Human reproduction: a comparative background for medical hypotheses.

Successful reconstruction of any aspect of human evolution requires broad-based comparisons with other primates and other eutherian mammals. If quantitative data are involved, the scaling influence of body size must be taken into account, and bivariate allometric analysis is a very useful tool. Broad-based comparisons lead to recognition of general principles that can feed into hypotheses concerning human reproductive medicine. Here, this approach is applied to basic life-history parameters (age at sexual maturity, gestation period, litter size, lactation period, interbirth interval, maximum lifespan), testicular dimensions and male mating strategies, female ovarian processes and mating cycles, placentation and embryonic/fetal development, gestation periods and neonatal condition, and brain development in relation to reproduction. Comparisons reveal that some features claimed to be unique to human beings in fact occur more widely in primates, such as 'loss of oestrus', which probably evolved in the common ancestor of higher primates (monkeys, apes and humans). Although humans show an extreme condition, in most higher primates mating is not strictly confined to the periovulatory period. Because this condition introduces the danger of fertilization with ageing gametes, it must have been favoured by strong selection and accompanied by the evolution of compensatory mechanisms. Overall, it seems likely that the high incidence of pre-eclampsia in human pregnancies is attributable to a partial failure of compensatory mechanisms associated with highly invasive placentation.