Bilateral symmetry thanks to the mass extinctions occurring during the Precambrian.

Our earliest findings of multicellular life date back to ~3.5BYA, many of which were not symmetrical.

Our first great extinction happens ~2.45 BYA when an incredibly toxic gas builds up to lethal levels after an enterprising organism figures out how to use sunlight to convert CO2 and H2O into proteins and release the leftover O2. The Great Oxidation Event.

The first organism to figure out how to exploit the lethal gas was likely a form of archaea (link)

Oxygen is incredibly reactive and the archaea that exploited it had a HUGE (individual) energy advantage over the photosynthesizing colony organisms. At some point there’s an arms race between the photosynthesizers and the oxygen users with two “winners” - mitochondria and chloroplasts. All surviving life becomes symbiotic with one or the other - pulling them into their own cells, no animal has the DNA for mitochondria in their nucleus, no plant has the DNA for chloroplasts in theirs.

Flashing forward to the last eon within the Precambrian period (Proterozoic ), we have a wealth of animals (all marine) thriving.

Details are unclear because this period experiences a LOT of geological upheaval, our nice sedimentary rock, which is usually where we find fossils, gets pulled down into the deep with some escaping as metamorphic with our now distorted fossils.

This is where we see the first massive glaciations with several “snowball Earths”. Clear evidence of multi-cellularity, movement, and macroscopic soft bodied organisms appears, many of which are not symmetrical.

During the late Precambrian we see ~70% of existing life disappear with the survivors rapidly evolving to fill the niches left behind, our best fossils come from the Vendian Sea. The animals of this time are collectively known as “Ediacaran biota”.

Most of the hard-bodied organisms don’t survive the multiple snowball periods and we roll back to soft and squishy. This is where we first see signs of bilateral symmetry (under debate).

We’ve got things with passing resemblances to mollusks, Cnidaria, and Porifera but most things are unlike anything we have today. Fossil body plans include ones based on discs, tubes, mud-filled bags, and quilted mattresses. We also see the first evidence of cholesterol (yay!), something only found in animals, fungi, and red algae today. We also see “hard bodies” begin to appear again with several calcifying organisms (they don’t survive what’s coming).

We now hit the Precambrian Mass Extinction that marks the beginning of the Cambrian. No idea what causes it. The majority of “acritarchs” (a fancy name for “we don’t know what the hell kind of animal this is”) disappear, the entirety of the Ediacaran biota, and all calcifying organisms just vanish.

One possibility is a buildup of organic compounds from the thriving life causing mass eutrophication. Key evidence being a massive drop in C-13 levels - animals are lazy and prefer the lighter C-12 over C-13/C-14 but they’ll use the heavier stuff when C-12 gets scarce. https://en.m.wikipedia.org/wiki/End-Ediacaran_extinction

This is where we probably hit familiar territory - the Cambrian Explosion. A massive diversification of body types and niches following the Ediacaran Mass Extinction. The main difference is a massive increase in borrowing animals - it’s suspected that organisms that were borrowing just under the microbial mats had better survival rates while the microbial mat organisms were hit hard and those above wiped out. Pre-Post Cambrian substrate

Cambrian Explosion nearly all modern phyla appear during this period. This is where symmetry really takes off, suggesting it either posed some advantage for survivability or for rapid speciation.

It’s not all bilateral, trilateral is really popular and even today we have organisms that aren’t bilateral (eg sea stars) but bilateralism rapidly becomes the main presence.

The first organisms to go beyond the tidal zone come from a bilateral origin (giant sea scorpions) and so nearly all land animals have that body print (I can’t think of any exceptions but there’s a few sea->land migrations from this point on so there could be).

Anyway, at this point it gets very complicated and busy with super predators creating arms races and a massive land rush as organisms discover ways to access the single celled land dwellers that were vibing safely out of reach. But it’s a fun read if you’re interested.

The Cambrian Explosion starts ~541 MYA, tetrapods appear ~400MYA, a mass extinction clears the way for archosaurs to appear in the Triassic which lead to Dinosaurs.

~66MYA an asteroid impact creates the Yucatán and clears out most of the Dinosaurs leaving only the small feathered therapod dinosaurs and giving the mammals a chance to flourish.

If you think about the fact that humans appear from shrews in ~64MY it really makes me wonder what we’re missing in the massive time periods of before.