A new study indicates that ancestors of modern humans mated not only with ancient Neanderthals but also with a hominid species known as Denisovans that lived in Siberia 40,000 years ago.

The researchers uncovered clues that ancient humans and Neanderthals mated. In addition, they have also found genetic evidence of the Denisovans in modern residents of the Pacific islands of New Guinea and the Southeast Asia.

Professor Mattias Jakobsson of Uppsala University in Sweden, who conducted the study with graduate student Pontus Skoglund, said hybridization took place several different times in the course of evolution and the genetic traces can still be found in several regions around the world.

According to Jakobsson, previous studies have found two separate hybridization events between so-called archaic humans -- different from modern humans in both genetics and morphology -- and the ancestors of modern humans after their emergence from Africa.

However, the researchers recently discovered that there was hybridization between Neanderthals and the ancestors of modern humans outside of Africa and hybridization between Denisovans and the ancestors of indigenous Oceanians.

Little is known about Denisovans, who likely splitted from the Neanderthal tree around 300,000 years ago, except from a few bone fragments, a tooth and a possible toe bone. According to Jakobsson, it's hard to tell when the Denisovan and human interbreeding occurred, but since Europeans don't have Denisovan ancestry, it's likely the mating occurred between 23,000 and 45,000 years ago, after Southeast Asians and European populations diverged.

Jakobsson's research is published in the journal Proceedings of the National Academy of Sciences.