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Because of the implications for behavioral, social, and cultural evolution, reconstructions of the evolutionary history of human parturition are driven by two main questions: First, when did childbirth become difficult? And second, does difficult childbirth have something to do with infant helplessness? Here we review the available evidence and consider answers to these questions. Although the definitive timeframe remains unclear, childbirth may not have reached our present state of difficulty until fairly recently (<500,000 years ago) when body and brain sizes approximated what we have now, or perhaps not until even more recently because of agriculture's direct and indirect effects on the growth and development of both mother and fetus. At present, there is little evidence to indicate that difficult childbirth has affected the evolution of gestation length or fetal growth, selecting for infants that are born in a supposed underdeveloped state, although these phenomena likely share causes.

The Evolution of Difficult Childbirth and Helpless Hominin Infants