  Cooperative behavior is widespread in nature, even though cooperating
individuals always run the risk to be exploited by free-riders. Population
structure effectively promotes cooperation given that a threshold in the level
of cooperation was already reached. However, the question how cooperation can
emerge from a single mutant, which cannot rely on a benefit provided by other
cooperators, is still puzzling. Here, we investigate this question for a
well-defined but generic situation based on typical life-cycles of microbial
populations where individuals regularly form new colonies followed by growth
phases. We analyze two evolutionary mechanisms favoring cooperative behavior
and study their strength depending on the inoculation size and the length of a
life-cycle. In particular, we find that population bottlenecks followed by
exponential growth phases strongly increase the survival and fixation
probabilities of a single cooperator in a free-riding population.
