ELI5- Human Ancestors



I feel like there are subtle variations in size and shape of bones that vary from individual to individual of the same species. When finding remains of ancient human ancestors- how do scientists know they have found a new species as opposed to a subtle variation of the same species?

In: Biology

There’s no definite line between species. Scientists have to make a judgment call to decide when one species becomes another.

I like this image, which gives a good explanation of the concept: https://i.imgur.com/oAnfA.jpeg

To distinguish between species there’s one basic rule biologists follow: reproductivity.

In simple terms if they mate AND produce fertile offspring, they’re the same species. If they won’t mate, cannot make, OR mate and produce sterile offspring then they are two separate species.

Now you can see how that can cause problems with finding fossil remains of species since extinct. We don’t know if they could, would, or did mate with each other so to be clear they don’t know when they find a new species vs a variation of the same.

Then it comes down to just number of differences. Basically they analyze what they can and see if there are enough differences to argue that its a new species. They might be right or they might be wrong. Later someone might even prove them right or wrong. Sometimes you’ll hear in the news about a “new” dinosaur that ended up being the bones of another, or someone who thought they had a certain dinosaur and it turns out to be something entirely different.

It’s the same with proto-human species. You don’t always know when they’re close, but usually you can find some that are different enough to say for sure that they are. That’s when they compare differences to see how they change to place them in chronological order.