Back before there were land animals, we know that most life on Earth lived in the seas. Fish may have evolved from corals, which, like most things, probably came from microorganisms. In the very beginning, life probably started with single-celled organisms like bacteria.
Unfortunately, most early life forms are too small to be preserved in the fossil record. And too old — there are only really a few places on the planet with rocks ancient enough to carry that kind of fossil evidence.
As a result, some of the earliest traces of life we have detected on our planet are so faint that the fossil evidence that remains only reveals the movements of these microscopic organisms.
