Periodical cicadas (Magicicada spp.) are known for their strikingly synchronized emergence, strong site tenacity, and unusually long (17- and 13-yr) life cycles for insects. Several explanations have been proposed for the origin and maintenance of synchronization. However, no satisfactory explanations have been made for the origins of the prime-numbered life cycles. I present an evolutionary hypothesis of a forced developmental delay due to climate cooling during ice ages. Under this scenario, extremely low adult densities, caused by their extremely long juvenile stages, selected for synchronized emergence and site tenacity because of limited mating opportunities. The prime numbers (13 and 17) were selected for as life cycles because these cycles were least likely to coemerge, hybridize, and break down with other synchronized cycles.