Our DNA suggests that ancient humans interbred with both Neanderthals and Denisovans, two species of now extinct hominids. However, a recent study has brought a new puzzling piece to the story of man. In the DNA of Pacific Islanders, scientists have found strains of genetic material from an “unknown source” suggesting that there was a third unidentified hominid species that also bred with humans to create modern man.

The study found that people from Papua New Guinea and north-east Australia carry small amounts of DNA of an unidentified, extinct human species, ABC News reported. The finding, though shocking, is not that far-fetched; in 2012, another group of researchers suggested that some people in Africa have remains of DNA from an extinct hominid species, Science News reported. In addition, scientists didn't discover the existence of Denisovans until 2008 when paleoanthropologists discovered a 40,0000-year-old tooth and pinkie bone from a young girl in a Siberian cave.

Now, the new finding suggests that a third still unidentified extinct species of hominid also interbred with ancient humans.

"We’re missing a population, or we’re misunderstanding something about the relationships," Ryan Bohlender, a statistical geneticist from the University of Texas, told Science News, adding that “human history is a lot more complicated than we thought it was.”

It’s only recently through the help of DNA sequencing that scientists noted the possibility that ancient humans interbred with their other hominid cousins. All human ethnic groups, apart from Africans, have very small traces of Neanderthal DNA. This suggests that encounters between humans and Neanderthals occurred just after small groups of ancient humans left Africa, but ceased before humans began to evolve into their own separate ethnic groups, National Geographic reported.

“Modern humans and archaic humans have met many times and had many children together,” Mattias Jakobsson, an evolutionary geneticist at Uppsala University in Sweden told Science News.

While research suggests that this Neanderthal DNA helped ancient humans better cope outside of Africa and adapt to their new environments, today it may only be a hindrance. For example, one study linked Neanderthal genes in humans to a number of modern-day human health problems such as conditions affecting our skin, immune system, metabolism, and mental health, including depression, myocardial infarction, and blood disorders.

There are even fewer remainders of Denisovan DNA in human genomes, with Europeans having no trace, and those of other non-African descents having very small amounts.

The evidence may suggest that there was another unknown hominid species that contributed to modern-day human DNA, but there could be another suggestion. Rather than being another species entirely, the DNA could be from another group of Denisovans that were separated for so long they became genetically different, although still belonging to the same species — similar to the genetic diversity between Europeans and Asians, Science News reports.

Source: Malaspinas AS, Westaway MC, Muller C, et al. A genomic history of Aboriginal Australia. Nature . 2016

Read More:

Our Immune System Genes Tell An Ancient Human/Neanderthal Love Story: Read Here

Neanderthal DNA Linked To 12 Human Disorders, Including Depression: Read Here