In a given environment, human groups fit into a trophic pyramid of biomass production. An essential aspect of their adaptive success depends on the carrying capacity of this environment, i.e. the maximum population size it can support per unit area. Variations in this parameter explain the significant variations in the size of territory exploited by hunter-gatherers from one region to another. As hominins evolved, their reproductive success depended not only on environmental and biological factors, but increasingly on technical and social ones. Energy extracted from the environment is consumed by the basal metabolism of individuals, by their physical activity and by the thermal effect of food itself.
As far as basal metabolism is concerned, each system and organ has very different levels of consumption. Physical activity aside, over the course of each individual's existence, the available energy must be allocated to three major functions - growth, maintenance and reproduction - and a compromise must be reached between these different needs. This is what is meant by the term life history theory. From one living organism to another, very different adaptive strategies have been adopted. Energy investment also has a temporal dimension, and these adaptive strategies are reflected in demographic traits (fecundity, growth duration, longevity, etc.) that vary considerably from one organism to another. Large primates, and man in particular, are characterized by slow maturation, late first reproduction, long longevity and high parental investment in a limited number of offspring with a high survival rate. The human life history amplifies these traits and the sequential nature of the growth trajectory, with strong investment in cerebral, then dental, then somatic development until access to reproduction.
Another peculiarity is the very early age of weaning, which allows the mother to have another child even though the first has not completed its cerebral development. This early weaning allows individuals other than the mother to participate in the high energy investment represented by the developing brains of young children. Generally speaking, the high level of human basal metabolic rate and the ever-increasing energy consumption of individuals have led to a decline in fertility in many human societies.
