[Ygg2]

You're overlooking the fact that even if easily breedable pool of foreign species existed along H. sapiens, given enough time they would either be purged or become common ancestors to everyone by now.

Since they were purged and exist as remnants in individuals' DNA.

[enkid]

If they are remanants in people's DNA, they did become common ancestors, no? Everyone's family tree collapses to the same individuals at 5500 BC, after the other humans went extinct.(1) That means we are all descended from non Homo Sapiens humans. What I'm trying to say is that they were not "purged," we are their descendants. We got most of our genetic material from the Homo Sapiens side, but there is continuity with the other species as well.

(1) https://www.scientificamerican.com/article/humans-are-all-mo...

[Ygg2]

Common ancestors are in all humans IIRC. While say Neanderthal DNA is only in subset of populations.

[enkid]

You don't necessarily have any DNA from any particular ancestor, but they would be your ancestor. This is explained in the article I linked. That means that if everybody's ancestors converged in 5500 BC, and at least one person had a Neanderthal as their ancestor, everyone has a Neanderthal ancestor.