It’s the question that keeps anthropologists up at night: Who had sex with whom, and when? Now, an ancient femur bone (pictured above) is helping them get closer to the answers. Uncovered from an eroding riverbank near the village of Ust-Ishim in western Siberia, the femur belonged to a man who lived 45,000 years ago. His DNA was so well preserved that scientists were able to sequence his entire genome, making his the oldest complete modern human genome on record, the team reports online today in Nature, following up on a meeting report in March. Now for the sex: Like present-day Europeans and Asians, the Ust-Ishim man has about 2% Neandertal DNA. But his Neandertal genes are clumped together in long strings, as opposed to chopped up into fragments, indicating that he lived not long after the two groups swapped genetic material, as Science reported from the meeting. The Nature paper uses further analysis of the length of the strings to propose specific dates: The Ust-Ishim man likely lived 7000 to 13,000 years after modern humans and Neandertals mated, dating the mixing to 52,000 to 58,000 years ago, the researchers conclude. That’s a much smaller window than the previous best estimate of 37,000 to 86,000 years ago. But what does the Ust-Ishim man tell us about post-Neandertal sex—I mean, population dynamics? He’s equally related to two other ancient skeletons: a 24,000-year-old boy from Mal’ta, also in Siberia, and an 8000-year-old man from La Braña, Spain. That means he probably belonged to the population of modern humans that first moved out of Africa and spread across Europe and Asia. However, among present-day populations, the Ust-Ishim man is more closely related to East Asians than to Europeans. This adds support to the idea that living Europeans inherited some of their genes from a different, unknown source, presumably a population that left Africa later in a separate wave. Whom they had sex with once they arrived is a question that scientists are only beginning to answer.