The five biggest evolutionary transitions?

Have been thinking recently about what the five biggest evolutionary transitions might be.  Naturally, there are several transitions and indeed it is arguable that at every speciation event, there has been a transition - indeed at every mutational event, even.

But which ones are the biggest?  Also, in this vein - what evolves and what is selected?

We can consider something to be a paradigmatic darwinian unit if it (a) can show variation, (b) can be selected and (c) can be inherited.  So, a gene or fragment of gene in a replicating system might be a paradigmatic darwinian unit. An individual can be one too and even perhaps an extended family, though many will argue against group selection, let's not completely shut the door.

So, now that we have defined paradigmatic darwinian units (yes, I know it is a loose definition), we can consider transitions.


Candidate 1:

OK, first off, we should probably include the first self-replicating molecule.  The consensus is that this was likely to have been RNA - in part because we have catalytic RNA molecules in modern organisms, but also for a variety of other reasons.




Candidate 2:

This would have to be the first recognizable cell.  It is reasonable to think that as self-replicating units became more complex - as might be expected on a planet where there are no predators to gobble you up - then any mutation that increased the likelihood of compartmentalization of biochemical reactions and self-replication would have been advantageous and so the first cell would have formed.



Candidate 3:

I think this might have been the separation of the two main prokaryotic cell types - archaebacteria and eubacteria.  They have fundamental differences in their cell walls, but also in ribosome structure, co-factors, etc.





Candidate 4:

The origin of the eukaryotic cell.  But I would say that, wouldn't I?






Candidate 5:

The origin of sex.  This is the separation of somatic cells from germ-line cells and the packaging of the reproductive cells into specialized structures.






So, what have I left out and why?

Multicellularity has arisen a few times, so it was not one single transition from single-celled to multicellular and therefore, if I had to include one, I would have to include them all.  If multicellular life only arose once, then it would have made it into my top  5.

The origin of animals - not a big enough transition.

The origin of plants or flowers or algae? - not as big as the other transitions I have mentioned.

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