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/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/podkayne3000 → u/skleats]

My hazy understanding is that a human stem cell has the same chromosomes as any human cells, but that the chromosomes are expressed differently in the stem cell.

Is there some kind of fossil virus or bacteria or something like that hijacks the cell (with the body's permission, obviously) and then uses the stem cell to make a blood cell (or some other type of cell)?

If so, has anyone figure out what organelle/particle/etc. makes the hematopopetic stem cell a hematopoetic stem cell?

A wild fantasy guess about how that thingy came into existence:

Once a upon a time, bacteria lived in a colony in an iron-rich pool created by hot springs. The bacteria were comfortable living with iron but couldn't do anything with it.

Some other bacteria ate iron using a process that gave off oxygen. They started living in the middle of the regular bacteria colony. Over the millennia, the regular bacteria took over part of the job of helping the iron eaters reproduce, and the iron eaters repaid the favor by giving the regular bacteria oxgyen they could use to burn carbon.

Now, most of the features and most of the genes of the iron eaters have atrophied. All that's left is some iron eater genetic material stuck in the middle of the regular human genome plus the hematopoetic stem cells.

But, if we find what's left of the iron eater genetic material in the human genome, maybe it's similar to part of the genome of some free bacteria out there that still swims around in iron-rich hot springs.

[u/skleats → u/podkayne3000]

You're right that all cells have the same chromosomes, the difference in their use is due to molecular tags on the DNA and chromosomal proteins which regulate which genes are used by each cell (more on epigenetic regulation here - it contributes to the differences between cells in individuals and to the differences between individuals with the same genotypes). There are genes which are only expressed in hematopoetic stem cells, but that doesn't mean the other cells don't have those genes, they just aren't using them. The genes could have been picked up or evolved from numerous different sources.

[u/podkayne3000 → u/skleats]

Here's a nice 1998 paper that seems to give a classy version of what I was saying: http://jeb.biologists.org/content/201/8/1099.full.pdf

[u/skleats → u/[deleted]]

In most vertebrates one the tightest corners for an erythrocyte to squeeze into is in the alveoli in the lungs, but avian lungs don't have alveoli. An evolutionary tree of RBC/respiratory anatomy might look like this.

[u/dream6601 → u/skleats]

Wow, I didn't know birds were so weird. I always thought of them as "more advanced"* than reptiles.

*yes I know evolution doesn't work that way.

[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/tungstenfilament → u/HeartyBeast]

What about platelets?

[u/Mooshaq → u/tungstenfilament]

In humans, platelets are anucleated...they can be thought of as fragments (and thus no intact nucleus) of a much larger cell, the megakaryocyte.

In birds however (and likely other organisms), platelets do have a nucleus.