They can infer certain physical characteristics from well-characterized genetic variants.

And one of the most powerful tools they have developed is using the length of Neanderthal DNA segments preserved within a modern human genome as a proxy for the recency of Neanderthal admixture.

The logic is straightforward once you understand it: When humans and Neanderthals interbred at some point in the distant past, the resulting offspring carried chunks of Neanderthal DNA interspersed throughout their otherwise modern human genome.

With each subsequent generation, those chunks get progressively broken up by a process called recombination — the shuffling of genetic material that occurs during reproduction when corresponding chromosomes exchange segments.