Modern human’s ancestors interbred with Neanderthals and another species of early humans on multiple occasions, as per a new study. It suggests our ancestors interbred with Neanderthals and Denisova hominin on at least four separate occasions in the past.

Our improved immunity to pathogens is the result of that prehistoric mating, said the study authors. “This is yet another genetic nail in the coffin of our over-simplistic models of human evolution”, said Carles Lalueza-Fox of the Institute of Evolutionary Biology. Lalueza-Fox wasn’t part of the study.

For the study, researchers examined a series of findings from the past. These findings support a theory which states that our ancestors lived along with other human species, such as Neanderthals and Denisovans, which disappeared thousands of years ago. However, before becoming extinct, they interbred with modern human ancestors and gifted immunity to pathogens.

As per the new study, our ancestors first interbred with Neanderthals over 50,000 years ago when they expanded out of Africa. It also stated that Neanderthals might not be the only species that interbred with the modern humans’ ancestors.

Remains unearthed from a Siberian cave showed that there are another human species that shared the world with our ancestors. Three different human groups- Neanderthals, modern human and Denisovans- lived with each other about 600,000 years ago. They also interbred at a number of occasions, as per the study.

More than 1,480 genomes of people across the world were examined, said Joshua M. Akey, a geneticist from the University of Washington. To know about Denisovans, the researchers studied 35 genomes from humans from New Britain and other Melanesian islands. They concluded that the non-Africans in the research had DNA of Neanderthals. The study has also supported previous findings.