Over many generations, the Neanderthal chunks become shorter and shorter as recombination repeatedly cuts them into smaller pieces.

The longer the Neanderthal segments preserved in an individual’s genome, the more recently the admixture occurred.

Short, highly fragmented segments mean many generations of recombination have passed — ancient admixture.

Long, relatively intact segments mean fewer generations of recombination — recent admixture.

This segment-length analysis acts as a molecular clock.

It doesn’t tell you the absolute date of admixture directly — that requires converting generation counts to years, which depends on assumptions about generation time — but it provides a relative measure of how recent the admixture was.

And crucially, it allows scientists to compare different ancient genomes and determine which individual’s Neanderthal admixture was more recent.