The Emergence of Humans: The Coevolution of Intelligence and Longevity With Intergenerational Transfers

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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.

We present a theory of life history evolution, which integrates biological theory and the economic theory of capital. We first review the evidence on the evolution of brain size and longevity and present our theory informally. The formal model is outlined next, including its key implication that a more challenging environment, one in which learning by doing plays a larger role, selects for both a larger brain and lower mortality. Three lines of relevant empirical evidence are then considered, and the paper concludes with a brief discussion of the implications of our theory.


This article was originally published in Proceedings of the National Academy of Sciences of the United States of America, volume 99, issue 15, in 2002.

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National Academy of Sciences