Adolph Schultz's famous diagram (Fig. 6.1) aged well: human life history is characterised by a long juvenile period (weaning to reproductive maturity), and a long post-reproductive lifespan in females. Table 6.1 compares human and great ape life history parameters and invariants of Charnov (1993). Notable here are low adult mortality, high fertility squeezed into a reproductive span similar in length to that of Pan (thus, shorter interbirth intervals), late age at first reproduction and great length of the juvenile period.
How do we explain these differences between our nearest relatives and ourselves? This chapter summarises some recent attempts to use life history models on data from contemporary hunter–gatherers, and other noncontracepting populations with little access to modern medicine (see also Borgerhoff Mulder, 1991; Hill, 1993; and for a comprehensive review of hunter–gatherer research, Kelley, 1995).
Trade-off between numbers and care of offspring
Hill and Hurtado (1996) examine interbirth interval and the trade-off between increased fertility and increased infant and child mortality among Ache foragers in Paraguay. As in other populations, after controlling for early death of a previous infant, mother's age, and mother's weight, shorter interbirth intervals are accompanied by higher infant and child mortality. But, in contrast to Blurton Jones (1986; for discussion of Harpending's (1994) critiques see Blurton Jones, 1994; for Hill and Hurtado's results, see Blurton Jones, 1996), they found that this effect was much too weak to render the observed intervals optimal. Ache values predict that the optimal interbirth interval would be much shorter than is observed. Hill and Hurtado discuss possible reasons, suggesting that there must be costs to very short intervals that we have yet to appreciate.