on the tree of human evolution, starting with an Unknown Ancestor some 600,000 years ago. Homo sapiens and Neanderthals split off from that Ancestor, and Denisovans later split off from the Neanderthal branch.

Thing is, it now seems that a third species, the newly discovered one, also split off from that Unknown Ancestor.

Later in the course of history, Homo sapiens who left Africa mated with Neanderthals. Members of that human-Neanderthal mix continued on to Asia, where they split into a host of peoples. And at least some of those people, the ancestors of the Andaman islanders checked in this study, mated with the third ancestral species.

Flash forward to now: The only population not to contain any Neanderthal or Denisovan genes (insofar as has been tested so far) are Africans. In other words, the interspecies sex with both happened after the split between the early Homo sapiens who left Africa and those who remained.

Open gallery view Denisova Cave in Siberia, Russia, where the findings the research relies upon were found. Credit: Bence Viola

Now we have evidence of a third human species interbreeding with man – with a very specific group of men. Or at least that's all that has survived. "This ancestry is absent from Europeans and East Asians," writes the team in their paper.

Denisovan genes have been found throughout populations in Asia, chiefly southeast Asia and most strongly, in Papua New Guinea – but are nonexistent in the Andaman islanders.

Genetic ghosts

The intricacies of Homo sapiens' merrymaking with other human species remain speculative. In fact not everybody buys the theory – some think the genetic signals of Neanderthals in humans are "genetic ghosts" of the ancient common ancestor.

Others are believers and are getting highly specific. A discovery this February of human genes in Neanderthals (as opposed to Neanderthal genes in humans) birthed, for instance, the theory that a group of Homo sapiens split off, left Africa at least tens of thousands of years earlier than the group that founded Modern Man), mated with Neanderthals at least 100,000 years ago and then went extinct.

Anyway, it is becoming clearer that human evolution is messier than we like to think.

Note two other human species that coexisted with modern men in fairly modern Asia. One is the so-called "hobbits", Homo floresiensis, a diminutive species about a meter in height that had existed on the Indonesian island of Flores for at least 700,000 years and survived until, it seems, about 60,000 years ago. Its origin remains hotly debated.

Another human that coexisted with us is Red Deer Cave Man, which survived until some 10,000 years ago. All the scientists found of Red Deer Cave Man was a thigh bone, which showed that like the so-called hobbit, it was also pretty small. But, its finders say, that bone seems like the thigh bone of Homo habilis and early Homo erectus, which predated man by some 1.5 million years, not like a leg bone of man himself.