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This question already has an answer here:

I can imagine several specialized unicellular beings collaborating. But how does life get from that point to a unique pluricelular being that reproduces into another pluricelular being?

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marked as duplicate by Remi.b, March Ho, Chris, fileunderwater, AndroidPenguin Feb 24 '15 at 22:21

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The key is specialization: e.g. some cells specializing in getting food, others in producing something sticky to keep them together. Which cell does what is usually determined by chemical signals: depending on their specialization they produce particular chemical compounds. So if a non-specialized cell notes that one compound is rather low because the specialized type of cell producing it is in low supply then it specializes that way.

While initially all cells are capable of reproduction, at some point there also will be cells specialized for reproduction, whereas the others lose the ability to reproduce to better concentrate on their specialized duty. Hence the other cells do not have an independent existence anymore but exist only as part of the larger organism. When such an organism is then to be considered a separate pluricellular entity is a matter of definition: if it is an entity independent of the original (e.g. capable of moving away)? if sexual reproduction happens?

Wikipedia also has some things to say.

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  • $\begingroup$ Welcome to Biology SE and thanks for your interesting answer. At Biology SE some references are appreciated to substantiate answers to allow people to validate the answer or for further reading. Thanks. $\endgroup$ – AliceD Feb 24 '15 at 20:15
  • $\begingroup$ Ps: although wikipedia is a much referenced source, it is unfortunately one of the most unreliable sources as its contents change continuously. +1 for your answer. $\endgroup$ – AliceD Feb 24 '15 at 20:33

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