So let's talk about very early creepy-crawlies.
First, creepy-crawly is a wild misnomer here because I want to start at cyanobacteria, and it would be 2.5 billion years after their appearance that even the earliest cilia evolved. Cyanobacteria are the earliest photosynthetic prokaryotes we know of. (Prokaryotes are single-celled bacteria lacking a nucleus or other organelles) The life we're talking about is so early that a cellular nucleus would be considered advanced technology, and it took nature almost another billion years to think one up.
Now where were we?
The oceans formed (probably via comet bombardment) about 4 Gya (Gigayears ago). Anaerobic (i.e., non-oxygen using) bacteria are probably on the scene shortly thereafter, and the first cyanobacteria show up half a billion years later, about 3.5 Gya. We know when cyanobacteria arrived at the party because they left behind a sticky gelatinous substance that trapped and bound together sediments in structures known as stromatolites. Fossilized stromatolites are numerous and readily available for study with notable concentrations in Australia, Mexico, and South Africa, and their ubiquitous nature has lent itself to fairly consistent radiometric dating.
So cyanobacteria are on the scene 3.5 Gya when the earth's atmosphere is largely composed of carbon dioxide. Sunlight and as much carbon dioxide as a young cyanobacterium out-on-the-town could possibly drink results in the obvious effect: the dominant life-form on earth becomes vast mats of cyanobacteria, using CO2 and photosynthesis to produce energy and expelling O2 as a by-product. The recent discovery of tiny fossilized hats and beards indicate that these cyanobacteria were also probably the first hipsters.
The next billion years or so are excruciatingly dull from an evolutionary standpoint... not much happens.... just vast mats of green bacteria photosynthesizing away, sucking in CO2, releasing the resulting oxygen into the atmosphere, and boasting that they discovered your favorite band back before you had even heard of them.
The funny thing about oxygen is that, as an electron-acceptor, it will bind with almost anything. Early earth had a great deal of iron lying around, so for quite some time that iron served as an oxygen sink, binding to the oxygen as fast as it could be created and forming layers of iron oxides that can still be seen all over the world in banded iron formations. Iron was the major oxygen sink, and there were a few others, but for almost a billion years, those oxygen sinks were able to bind virtually all of the oxygen that the mats of cyanobacteria produced.
Then... slowly but surely, those oxygen sinks filled up, and atmospheric oxygen concentrations began to rise. Did I mention that oxygen is toxic to most of the anaerobic bacteria that almost certainly occupied the earth at that point? This rising concentration of oxygen suddenly (OK, over several million years) accumulating in the atmosphere killed virtually all life on earth in an event known as the Great Oxygen Catastrophe. Between this massive build-up of oxygen wiping out virtually all anaerobic life and the lowering concentration of available CO2 due to the increasing oxygen, something like 99% of all living things on the Earth's surface died.
We all know the story after that: with no competition from anaerobic microbes, aerobic microbes take over, the Pre-Cambrian explosion occurs, evolution does its thing, a time traveller stumbles off the path, accidentally steps on a butterfly, hears a sound of thunder, and then a fascist gets elected.
Hey, did anyone notice me blithely passing right over those anaerobic prokaryotic bacteria that were here before the rise of the cyanobacteria? (Smooth, wasn't I?). Yeah, we should probably talk about that... and Mars.
There's an Angry Orchard Cider calling my name so... to be continued.