Anthropologists are pretty sure that Neandertals and humans interbred — and DNA analysis backs up this idea. But we don't know the date ranges involved — including when the two groups stopped hooking up.
A new study is offering some possible answers, indicating that Upper Paleolithic humans coming out of Africa stopped interbreeding with Neandertals about 47,000 years ago — which may have been before humans started to spread across the rest of Eurasia.
Neandertals reached Eurasia first. Our best estimates suggest that they first emerged in that part of the world roughly 230,000 years ago. Humans, on the other hand, made our first appearance in Africa over 200,000 years ago and we didn't start making our way up into the European continent until about 100,000 years ago.
Scientists are not exactly sure when the interbreeding between the two species first got started, speculating that it may have happened as early as when our mutual ancestors co-mingled in Africa. Genetic analysis, however, suggests otherwise.
Back in 2010, scientists successfully sequenced the Neandertal genome by using DNA from a well-preserved specimen. Analysis revealed that Neandertal DNA comprises anywhere from 1 to 4 percent of modern Eurasian genomes. Africans, on the other hand, don't share this genetic ancestry. Subsequently, the two prevailing theories suggest modern humans and Neandertals started interbreeding in Europe about 100,000 years ago, or that African populations ancestral to both Neandertals and modern humans remained subdivided over a few hundred thousand years, but then started breeding with Neandertals as they made their way into Europe during the Upper Paleolithic era.
To help solve this mystery, Sriram Sankararaman and David Reich measured the length of DNA pieces in the genomes of Europeans that are similar to Neandertals. When egg and sperm cells are formed, the strands of DNA within them form new combinations of genetic material. This process decreases the length of the chunks in each generation, thus allowing the geneticists to determine when the two populations last shared genes.
Based on their analysis, the scientists concluded that Neandertals and modern humans last exchanged genes anywhere between 37,000 and 86,000 years ago (with 47,000 years ago being the most likely). This is well after modern humans appeared outside of Africa, but likely before they spread east into Asia. The study strongly indicates, therefore, that Neandertals did have children with the direct ancestors of non-Africans.
You can read the entire study in PLOS.