I’ve read that all sorts of animals tend to evolve into crabs over time. It’s possible to prove that with the following experiment:
1. Throw a few fish or elephants or cookies into the sea.
2. Wait a few million years.
3. Take a look.
The odds are that they all will have evolved into crabs.
Maybe that’s slightly exaggerated, but in principle, it works just like that. I think. I mostly drew stick figures in my biology class instead of paying attention.