The origin of multi-cellular organisms out of single-celled organisms and of social groups out of lone individuals have both been important areas of study. They might also be more closely linked than anyone realized: recent research indicates that yeast cells will form clumps because those clumps confer advantages for the individuals. Those clumps are analogous to social groups as well as a precursor to multi-cellular life.
In yeast, simple mutations that alter the completion of cell division or prevent final digestion of the cell wall are sufficient to cause yeast to form aggregates or clumps of cells. Indeed, wild yeast grow in such multicellular forms, although common lab strains have been selected to grow as individual cells.
Mathematical modeling and experiments show that when concentrations of sucrose are limiting, growing in clumps is advantageous because the cells secrete the enzyme invertase, which splits sucrose into the glucose and fructose the yeast use to fuel metabolism. Thus, it pays to be near other cells to have access to the sugars that are available in their immediate vicinity.
In their experiments, clumps of yeast cells grew in concentrations of sucrose that were too low to support the growth of equivalent numbers of individual cells. Furthermore, clumping also helped cells compete with "cheater" yeast cells, which freeload off other cells and do not make any invertase themselves, thus gaining a small survival advantage.
Source: Science, September 9, 2011
So it sounds like there is a selective, evolutionary advantage for loner cells to join up with others and form colonies -- colonies which, over time, develop specialization and eventually multi-cellular life. Sounds an awful lot like the advantages that exist for lone humans to join up and form communities. People can survive when alone, but they are much more likely to survive and even thrive when in groups.
Maybe there is a drive towards group-formation that exists within us at even a cellular level.
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