Multicellular Organisms

[DarkWaters]

A common misunderstanding that creationists hold is that evolution is unlikely because there are no multicellular organisms with just a handful of cells. While the flaw in this argument is obvious, it nevertheless compelled me to ask the following questions:

1. Are there any documented cases of multicellular organisms? If so, what is an example?
   
2. Is there an evolutionary game theoretic reason for why an organism with just a few cells is very unlikely?
   

I was wondering if there existed a fungus or something similar that would serve as an example to the first question. Even this seems very unlikely as fungii probably have thousands of cells at least. Furthermore, any organism that would meet this criteria that might have existed around two billion years ago, would lack any rigid structures and would therefore have an infintesimal chance of surviving in fossil form to this day.

With regards to the second question, I suspect that once an organism has multiple cells, the advantages of evolving a multitutde of cells is enormous as it would then allow the formation of specialized structures.

I am curious if anyone has anything to add to either of these.

[Maarten]

One of the smallest multicellular organisms I know of is Caenorhabditis elegans, which has around 1000 cells. It has been extensively studied as a model organism for apoptosis, because it is quite easy to follow where the different cells go during the development of the worm.

A thousand cells is quite a small amount, seeing how humans have somewhere in the vicinity of 10^13 cells if I remember correctly. I do not know of any organisms that only have, say, 3 cells. My suggestion is that perhaps being multicellular only starts to pay off beyond a certain number. I do know some unicellular organisms can cling together and form small balls of biomass, but they still remain individual organisms in that case so it wouldn't count as multicellular.

I've been looking through wikipedia pages but only very few of the pages about multicellular organisms say how many cells it has. I'll try to look a little further, but the answer may not be very easy to find I think.

Edited December 10, 2006 by Maarten

[LaszloWalrus]

Incidentally, a good book on the subject of evolution in general (which addresses your question as well) is The Emergence of Life on Earth by Iris Fry.

[Qwertz]

I read somewhere (can't remember where, unfortunately) that some people view eusocial insect colonies (many infertile individuals 'altruistically' serving the greater needs of the hive) as analogous to single multicellular organisms - the queen like a brain commanding an army of clones like separate parts of her vast body. Interesting idea conceptually, but I don't think it holds water evolutionarily, because the clones are still themselves highly complex multicellular organisms.

Though perhaps something like this happens on a small scale, too? with individual single-celled organisms partially adapted to specialized tasks in symbiosis with other individual single-celled organisms? And it's not until a large number of individuals are living in symbiosis that the specialization becomes specific enough for the collective to be considered a whole...

[Maarten]

One potential group of very small organisms is the parasites that live in insects. Small insects are usually around a few millimeters in length, so the parasite has to be much smaller still. It's difficult to find how large they are in terms of cells, though... I know the C. elegans I mentioned earlier is about 1mm in length and it has 1000ish cells, so I would assume that a parasite half that length has around 400ish cells...

[Liriodendron Tulipifera]

Dark Waters,

The claim that there are no multicellular eukaryotes that have just a few cells is complete and utter bunk. The creationists simply get away with this crap because they are allowed to.

First of all, you are correct - there is no reason to think that multicellularity is a more advanced evolutionary condition. Yes, the trend overall has been from unicellularity to multicellularity - but there have been plenty of reversals in all sorts of eukaryotes: there is an entire phylum of fungi that are unicellular - the Chytridiomycota, and the Glomeromycota are slightly more complex, consisting of arrays of a few hundred cells. (The most advanced sister groups on the evolutionary tree are fungi and animals.) There are also all kinds of protists that are unicellular to simple multicellular - colonial green algae come to mind.

Nor is there any reason to believe that intermediate states between unicellularity and very complex multicellularity have to exist in order to provide evidence for evolution. This is analagous to the argument creationists use when they claim that there are no intermediates between fossils in the fossil record.

There certainly are documented cases of multicellular organisms with just a few cells, including the group of fungi that I work on, the Laboulbeniales (Ascomycota). These organisms have a very simple body plan. At the simplest, roughly 20 cells at sexual maturity. There are all sorts of obscure organisms out there unknown to most scientists. People making these arbitrary claims of whether certain levels of complexity exist or not are simply willfully ignorant (the creationists), or they are honestly seeking but don't know where to look. Sometimes there are only a handful of experts around the world that work on groups of organisms that number in the tens of thousands - with 1.5 million described species, it can be difficult to know where to look.

I suggest tolweb.org (Tree of Life website) to examine current classification systems and characteristics across all known organisms. The molecular phylogenies are updated regularly.

P.S. Maarten is quite correct on the insect parasites. There are fungal parasites that live inside and outside of insects - including the Laboulbeniales, which I mentioned above, and the Trichomycetes that live in guts. They range anywhere from around 20 cells up to several hundred. There may be multicellular organisms with even fewer numbers of cells but I'd have to wrack my brain and do some serious literature searches to come up with them.

Edited December 11, 2006 by Liriodendron Tulipifera

[Liriodendron Tulipifera]

Perhaps there is a very simple answer to this question of why complex multicellularity is common amongst multicellular organisms - rather than very very simple multicellularity, although I wouldn't want to make too many speculations right now as to numbers of species or sheer numbers of individual living organisms with these body plans. Many organisms are multicellular so that the cells carry out division of labor, right? I can't see a whole lot of division of labor in an organism that has only three cells. In the simple 20 celled organisms I mentioned, two cells are for structural support of the organism, another two for male reproductive function, another couple for female reproductive function, and the rest as a structure to house the spores - which are released to make more adult individuals that develop to form the same 20 celled structure.

Also, I just remembered - I don't know this slipped my mind before - that in the simplest genera of fungi that I study, there are male and female individuals that live side by side on the insect. The males are two-celled. The cell on the bottom is for structural support; the one on top is for release of spermatia which fertilizes the female ascogenic cell in the female next door...