The Extended Evolutionary Synthesis and Distributed Adaptation in the Genus Homo: Phenotypic Plasticity and Behavioral Adaptability

The extended evolutionary synthesis (EES) emphasizes integrative approaches to understanding distributed adaptability, incorporating development and intergenerational effects, inclusive inheritance and niche construction. We have previously argued that components of the EES, including enhanced phenotypic plasticity, life-history variation, social learning, behavioural flexibility, and niche construction, are characteristics of the hominin lineage that accommodated both environmental variation and the colonization of new environments (Wells and Stock, 2007). Over the past decade we have gained considerable resolution in our understanding of spatio-temporal variation in fossil hominin phenotypic variation, material culture, and behaviour, and a refined understanding of the intergenerational and developmental mechanisms driving phenotypic diversity within our species. This paper reviews evidence for phenotypic and behavioural diversity within the genus Homo to evaluate the hypothesis that our evolution is characterized by a shifting distribution of adaptability across different adaptive systems described by the EES, which we broadly group as physiology and development and cultural evolution. Adaptation that is distributed towards physiological and cultural mechanisms allows for more rapid adaptability in stochastic environments and protects the genome from the costs of adaptation which generally involve a reduction in genetic diversity and hence potential future adaptability. Predictions of distributed adaptability are proposed in relation to: a) biology and morphology, b) habitual behaviour, and c) feedback between behavioral change and biology. To evaluate these predictions in relation to a) we consider evidence for shifts in phenotypic plasticity and morphological variation, including the emergence of body and brain size variation, limb proportions, skeletal robusticity, and regional variation in plasticity and canalization within the body, and how these relate to environmental factors and dispersals. Predictions of behavioral change b) are considered in light of the emergence of spatial, temporal, and environmental variation in archaeological assemblages in the late Middle and Late Pleistocene as indicators of local adaptability, cognition and niche construction. Finally, we consider the relationships between dispersals and material culture, plasticity in response to cultural change, and variation in the microbiome in relation to c). Current evidence suggests a mosaic pattern of the evolution of distributed adaptation and selection within our genus. In early Homo there is evidence of phenotypic diversification and increasing plasticity that precedes evidence of increased cognitive, behavioral, and cultural variation of Middle and Later Pleistocene Homo. This can be interpreted as representing a shift towards the distribution of adaptability, first onto mechanisms based on phenotypic plasticity, and later onto cognition, cultural buffering of environmental stress, and enhanced niche construction. Phenotypic plasticity was likely an important component of adaptability to environmental stress, but also served to buffer development of the brain, an extended life history, and social learning.