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Why don't complex multicellular prokaryotes exits and also what made eukaryotes to become multicellular ?

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    $\begingroup$ Prokaryotes are called so because they are unicellular. If they were multicellular, they would be called eukaryotes. Similarly, if unicellular, it would be a prokaryote. That is just how these cells have been named. $\endgroup$ – The Last Word Aug 14 '18 at 16:24
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    $\begingroup$ @TheLastWord "If they were multicellular they would be called eukaryotes" - this is false. The more specific definition of prokaryote vs eukaryote has to do with the presence of membrane-bound organelles in eukaryotes. There are many many single-celled eukaryotes. $\endgroup$ – Bryan Krause Aug 14 '18 at 21:16
  • $\begingroup$ @BryanKrause Yes, I got that from the answer. I did not think about single celled eukaryotes. $\endgroup$ – The Last Word Aug 14 '18 at 21:27
  • $\begingroup$ @TheLastWord True, just don't want a misleading comment to confuse future users that come here, there is no way to downvote a comment, and someone already upvoted yours. $\endgroup$ – Bryan Krause Aug 14 '18 at 21:28
  • $\begingroup$ @BryanKrause Yes, I noticed the error much earlier but let it stay so that someone else does not make the same error. $\endgroup$ – The Last Word Aug 14 '18 at 21:30
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Why are there no multicellular prokaryotes?

From a Molecular Cell Biology introduction course web resource from the University of Illinios, which explains the general sentiment in microbiology over this question:

There are lots of unicellular eukaryotes, including amoebas, paramecium, yeast, and so on. As to whether there are multicellular prokaryotes, the standard answer is No, but there is a lot of evidence that some bacterial species can aggregate together and divide labor so that the "colony" is working more efficiently. This is characteristic of any traditional multicellular organism, but there's still a lot of resistance to the idea of calling these prokaryotes "multicellular."

Here's a Scientific American article on the subject, which is good for the layman and will add to your understanding. Note that the most complex 'multicellular prokaryotes' known do not form comparably large or complicated structures when contrasted with eukaryotes. Even though prokaryotes have more biochemical tools available to them, their largest multicellular specimen pales in comparison to our small multicellular eukaryotes, let alone out large eukaryotes like the Armillaria ostoyae fungus in Oregon (nicknamed "the largest organism on Earth"), giant sequoia trees, the dinosaurs, or blue whales. But I digress! Total organism size is not a requisite for multicellularity, which is why some bacteria can be termed so.

Here is a paper I found in the literature on the first reported 'true' multicellular prokaryote, from 2004. Similar organisms have been observed since. See Pubmed link.

The same question has also been answered extensively by specialists on Quora and on ResearchGate.

What made eukaryotes to become multicellular?

Your question implies external influence. Biological evolution does not proceed teleologically, with an end goal in mind. Instead, traits gradually accumulate if they are not detrimental to the replication of the genes. The evolution of multicellularity is a complex and active field of research but there is no reason behind this major switch in cellularity other than that it works as a strategy for the replicating organism and its genes. Multicellular organisms survive. That's all.

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