Personal and evolutionary destinies

Inevitably and normally, we all are born, develop, mature, age and die. This simple fact, however, has important evolutionary consequences, since survival and reproduction are the key factors which determine individual and species fitnesses. The main life history traits, as indicated by Stearns (1992), are: (a) size at birth; (b) growth pattern; (c) age at maturity; (d) size at maturity; (e) number, size and sex ratio of offspring; (f) age and size-specific reproductive investments; (g) age and size-specific mortality schedules; and (h) length of life. All of them are interrelated, so that disturbance in a given stage of the life cycle may affect the other components in multiple ways.

In humans, culture acts as a buffering system between ourselves and the environment, and the ensuing cultural–biologic interactions often make the interpretation of a given process difficult. The environment has to be partitioned in its physical, biotic and social components; humans' resources, constraints, and specific stressors may lead to short- or long-term responses, at different (individual, populational) levels. These responses can be classified into seven categories: avoid, modify, buffer, distribute, resist, conform, or change. In its most simple characterization, what happens is energy flow (Thomas et al., 1979; Thomas, 1998).

As examples, let us consider the extremes of the cycle. Humans are different from most mammals (including nonhuman primates) by the fact that significant quantities of fat are deposited in the fetus in utero. Consequently, Homo sapiens babies have a fat mass approximately four times that predicted for a mammal of their body size at birth.