Growth and Investment in Hominin Life History Evolution: Patterns, Processes, and Outcomes

Abstract

The transitions from apes to lineages allied to humans are marked by shifts in the allocation of parental effort, associated with discontinuous changes in rates of infant and juvenile growth both prenatally and postnatally. Here, I assess growth and life history characteristics of apes within a general mammalian / primate paradigm, using time and energy expenditure as 2 fundamentals that covary with infant survival and success probabilities. I suggest that these survival probabilities depend on the quality, amount, and timing of parental care allocated to infants. Growth to birth, growth to weaning, and growth to reproductive onset are partitioned as separate periods within a life history on the basis of comparative mammalian data. Growth problems such as sexual dimorphism can be incorporated into an investment perspective by assessing when and how sex-specific parental care affects growth rates and the onset of reproduction. I compare features of the hominoid life history with developmental rates for hominin lineages as seen in dentition, and the fossil record of body and brain size changes over time. The links between parental effort and allocation of care to infant growth and survival generate speculative scenarios of sex-specific parental care allocation; I then explore hominin social evolution - mating system and childhood - for the lineages thought to lead to modern Homo, and for those that coexisted with ancestors of Homo.