Although lifecycles with a conditional social state are known in a wide range of taxa, evolution of the solitary-to-social switch is poorly understood. The social amoeba Dictyostelium discoideum switches between solitary growth and social fruitification depending under nutrient-rich and starved conditions, respectively. In the social state, cells aggregate and form fruiting bodies consisted of spores and non-viable altruistic stalk cells. Once cells are in the multicellular structure, they complete fruiting body formation even under renewed source of nutrient. The irreversibility of the solitary-to-social switch is puzzling, because it deprives individual cells of benefits from further solitary growth. One idea posits that selection for traits related to respond to nutrient recovery is hindered in a social context. Our chimera experiment revealed that cells switching back to the solitary were disadvantageous, because they were forced to be altruistic when interacting with social cells. Furthermore, theoretical analyses revealed a valley on the fitness landscape that prevents evolution to acquire adaptive switch-back ability, thus the irreversible switch as observed in Dicyostelium is an evolutionarily stable strategy. The penalty that inheres in the division of labor explains why cellular cooperation could override selfishness. We also introduce our ongoing experimental evolution for the fitness-valley crossing.