The origin of the anus is still a mystery. It's hard to study something that must be millions and millions of years old and doesn't fossilize. The short answer is, we have no idea. But there are at least a couple competing theories.
Once upon a time, there were no anuses. These ingenious organs allowed our primordial ancestors to grow bigger and more complex, but scientists still don’t understand how they evolved. And they’re still grasping at a mystery that literally surrounds it: Why is the human butt so big?
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