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Evolution of life histories: key points
- Life history theory describes why species have particular patterns of growth, development, reproduction and mortality.
- The life history strategy of a species is determined by an evolutionarily optimised allocation of limited resources between growth, reproduction and tissue repair to maximize reproductive success. This requires trade-offs among life history traits, such as age versus size at maturity, number versus quality of offspring, current versus future reproduction and fecundity versus lifespan.
- Humans are characterised by large brains and long lives. Their development is typified by a singleton pregnancy, a long phase of postnatal nutritional dependency, a prolonged juvenile period, delayed sexual maturity, and only modest sexual dimorphism. Females terminate reproduction before the end of their intrinsic lifespan. Humans produce very few offspring, who benefit from high parental investment, ensuring a comparatively high rate of survival to adulthood.
- Evolutionary considerations can offer explanations for the unusual characteristics of human life history, including the relatively long childhood phase, the pubertal growth spurt, and menopause.
Secular trend in age of menarche in two European countries
The age at menarche in Europe has fallen by about 4 years over the last 200 years. This is secondary to improvements in maternal and child health (better nutrition and reduced infection). But this seems paradoxical, operating in an opposite direction to the effect of extrinsic mortality, which has also fallen because of these improvements. The resolution of this paradox lies in understanding that the age at puberty is the outcome of a hierarchy of influences and controls. The mortality/pubertal age relationship operates in populations which are in a relatively steady state. But fitness has to be sustained in the face of variable environments and this adds another layer of control.