Why prokaryotes never evolved to the level of organization of complex multicellular organisms like humans? They can only change behaivior when in colonies, and some species can differenciate somewhat in colonies, but nothing further. Is there something that constrains their evolution into complex multicellular organisms? Small cell size? Relative ease of horizontal cell transfer? They had billions of years of time before the ascendance of multicellular eucaryotes to make the transition, but never did.

The simple answer is two fold:
1) Prokaryotes have evolved to levels of high complexity - our remote ancestors were prokaryotes, one lineage went eukaryotic, evolved multicellularity and over time evolved complexity.
2) Unicellular prokaryotes are extraordinarily successful. They make up the vast majority of species and cells on the planet. They typically have fast reproduction and have evolved the ability to exploit an enormous range of ecological niches.

But there is another question you are getting at:
Why do membrane-bound organelles seem to be a prerequisite for complex multicellular individuals?
The hypotheses I've seen for this typically hinge around signal transduction and metabolic scaling.
Prokaryotes are size limited due to the smaller amount of membrane space for metabolism.
Prokaryotes lack some mechanisms for signal transduction since they are missing membrane-bound organelles (the golgi and ER) that process proteins for multiple signal transduction pathways.
Small size and less ability to communicate with other cells may be limiting prokaryotic cells to a solitary or simple communal lifestyle.

Last edited by Ajna Rivera (31st Mar 2015 17:27:47)