The sea floor is not implemented in the game yet, but it is very important for evolution.
First off, the sea floor would start having use as protection from predators. You could go through a small crack that the predator can’t fit through to wait them out. As well as this, small Cyanobacteria could attach to the floor to provide phosphates, that can’t be made by organelles. As a downside, you can’t move so predators can also easily attack you, and other species could attack to compete with you to get phosphates.
Fish also evolved from sea sponges, from something like this: choanofloggates form colonies and become multicellular, and begin forming colonies on the sea floor to make ancestors of sea sponges and starfish, and some sponges keep their larval state into adulthood as a mutation, resembling fish. This gene slowly fills the gene pool due to natural selection, and the fish is born.
How does this idea sound? Tell me your own ideas!