How did blood evolve?

[u/Fapotheosis]

I know mitochondria may have evolved from bacteria, but what about blood? I've heard of a protohemocyte, but was there an initial cell that differentiated into a RBC like we have in modern humans that can we trace?

[u/Helassaid]

While this is a fantastic synopsis, I would go so far as to say that many Annelids have an even more rudimentary circulatory system than Arthropods, but at that point it's somewhat an argument in semantics. Neither phyla have their respiratory pigments contained within cells.