[iacopo.russo (OP)]

Alexander from Auckland (New Zealand) wrote to us to ask:

If we accept that Neanderthals and human are different species, interbreeding resulting in fertile offspring should, by definition, be impossible or at the very least extremely rare. How then do you account for the fact that up to 15% of the human genome is considered to be of “Neanderthal origin”?

What do you think?

[alancalverd]

There is no rigid definition of species. Indeed if there were, evolution would be impossible.

[evan_au]

Another group of modern humans have inherited DNA from the Denisovans - apparently another group of ancient humans living in Asia (compared to the Neanderthals in Europe).

With the ability to travel to the other side of the world in 1 day (before COVID), we often forget that just a few centuries ago, most people didn't move far from the village where they were born. That can lead to a lot of isolated populations, and, if taken to extremes, development of subspecies.

I expect that the human family tree is not just a linear thing leading to us, but is a lot more "bushy" than many people think.
 https://en.wikipedia.org/wiki/Denisovan