How difficult is the transition to multi-cellular life?

[Avalancheon]

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.

You're right, I was conflating 'multi-cellular' with 'eukaryote.' That was an embarassing flub on my part.

Just an FYI, but there is a theory (not widely accepted) that chloroplasts actually had a single origin. In other words, they used to be homologous to all eukaryotes, until they started diversifying into separate phylums. Those that didn't evolve to use photosynthesis eventually lost their chloroplasts, because they were functionally useless and vestigal.


Nick Lane contends that the evolution from prokaryote to eukaryote was very difficult: "The emergence of complex life, then, seems to hinge on a single fluke event - the acquisition of one simple cell by another. Such associations may be common among complex cells, but they are extremely rare in simple ones. And the outcome was by no means certain: the two intimate partners went through a lot of difficult co-adaptation before their descendants could flourish."

"This does not bode well for the prospects of finding intelligent aliens. It means there is no inevitable evolutionary trajectory from simple to complex life. Never-ending natural selection, operating on infinite populations of bacteria over billions of years, may never give rise to complexity. Bacteria simply do not have the right architecture. They are not energetically limited as they are - the problem only becomes visible when we look at what it would take for their volume and genome size to expand. Only then can we see that bacteria occupy a deep canyon in an energy landscape, from which they are unable to escape."