E.V.O. is all about evolving your creature to survive in the different eras of prehistory, and there are a few hidden surprises waiting for you.

For instance. Make it pretty far in to the game and you'll hit the stage Final Ocean

Hop in and you'll be given the ability to swim. But check out the Hands and Feet option in your E.V.O. menu.

Try to evolve and you end up looking like a fish thing.

Go back to Hands and Feet and choose 'Evolve Further' to... evolve further (natch).

And again

And again

Boom! Mermaid!

Why would you want to do this? The mermaid has 100 HP, the strongest bite of any of your creatures so far, and is the fastest swimmer, so there's really no reason to not do it.

Also, once you're finished with this stage, you get your old body back, so enjoy it while you can!