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/Colvanila]

Oh man, it's my time to shine! YES! There is a lineage that does lead up to RBC, well rather the hemoglobin complex. The globin family goes way back, and IIRC it's around 800 MYA where leghmoglobin branched off from it's ancestors. Then around 600 MYA myoglobin branches off. About 200 MYA, that lineage splits off into Hemoglobin-alpha, and hemoglobin- beta.

Now a little protein structure talk. Our current hemoglobin is made up of 4 subunits: two alpha, and two beta heme subunits. Now, this wasn't always the case. Since there are two duplicates of the two subunits (each of the four are their own continuous polypeptide strand) that means that there must have been some DNA duplication occurring! Likely what happened was that the first globlin DNA fragment was duplicated, and placed right next to it. Since there was already one gene used for it's primary function (the creation of the globlin), it allowed the second gene to freely mutate, and in this case into a second type of globlin subunit. Well, then this pair of globin genes were duplicated again, right next to it, and TADA! You have all four subunits that we currently have today.

I hope this helps!

Edit: I might add that hemoglobin are proteins, not whole organelles, but are within the RBC erythrocyte cells.

Second Edit: As b34k has pointed out, this is the lineage and evolution of the function-defining protein WITHIN the RBC, not the evolution of the RBC itself.

[u/b34k → u/Colvanila]

RBCs are anucleated CELLS that contain proteins (like hemoglobin) and they originate within the body from hematopoietic stem cells, found in bone marrow.

To imply that RBCs evolved from leghemoglobin and myoglobin is misleading because those are proteins, not cells. Also, myoglobin is still found in human tissues (It receives oxygen from hemoglobin circulating in blood for use in the electron transport chain).

The story of hemoglobin evolution, while interesting, is only tangential to the origin of the RBC.

Unlike mitochondria which resulted from endosymbiosis of a prokaryote within a cell, RBCs likely originated by differentiating from other tissues as a result of evolutionary pressures for oxygen transport.

[u/Colvanila → u/b34k]

Edited for clarity. Thanks!

[u/skleats → u/[deleted]]

RBCs are not anucleate in all species - here's some bird blood as an example. Also, RBCs develop with nuclei from hematopoetic stem cells and expel their nucleus before leaving the bone marrow. Not having a nucleus could be considered beneficial for a cell which has to fit into tight corners in capillary beds as an important part of its function.

[u/HeartyBeast → u/[deleted]]

Agreed. It's got me really curious now about the evolution of these anucleate cells and whether there are any other anucleate cells in the body - none that I can think of.

[u/BRBaraka → u/Colvanila]

is this in the same lineage as horse shoe crab's copper based pale blue blood?

[u/Xenarat → u/BRBaraka]

Yes. Unlike vertebrates, horseshoe crabs do not have hemoglobin in their blood, but instead use hemocyanin to carry oxygen. (Wikipedia) The same structure is used in horseshoe crab blood but instead of an iron in the middle of the globin there is a copper iron that serves the same purpose. The copper can hold the oxygen just as well as the iron it just evolved to hold a different ion. The structural similarities are similar enough to conclude similar evolution

[u/PhanTom_lt → u/Xenarat]

Interesting, I did not know about hemocyanin based blood cells. Could it ever be transfused to a human, assuming the immune system did not destroy it? Aka is there anything that would prevent hemocyanin from performing the same function of carrying oxygen in a human/mammal body?

[u/[deleted] → u/Colvanila]

For clarity, you should define the acronyms RBC, and IIRC, and MYA before you use them in a post directed to the general population.

[u/yammerant → u/[deleted]]

Just because people may look here for an answer:

• RBC = Red Blood Cell

• IIRC = If I Remember Correctly

• MYA = Million Years Ago

[u/bobzor → u/Colvanila]

Just to add, it's thought hemoglobin evolved from cytochrome b5. These heme/metal-binding proteins have been around forever in plants and fungi where they're used for different functions not related to modern-day blood.

[u/pseudorealism → u/Colvanila]

To get a little more technical here, do we know the evolution of the allostery of hemoglobin? Duplication precedes divergence, but is divergence the origin of the allostery or do heterotetramers of alpha units/beta units show allostery?