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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 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 2 days ago
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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 are 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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