The life history of Homo sapiens is characterized by a lengthy period of juvenile dependence that requires extensive allocare, short interbirth intervals with concomitantly high fertility rates, and a life span much longer than that of other extant great apes. Although recognized as species-defining, the traits that make up human life history are also notable for their extensive within- and between-population variation, which appears to trace largely to phenotypic and developmental plasticity. In this review, we first discuss the adaptive origins of plasticity in life history strategy and its influence on traits such as growth rate, maturational tempo, reproductive scheduling, and life span in modern human populations. Second, we consider the likely contributions of this plasticity to evolutionary diversification and speciation within genus Homo. Contrary to traditional assumptions that plasticity slows the pace of genetic adaptation, current empirical work and theory point to the potential for plasticity-induced phenotypes to "lead the way" and accelerate subsequent genetic adaptation. Building from this work, we propose a "phenotype-first" model of the evolution of human life history in which novel phenotypes were first generated by behaviorally or environmentally driven plasticity and were later gradually stabilized into species-defining traits through genetic accommodation.
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