Nature hints that modern humans have a mysterious X factor ancestor:
Genome analysis suggests there was interbreeding between modern humans, Neanderthals, Denisovans and an unknown archaic population. Updated genome sequences from two extinct relatives of modern humans suggest that these ‘archaic’ groups bred with humans and with each other more extensively than was previously known.
The ancient genomes, one from a Neanderthal and one from a member of an archaic human group called the Denisovans, were presented on 18 November at a meeting on ancient DNA at the Royal Society in London. The results suggest that interbreeding went on between the members of several ancient human-like groups in Europe and Asia more than 30,000 years ago, including an as-yet-unknown human ancestor from Asia.
All modern humans whose ancestry originates outside of Africa owe about 2% of their genome to Neanderthals. Certain populations living in Oceania, such as Papua New Guineans and Australian Aboriginals, share about 4% of their DNA with Denisovans.