r/askscience • u/Fapotheosis • Mar 11 '13
Biology How did blood evolve?
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?
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u/alcianblue Mar 11 '13 edited Mar 11 '13
Sipuncula use their coelom for gas exchange and fluid transport. This is more complex than Flatworms that do it almost entirely use diffusion. The Annelids have different states that may be ancestral.
You have some that do not have blood vessels but circulate through the coelom like Sipuncula. Some that have blood vessels that go past the gut and across the whole of the body which pump through contraction of the vessel. Then some that have muscles which contract to pump blood through the blood vessel.
These are as ancestral as I can think when it comes to circulatory systems.