[ExI] evolution problems

[Stuart LaForge]

Darin Sunley wrote:

> One thing that's always puzzled me about the standard [evolution]
> narrative: how do unicellular creatures evolve into multi-cellular
> creatures with tissue differentiation, that also use gamete cells for
> reproduction that contain the DNA of all of the tissue types?

Sexual reproduction predates multicellularity. If you want to understand
sex by some of the first colonial microorganisms, a pre-cursor to true
multicellular organism with differentiated gametes then check out volvox.

It is a flagellated colonial algae predating the split between plants and
animals with characteristics of both.

http://www.sciencemag.org/news/2004/06/sex-gets-algae-through-hard-times

> It's easy to
> see how you can go from unicellular creatures to colony creatures with
> tissue differentiation. And it's easy to see how you go from unicellular
> creatures reproducing via fission to unicellular creatures reproducing
> sexually. The problem seem to be that, once you're a multi-cellular colony
> creature with differentiated tissues, it's very hard to see how you get
> the DNA from all the subspecies back into a single subspecies of gamete
> cell.

The chromosomes contain the DNA for all the cell types. Meiosis is how it
is ensured that each gamete gets just one copy of each chromosomes. The
full mechanism of meiosis is still a field of active study, but what we
know so far is that there is some pretty sophisticated naturally-evolved
molecular machinery that manages to sort your chromosomes during meiosis
to make sure that each gamete gets one and just one of each type of
chromosome from either mommy or daddy with a 50% chance of either. Spindle
fibers are what actually do the separating of chromosomes.

> Conversely, it's also difficult to see how, if you're a unicellular
> species that reproduces sexually, how to you accumulate and absorb other
> species for tissue differentiation?

You accumulate the additional genetic information through sloppy DNA 
recombination that allows genes to occasionally get duplicated. Once a
gene a gets duplicated, the cell can have one copy of the gene perform its
original function while the other is free to evolve a whole new
differentiated function. Microorganisms can also absorb novel genes from
other species but gene duplication is equally if not more important to
cell differentiation, multi-cellularity, and the evolution of novel traits
in general.

> Summing up, it's easy to see how one
> feature (tissue differentiation) or the other (gametes) evolves, but I've
> never heard an even remotely convincing narrative about how you end up
> with both features in one species.

https://en.wikipedia.org/wiki/Hydra_(genus)
https://en.wikipedia.org/wiki/Bryophyte#/media/File:Moss_alternation_of_generations_03-2012.png

In the most primitive of multicellular organisms, gametes are typically
used to create diploid cells that become seeds or spores or other
drought-resistant forms allowing for suspended animation of sorts. So the
short answer to your question is that so those species could go into a
dormant stress-resistant state to wait out hard times.

> Speculation: however it ended up
> happening, I strongly suspect it's the single innovation that caused the
> Cambrian explosion. And given how late the Cambrian explosion was in our
> planet's prehistory, it may have been a seriously difficult hump for
> evolution to get over. And may therefore be a prime candidate for the
> Great Filter.

That is a feasible hypothesis.

Stuart LaForge