How difficult is the transition to multi-cellular life?

[iancampbell]

Our galaxy is over 13 billion years old, and contains 400 billion stars. This strongly implys that many advanced civilisations ought to have come into existence by now. And yet, we find no real evidence of intelligent life beyond our own. Therefore, there must be some barrier that prevents life from evolving to the point where it can spread among the galaxy.

Lately, I have been wondering what kind of evolutionary step could be so incredibly difficult as to leave the galaxy barren and lifeless for this amount of time. Even with billions of potential biospheres that could spawn intelligent life, it has ended up with nothing. In my opinion, therefore, the great filter must make an appearance at a very early stage. It stops the progress of life that is young and primitive.

What I am undecided on is whether or not the transition from single cell to multi cellular life could be a candidate. I have found two different sources making opposite arguments for this. Richard Grosberg claims that the transition isn't that hard, as it has independently happened many times in earths history. However, Nick Lane argues that the transition was a freakish anomaly, and that it has only happened once in earths history.


I would like to get a discussion going on this subject. Of the two authors, who do you think made the better case? Just how likely was the eukaryotic transition? Could it have realistically happened in other biospheres?

You seem to be talking about two different things here, and possibly confusing the two.

There are almost certainly more single-celled eukaryotes (in terms of species) than multicellular ones. In terms of biomass, there is an even larger difference. And, BTW, the transition has happened at least twice, in succession, to any eukaryote that has chloroplasts as well as mitochondria; this argues against it being difficult.

The transition to multicellular life appears to be easy, because many distinct evolutionary lines have made it. The simplest version appears to be that used by various algae, which merely stick together in a line. The next simplest is probably Volvox.

I would also like to make another point. On Earth, as far as I know, there are no multicellular prokaryotes with any form of specialised cell types or mobility; bacterial biofilm mats probably don't count. However, this does not mean that the idea is impossible; the probable reason it hasn't happened on Earth is that eukaryotes are better at it and out-competed them long ago.