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I read a story this week on Richard Lenski who has been 'evolving' E. coli for more than 50,000 generations now. One comment I read was from someone who doesn't accept Evolution who pointed out that we haven't seen a single celled organism 'evolve' into a multi-celled organism. Another person responded and said that a bacteria is not going to evolve into something that isn't a bacteria.

So, if Evolution created single celled organisms and then multi-celled organisms how might that change have happened? And is it possible to recreate that set of driving forces to make a bacteria something other than a bacteria?

To that end, what advantage does being multi-cellular have over being unicellular (if that's even a word)?

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There are quite a lot of books addressing this question. Here are some few examples: Major transitions, levels of selection and Major transitions revisited. Note concerning the question of level of selection versus kin selection one might be interested by this article –  Remi.b Nov 21 '13 at 8:42
Two things: 1. Although there are a number of hypotheses, as others have pointed out, this is not an entirely solved problem. Attempting to figure out which hypothesis (or hypotheses) are most plausible is an ongoing area of research. 2.Clearly, saying that "a bacteria is not going to evolve into something that isn't a bacteria" is an oversimplification. Eventually this is possible. But it would take orders of magnitude longer than we can test in a laboratory - even with such impressive experiments as Lenski's. –  seaotternerd Nov 21 '13 at 10:51
@seaotternerd, If we use 20 years as a human generational time period then 50,000 generations is 1,000,000 years. Looking at the 'conventional' timeline for human evolution we have progressed quite a bit in the last 1,000,000 years - easily changing 'species' more than once. Given that, why do you say that 'orders of magnitude' more generations are required to see a change in bacteria? –  CramerTV Jul 18 '14 at 23:41
@CramerTV - While it's true that the lineage that lead to humans has transitioned between "species" multiple times within the past 1,000,000 years, the evolutionary distance between two different species (say Homo sapiens and Homo erectus) is orders of magnitude smaller than the evolutionary distance between two different kingdoms (bacteria vs. non-bacteria). Bacteria would have to undergo multitudes of changes in order to be considered something other than bacteria. –  seaotternerd Jul 21 '14 at 22:47

4 Answers 4

up vote 11 down vote accepted

How did multicellularity evolved?

It is an ongoing field of research - Some insights about the origin of multicellularity

This is a big ongoing field of research. To start with an example, there was relatively recently (2012) an important article by Ratcliff et al. that shows that yeast can quickly evolve multicellularity under selection on the speed they sink to lower water layers. This article is one among many others and is far from being able to explain everything we would like to understand about the evolution of multicellularity. Typically, I think that this yeast species had a multicellular ancestor and we might think that this species would already have fixed alleles (=variants of genes that is fixed meaning that the whole population is carrying this variant today) in the population predisposing this species to easily (re-)evolve multicellularity. Also they may have kept some standing additive genetic variance in their genome from their past and they would therefore very quickly respond to selection as they don't need de novo mutations. (Sorry if this last sentence was slightly technical).

One of the first trait that we usually refer to when talking about the evolution of multicellularity is the presence of sticky proteins allowing individual cells to paste to each others.

Some insights about the evolution from simple multicellular to more complex multicellular

Then, we could talk about more complex multicellular and argue how do these simple multicellular evolve into some more complex organisms. A common argument is that multicellular can have specialized cells are are very could at doing what their doing as they are specialized. Also, some level of complexity is thought to have raised due to the fact that multicellular organisms tend to have smaller population size than unicellular (see Lynch and Conery, 2003). It is important not to confuse evolution of complexity with evolution of multicellularity although these two notions are somehow related.

What do you mean by multicellularity?

The evolution of multicellularity can be discussed in the context where sister cells form an organism together or when unrelated cells (among the same species or even cells from different species) come together to form an organism. Also, the multicellularity can be discussed at different level depending on how we want to define multicellularity. Is a stack of cells reproducing individually, working for their own benefit a multicellular? Do we need division of labor? Do we need division between germline (reproductive caste) and soma line (non-reproductive case)?

How many times did multicellularity evolve independently?

Some people consider that there are multicellular bacteria (biofilms) but we will avoid discussions that are based on limit-case definitions. Let's talk about eukaryotes. Most Eukaryotes are unicellular and multicellularity evolved many times independently in eukaryotes. To my knowledge, complex multicellularity however evolved only (only?) 6 times independently in eukaryotes.

  • Metazoa (animals)
  • Ascomyceta (fungi)
  • Basidiomyceta (fungi)
  • Embryophyta (land plants)
  • Florideophyceae (red algae)
  • Laminariales (brown algae)

Model organisms and interesting cases to study multicellularity

There are a bunch of specific clades that are particularly interested in studying multicellularity because they present transition states. For example Volvox is a chlorophyte genus and the species in this clade present different stages of multicellularity; Some species are exclusively multicellular, some form small groups, some create big colonies, some have some division of labor and some even have separation between the germline and the soma (Some castes don't reproduce). (ref1, ref2, ref3, ref4, ref5, ref6). Yeasts are also good model organism for studying the evolution of multicellularity.

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For one thing, larger organisms are much more energy efficient. This is what is known as Kleiber's Law where the caloric requirement scales as the 3/4 power to the body mass.

Another thing is that when all the cells cooperate to form a multicellular organism, each given individual is more likely to reproduce and less likely to die due to environmental variation because cooperation creates stability.

There are several theories about how this came about,but those are the elements of why. Collaboration and efficiency improve the chances of survival, which is to say that selection will favor multicellular organisms however they came to be.

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Disclaimer: Not my field of research, and not a field where I know the litterature well. See it as a complement to the other answers.

A distinct advantage of multicellularity is specialized functions of different cells. This can allow for higher efficiency of e.g. metabolic processes, and also that redundant functions can be removed from some cell lines, since they can be handled by other cells. Therefore, the constituent parts can become simpler, while the resulting organism becomes more complex at the same time. Mathematical modelling of cellular systems have shown how this type of division of labour can evolve from unicellular lines (Ispolatov et al. 2011), through the steps of aggregation and differentiation from preexisting functions.

An interesting intermediate step that can provide some clues to how multicellularity can evolve, is in cyanobacteria, where some unicellular species can show partial specialization e.g. when part of cellular biofilms. A phylogenetic study of cyanobacteria has also shown that they have reversed from multicellularity to unicellularity at least five times, and most extant cyanobacteria seem to descend from multicellular ancestors (Schirrmeister et al. 2011). This means that the evolution of multicellularity is not a one-way process, but seems to be a much more complex process.

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I STRONGLY encourage to read work from the lab of Nicole King - she studies Choanoflagellates, which are the "out group" for animals - they are, in some sense, the most animal-like single celled organism that exists.

Chaonos are also amazing because they go through a single to multicellular transition in there own life cycle, so they provide an amazing opportunity to understand when it is more beneficial to be single celled vs. multi-celled. Currently, on of the working hypotheses of the group is that on of the main drivers of the push towards multicellularity may have just been simple fluid dynamics: the flows around a spherical multicellular "rosette" of chaonos bring more food to them.

If you are interested in the evolutionary transition to multicellularity you must read work from the King Group.

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