how come humans could successfully breed with neanderthals to produce non-sterile offspring

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I’ve been thinking about this for awhile, living things have barriers that stop breeding between different species like they either can’t get pregnant, or their offspring is sterile (liger and mule). Then how come humans have neandertal DNA. How come humans and neanderthals could produce viable offspring. They are different species right?

Edit: spelling

In: Biology

Humans have mice dna too. That doesnt mean humans and mice had children it means they had a common ancestor.

It happens. It’s mostly because our definition of species isn’t consistent (regarding sterile offspring)

Much like the coywolves today that are part coyote and part wolf, they were two distinct species that came about because of Environmental pressures. In the case of homo sapien and neanderthal, the Neanderthal ancestor had migrated from Africa earlier than the ancestors that would become Homo Sapien however it was close enough in their evolutionary Divergence that they were still compatible. Descendants of homosapien and Neanderthals still survive today, Neanderthal DNA can be found in some groups of European descent but cannot found it be found and those of pure African descent. As you can probably understand coywolves themselves are not sterile, the categorization of species is very blurry the closer on the metaphorical evolutionary tree two species are. But often many species that are close enough to be seen as distinct species and not just a subspecies of one species biologically compatible but will not produce fertile offspring. But in the rare case since you get things like the coywolf where the offspring’s itself furtile it has advantages that allow it to propagate where is parents wouldn’t be able to.

This is a very condensed version of what can happen when two species comes together reproductively.

This is one of the reasons that some classify Neanderthals as a supspecies: *Homo sapiens neanderthalensis* (in which case we’re the subspecies *Homo sapiens sapiens*).

The term species is largely arbitrary and there are a few grandfathered in cases, like this one, where modern science doesn’t affect nonclamenture due to either academic inertia or political realities. The term species began in an era before genomes were really a thing.

The two most genetically different modern humans on earth are still closer to each other than some [chimpanzees on either sides of the river from each other.](http://www.ox.ac.uk/news/2012-03-02-chimps-show-much-greater-genetic-diversity-humans)

Meaning you can have some humans wildly different from each other or far removed from modern humans and still have them produce viable offspring with other types of humans. So you have humans and Neanderthals, and humans and Denisovians, and humans with other similar species. But those groups were also regularly reproducing with each other separate of homo sapiens. In fact we have some remains from humans that were combinations of three or more species but have no homo sapiens ancestry.

Though the degree to which these species were separate to begin with is also highly debatable. There are some remains of even earlier hominids found as a group that were more distinct in and of themselves than several remains found all over the world that were previously classified as separate species.