You realize fossilization is actually pretty rare, right? Like the conditions to create fossils only happen once in a while and so we only get little snapshots here and there through time. I believe the oldest fossil we know of that demonstrably had blood was an early arthropod called Marrella splendens from the Burgess Shale, around 500 million years ago. Based on the somewhat advanced development of Marrella, it’s assumed that the first emergence was probably significantly older, probably around 600-700 million years ago in the late Proterozoic but aside from that, we don’t have a lot of evidence of what the animal got there first was.
Animals like sponges, anemones, and cnidarians like jellyfish don’t actually need a circulatory system, each cell just absorbs what they need straight from the sea water because their tissue is so thin.
Educated guess? Probably something that looked a lot like a worm or a slug with a hydro-static skeleton. They had already developed muscles to move fluid around to wiggle. That’s backed up by other evidence.
Makes you wonder where the boundary was, like, the first organism to have blood whose parent(s) didnt have blood. How did such a crazy adaptation ackshualy play out
I mean, there likely wasn’t a sharp cut-off. When you get into the gritty details, “blood” is just water with nutrients and minerals, enzymes, hormones and various free-floating cells in it. Muscles had been around for a while when hydro-static skeletons developed. Then something started storing stuff in the interstitial cavity that those muscles pushed against and that would get circulated when the body moved. Can you say for sure where the line is between “liquid bones with stuff in them” and “blood” definitively is? First time a hemo enzyme formed that carried oxygen? First time a cell started floating free inside doing garbage clean-up?
Humans like to think in hard lines, but biology doesn’t work that way. It’s a bunch of tiny changes that don’t mean much by themselves slowly adding up to something unrecognizable to distant past generations. No borders, no hard edges, just tiny deviation after tiny deviation until one day the infinitely great-grand-child of a compsognathus wakes up a turkey.
https://onlinelibrary.wiley.com/doi/pdfdirect/10.1111/jth.12253
Existing evidence suggests that the blood vascular system first appeared in an ancestor of the triploblasts over 600 million years ago, as a means to overcome the time-distance constraints of diffusion.
Diffusion?
Oxygen, nutrients, waste products… If you’re small enough, diffusion is enough to spread everything throughout the organism
