ScienceDaily -- Any multicellular animal, from a blue whale to a human being, poses a special challenge for evolution.
Most of the cells in its body will die without reproducing; only a privileged few will pass their genes to the next generation.
How could the extreme degree of cooperation required by multicellular existence actually evolve? Why aren't all creatures unicellular individualists determined to pass on their own genes?
Joan Strassmann and David Queller, evolutionary biologists at Washington University in St. Louis, provide an answer in this week's issue of the journal Science.
Experiments with amoebae that usually live as individuals, but must also join with others to form multicellular bodies to complete their life cycles, show that cooperation depends on kinship.
If amoebae occur in well-mixed cosmopolitan groups, cheaters will always be able to thrive by free-loading on their cooperative neighbors.
But if groups derive from a single cell, cheaters will usually occur in all-cheater groups and will have no cooperators to exploit.
The only exceptions are brand new cheater mutants in all-cooperator groups, and these could pose a problem if the mutation rate is high enough and there are many cells in the group to mutate.
The scientists calculated just how many times amoebae that arose from a single cell can safely divide before cooperation degenerates into a free-for-all.
The answer turns out to be 100 generations or more.
Population bottlenecks that kill off diversity and restart the population from a single cell are powerful stabilizers of cellular cooperation, the scientists conclude.
"The leap from single-celled organisms to multicelled ones was a critical step in the history of life that paved the way for the world's plants and animals, including humans," says Sam Scheiner, program director in the National Science Foundation's (NSF) Division of Environmental Biology, which funded the research. "This study provides important clues that about the conditions necessary for that leap."
