Fish. They seem so innocent and harmless. But secretly, they're subverting scientific law and order.

That's according to a group of scientists nicknamed cladists for their support of a scientific classification system of species based on clades.

A clade is a fancy term for all of and only the modern species descended via evolution from a specific common ancestor.

To make that a little easier to understand, picture your own family tree: your maternal grandmother, your mom and all of her siblings, and you, all of your siblings, and all of your mom's siblings' children - they're a kind of clade.

Take away your siblings or add your father, though, and it's no longer a clade.

This is where fish get into trouble. A lot of trouble. Trouble the size of an elephant, a whale, and an emperor penguin all put together.

That's because all life evolved out of the water. Reptiles, mammals, birds - even dinosaurs - all came from something that we would say looked pretty much like a fish. And there's so much more diversity among what we call "fish" in every day conversation that they spread far around the outskirts of these subgroups.

Here's a simplified depiction of the problem at hand:

Petter Bøckman/Wikimedia Commons/Tech Insider

As you can see, there's no way to draw a clade that will encompass everything we call a fish without snagging a mouse or a manatee along the way.

So for the cladists, either there is no such thing as fish - or we're fish too.

Of course, the cladists' approach to species is useful for asking certain questions. When evolution has literally built everything you are thinking about, classifying all those things based on how evolution works makes a lot of sense.

But it's hard not to find the proclaimed death of the idea of a fish a little absurd.

This article was originally published by Tech Insider.

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