After the development of Eukaryotes 2.1 billion years ago, with the specialization of organelles for getting chemical or light energy into biologically useful forms, and sexual reproduction and assortative mating 1.2 billion years ago, life really got swinging.

Before sex, adaptation was really a series of bets. If a bacterium found a good store of resources, it multiplied copies of itself until the resources were gone, then died off. Any propagation of its lineage was accidental migration away form the store. It must have worked well enough, but a bacterium had little way for routinely transmitting its genome elsewhere without actually visiting that place.   Bacteria do have a method of genetic spread (plasmid transfer), but I don’t know when this evolved. It was also not routine the way sex is routine. For most modern animals, clonal reproduction is simply not an option. Plants and mushrooms routinely do it, but only because their way of growing is not as structured as animal growth. By the way, thaw last common ancestor between all of these was around 3.5 bya.

Evolution is the practice of traits becoming more or less common in populations. These traits can spread a lot more quickly if individuals can trade their genes with others. Sex allowed evolution to speed up. Just in time too, as we’ve only got a quarter of the history of life left.

Once we had energy, we could move around more daily. The first flagellates, using a whip to swim through the water, were just after sexual reproduction 1.1 billion years ago. These diverged from the bacteria to form the animal kingdom by 0.9 bya, forming the single celled, but complex, protozoa by 0.75 bya. It was after several hundred million years of constant animal movement that sleep evolved (0.7 bya), a way for animals to lie dormant for a while in the relative assurance that thy would not be eaten in the night.

The length of night has lengthening ever since the formation of the moon from the Earth’s rib (Theian Collision 4.5 bya). The moon has been dragging at the earth for all that time, slowing its rotation down, lengthening the day and leaving fewer days in the year. The length of year in in hours has not changed substantially, as the sun does not care. The length of a day has increased from 10 hours to 24, and the days in a year have decreased from almost 800 to less than 400. This is important when you start considering that animals and even plants and fungi set their schedules and their sleep by sunrise and sunset. We have a lot fewer dawns than we did then, and a lot fewer dreams.

The first multi-cellular animals with nerves and muscles, sponges, were around by 0.6 bya. Flatworms (0.55 bya) even had photosensitive cells at their head. They were also the foundation of bilateral symmetry and head-tail asymmetry, which is what we expect from every animal from bug to beast. The first animals with a circulatory system for getting oxygen from the first gills to their trillions of mitochondria in their cells was the acorn worm (0.54 bya). Having a circulatory system for those 50 million year old muscles also meant they could have a waste collection system, with kidneys for flushing away used amino acids. Which is why your pee smells like ammonia. Diversification of the animal kingdom blossomed after this, forming starfish, lobsters, and clams, By 0.505 bya, some species had even developed skeletons. Just in time for the 1st major extinction (0.488 bya).

Now just about 7/8 the way through the history of life.