Let's play an evolution game set in the primordial oceans, shall we?
The rules are simple:
1) evolve a creature by modifying their picture and giving a plausible reason.
2) only a single modification/upgrade may be made per 'update'.
3) you can choose different species to upgrade.
4) I'll be plopping some events which you can adapt your creatures to, and decide if any updates are invalid.

One of the creatures: the Sluck. It is a slow-moving bottom-dweller which eats primitive moss and detritus.