Statistical geneticist Ryan Bohlender and his team investigated the percentages of extinct hominid DNA in modern humans and found that as well as identifying Neanderthal and Denisovan DNA, there appears to be DNA of another unidentified extinct human species. This is the result of DNA testing of people from Papua New Guinea and north-west Australia.
To read more about this research and for an explanation of the differences between Neanderthal and Denisovans, go to ABC News:
Aboriginal Australians, Pacific Islanders carry DNA of unknown human species
Also reported in Science News:
DNA data offer evidence of unknown extinct human relative
Also, here is the abstract of the presentation given at the American Genetics Conference:
PgmNr73: A complex history of archaic admixture in modern humans