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. (read more)