Part Human, Part Neanderthal
Many anthropologists believe modern humans and Neanderthals interbred, a contention supported by the claims of molecular anthropologists who report detecting signatures for interbreeding in Neanderthal and modern human genomes. (11) But, not all the data supports this belief.
The first strong evidence for interbreeding came in 2010, when researchers completed a draft sequence of low-quality Neanderthal genome constructed using DNA isolated from three different Neanderthal individuals. A comparison of this composite genome with representative human genomes revealed that Neanderthal DNA has more in common with non-African people groups than samples from African populations.
Researchers concluded that this was more than likely due to limited interbreeding events occurring between humans and Neanderthals in the eastern portion of the Middle East as humans began migrating around the world roughly 45,000 to 80,000 years ago. (12) That means that non-African populations appear to display a 1 to 4 percent genetic contribution from Neanderthals, whereas African people groups evidence no contribution whatsoever.
Additionally, in 2014 researchers published a high-quality genome sequence for an individual Neanderthal (13) recovered in the Altai Mountains of Siberia; that confirmed a 1 to 2 percent Neanderthal contribution to the human genome, again attributed to interbreeding between humans and Neanderthals as humans migrated out of Africa.
Other research has identified distinct genetic (14) regions in the human genome derived from Neanderthal DNA, making the case for interbreeding between humans and Neanderthals stronger yet. Nonetheless, there might be another explanation for the supposed Neanderthal signature in the human genome.
There are researchers who believe the statistical association between the Neanderthal genome and non-African people groups could be due to the ancient population substructure of the African genomes—not interbreeding. Also, research into the population size and distribution of Neanderthal groups raises doubts about the likelihood that modern humans and Neanderthals would have ever encountered one another. Recent re-dating of two key sets of Neanderthal remains indicates that Neanderthals may have gone extinct earlier than thought, in which case, humans and Neanderthals would not have co-existed.
Also, a study conducted by a large team of collaborators from Europe and China observed that the genetic variability among Neanderthals was remarkably low, suggesting they lived in small populations, isolated from one another. Likewise, analysis of the Altai Mountain specimen’s high-quality genome indicates Neanderthals frequently mated with close relatives, supporting the contention that these creatures lived in small, isolated groups.
In fact, the effective population size of Neanderthals across Europe and Asia might have been as low as 3,500 at any one point in time. Such a low population density would greatly reduce the likelihood that migrating humans would have encountered Neanderthals, making it difficult to explain how interbreeding could have occurred. (15)