I think the filter was eukaryogenesis [1]. (Which in turn depends on the endosymbiosis of mitochondria.)
Put simply, I expect the universe is littered with single-celled life. I think multicellular life, on the other hand, is rare.
Put simply, I expect the universe is littered with single-celled life. I think multicellular life, on the other hand, is rare.
If your thesis is specifically that oxygen-powered metabolism fueling multicellularity is rare... that wasn't clear. And I'll still say that endosymbiosis being the shortest path to scaling oxygen metabolism is probably Earth happenstance (with a reminder that cellular endosymbiosis is also common). But some critter had to originally evolve oxygen metabolism before being turned into mitochondria. What if that had evolved in a cell prepared to directly take advantage of it?
But no multicellular prokaryotes. You're right that eukaryotes can be single celled. But my hypothesis is that multicellular life is rare because eukaryotes are rare.
Which in turn depends on existing enough atmospheric oxygen for mitochondria to make sense in first place. I believe the filter is atmospheric oxygen. If Earth had more iron in the crust, perhaps the cyanobacteria would never finish oxidizing all of it, and we would be doomed to only host microscopic life forever. Macroscopic life requires high-energy metabolism molecular oxygen allows.
We have lots of iron , but most of it sank to the core during the "iron catastrophe" . And Mars is red because of theres so much iron.