eli5 If it’s suspected that early humans interbred with other species of humans, why would they be considered different species since the offspring were obviously fertile?


eli5 If it’s suspected that early humans interbred with other species of humans, why would they be considered different species since the offspring were obviously fertile?

In: 54

It’s actually quite common. Being a different species (which is simply a scientific designation) does not necessarily mean that creatures from different species cannot interbreed to form a hybrid offspring. You may have seen stories in the news, of late, about a sudden increase in the number of brown bears interbreeding with polar bears. Each is of a different species, yet they are able to interbreed. The same thing obviously happened with early humans; despite being of different species, they were still able to interbreed, ultimately leading to the human variants that exist today.

Whether two populations can create fertile offspring is just one aspect to *consider* when defining species. If interbreeding was the only thing to consider, then we’d be shit out of luck for organisms that don’t sexually reproduce.

But also, we simply *don’t have a definition for “species”*

“Species” is mostly a man made concept. Humans like putting things into nice, distinct boxes because it can make things easier to understand and talk about. But the natural world is going to be messier than that.

The wikipedia article for species has two whole sections about how hard it is to define:

* https://en.wikipedia.org/wiki/Species#Definition
* https://en.wikipedia.org/wiki/Species#The_species_problem

And even it’s own article: https://en.wikipedia.org/wiki/Species_concept

This Darwin quote from that article sums things up pretty nicely

> I was much struck how entirely vague and arbitrary is the distinction between species and varieties

There is more to speciation than whether species breed and produce fertile offspring. Very closely related species can and do all the time. Domestic dogs and wolves or coyotes are a ready example.

Take the example of coyotes (canis latrans) and wolves (canis lupus); they can breed and produce fertile offspring. They typically don’t, but they can. Wolves are larger and hunt in packs. When people or other animals get into their territory, they leave or die out. Coyotes stay put and adapt. Coyotes are solitary or work in pairs.

To the best of our knowledge, the different human species that co-existed had enough differences physically and behaviorally to be considered different species despite being able to reproduce. This isn’t an exact science and based on how much neanderthal DNA is in modern humans, we didn’t just mate on occasion, we mated regularly. However, we typically find neanderthal and primitive sapiens separately from each other in the fossil record; as light as it is on the topic.

Think about dogs, wolves, and coyotes. Different species, because they would not interbreed without significant interference in their natural patterns.

And yet in some areas nearly all the coyotes show some degree of dog or wolf ancestry, because in a disturbed ecosystem the lines tend to blur.

And even in clean environments, some species will tend to hybridize. If there are two closely related species, one adapted for the hills and one adapted for river bottoms, maybe they will grade into each other with hybridization. Maybe the hybrids will even be the best adapted for the foothills.

Remember, a species is defined by reproductive isolation, not by reproductive impossibility.

Long story short: “species” is a somewhat arbitrary way to decide that two groups of organisms are different from each other. But that’s not really how evolution works. As one species diverges from another from a common ancestor, there’s a gradient – and often there’s a time period when they look/behave differently but can still interbreed. But overall, each population keeps to themselves and they end up on different evolutionary pathways. So it makes sense to call them different species even though they could still breed for a time.

Y’all have probably been taught the “biological species concept” in high school which is that two populations that can interbreed are considered the same species. That’s SOMETIMES true, but not always- there are many, many exceptions to that rule and other definitions of what makes a species a species. We can define species in many ways, and the ability to produce fertile/viable offspring is just ONE small part of it.

Source: I’m a biologist