The emergence of humans: The coevolution of intelligence and longevity with intergenerational transfers

doi:10.1073/pnas.152502899

The emergence of humans: The coevolution of intelligence and longevity with intergenerational transfers

Hillard S. Kaplana,b and
Arthur J. Robsonc

^aDepartment of Anthropology, University of New Mexico, Albuquerque, NM 87131; and ^cDepartment of Economics, University of Western Ontario, London, ON, Canada N6A 5C2

Edited by Kenneth W. Wachter, University of California, Berkeley, CA, and approved May 14, 2002 (received for review September 18, 2001)

Abstract

Two striking differences between humans and our closest living relatives, chimpanzees and gorillas, are the size of our brains (larger by a factor of three or four) and our life span (longer by a factor of about two). Our thesis is that these two distinctive features of humans are products of coevolutionary selection. The large human brain is an investment with initial costs and later rewards, which coevolved with increased energy allocations to survival. Not only does this theory help explain life history variation among primates and its extreme evolution in humans; it also provides new insight into the evolution of longevity in other biological systems. We introduce and apply a general formal demographic model for constrained growth and evolutionary tradeoffs in the presence of life-cycle transfers between age groups in a population.

Footnotes

↵ b To whom correspondence should be addressed. E-mail: hkaplan{at}unm.edu.
This paper was submitted directly (Track II) to the PNAS office.
↵ d For simplicity, t̄ is independent of K.
↵ e This assumes that V(K, t + 1, α) > 0; if V(K, t + 1, α) ≤ 0, then s(t) = 0.
↵ f We prove this claim for a continuous-time version of the model in ref. 21. Simulations illustrating this claim are also available at www.unm.edu/∼hkaplan/pnas/regress.htm.
↵ g The optimal K is further assumed to be unique, and the second-order condition is strengthened to p̄V _KK(K*, 1, α)e ^−r − C"(K*) < 0.
↵ h If σ′(0) were finite, it might be optimal to set s(t) = 0 earlier than this, where the value of life remains positive but is small enough.
↵ i Data such as that in Fig. 1 suggest that hunter–gatherers experience a final phase of life during which net production is negative. However, this phase is relatively short and might vanish if nonfood production were accounted for.
↵ j It is less clear how to maintain zero population growth now, however.
↵ k Because brain size is endogenous (it is the dependent variable in the first regression), the problem of simultaneity can be addressed by using predicted brain size in this second regression. Similar results are obtained, however, when measured values are used instead of predicted values.
↵ l Similar results are obtained by using path analysis and the method of independent contrasts to control for phylogenetic relationships (unpublished results available from H.S.K.).
↵ m Pleiotropic effects are not required here. That is, genes affecting only intelligence might coevolve with those affecting only longevity. However, the existence of suitable pleiotropic genes tends to validate the present approach, because it suggests that longevity and intelligence were typically subject to simultaneous evolutionary pressure.
↵ n In addition, at lower population densities, fire ant queens themselves grow larger before reproducing and live longer, producing more generations of workers (34).
Abbreviation:

apoE,

apolipoprotein