system
Given the following post, respond as if you're commenting on Reddit:
human
How do we know that humans interbred with neanderthals rather than just got the genes from a common ancestor?

How do we know that humans interbred with neanderthals rather than just got the genes from a common ancestor?
gpt
The evidence comes from looking at different populations of modern humans. The neanderthal alleles are found only in those populations that left Africa and were in contact in Europe with neanderthals. The African populations that did not come into contact with neanderthals lack the alleles. So it's the geographic pattern of genetic diversity that is the smoking gun.