If Homo sapiens and Neanderthals are different species, how did they interbreed and produce the fertile offspring needed for modern humans to have some Neanderthal DNA?

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I was told that two seperate species couldn’t interbreed and produce fertile offspring, so I don’t understand how we ended up with some Neanderthal DNA.

In: Biology
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Two different separate species cannot. But similar animals can. For example, you can mate some wild felines with domestic felines.

So the theory is that the early proto humans were close enough in DNA that they could mate. Its very possible that mating wasn’t successful all the time, but enough of the time for the genes to mix. And as they mixed, it would make sense that more and more compatibility would develop.

Would be more like breeding different dog breeds than completely separate species.

It’s quite common for separate species to be able to interbreed and produce fertile offspring, what you were told was just the very oversimplified school version. Cats can crossbreed with a number of related species, wolves can cross with dogs and coyotes, Bison can cross with cattle (despite being in different genera!) and dolphins can cross with false killer whales. There are many more examples.

The Neanderthal are sometimes classified as *Homo neanderthalensis*, and as *Homo sapiens neanderthalensis* by others.

Fun fact:. ‘th’ in German is pronounced as a ‘t’, and the Neander valley changed it’s spelling to ‘Neandertal’ when the Germans changed all the ‘th’s to ‘t’s around 1900, but the hominids retained the old spelling. It’s still properly pronounced Neandertal, but mostly by anthropologists.

Anyway, species definition isn’t so cut and dried. It sometimes has more to do with whether two species *do* interbreed, more than whether they *can*. Some bird species can, but have different birdsongs, so they don’t. Sometimes it’s physical distance or population isolation. Sometimes by DNA. There’s no general agreement and no one definition fits all cases. Particularly for extinct species.

Species within the same genus can generally interbreed and often create fertile offspring. The simplified school version doesn’t look at the classifications above species.

Both Neanderthals and Homo Sapiens belong to the genus Homo

The same way horses and donkeys reproduce mules, their genetics are just close enough to mix but just different enough to have to be considered different species, not just an adaptation