Human birthweight evolution across contrasting environments.

We explore from both theoretical and empirical perspectives the hypothesis that a significant part of the worldwide variability in human birthweight results from adaptive responses to local selective pressures. We first developed an agent-based model to simulate the process of evolutionary selection on life history strategy, and then we performed a comparative analysis across 89 countries worldwide. The model illustrates that optimal birthweight depends on which fitness-reducing risk locally predominates (somatic diseases, parasitic diseases or adverse environmental conditions). When fitness variations between individuals mainly result from somatic diseases (e.g. industrialized countries), or conversely from infectious and parasitic diseases (e.g. developing countries), selection is expected to favour individuals producing larger children. Conversely, when environmental risks increase in relative importance, selective pressures for producing children with high birthweight are reduced. The comparative analysis supports these theoretical expectations, in particular the finding that birthweight is higher than predicted in highly parasitized countries.