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other humanities, reflect in diverse ways upon what it can mean to be uniquely human.

With respect to life-history traits, humans tend to live longer and mature later than our nearest living relatives (the great apes); yet, paradoxically, we share similar ages at which females lose the last of their fertility. In other words, human females have exceptional postmenopausal longevity. In Chapter 11, Kristen Hawkes addresses the history of scientific speculation about this evolutionary conundrum, including an elaboration of senescence theories, resource allocation theories, and especially the “grandmother hypothesis” that emphasizes the key supportive roles that postreproductive women can play in rearing grandchildren. Hawkes then focuses on life-history comparisons between humans and chimpanzees, and describes variation in aging patterns within and among populations of both species that may seem inconsistent with some of the standard assumptions of life-history theory, such as that tradeoffs inevitably exist between current and future female reproductive success. To help reconcile these apparent contradictions, Hawkes proposes that individuals differ substantially in their overall “frailties,” such that those who are more robust can enjoy not only higher fertility but also better survival. Incorporating this idea into life-history theory may offer some fresh insights on human aging.

Culture, which can be defined as the deployment of socially learned information, has been a part of the “human condition” for more than 2 million years (as judged, for example, by the early appearance of stone tools) and it is the proximate reason for our remarkable success as a species. Cultural evolution emerged from biological evolution and the two processes are similar in some respects, but very different in others (such as in the speeds at which they operate and in their modes of information transmission). In Chapter 12, Peter Richerson and Robert Boyd develop the case that human genes and human culture coevolve, with cultural innovations often precipitating environment-mediated changes in natural selection and social selection with feedback effects on gene evolution. They further argue from paleontological and other evidence that geneculture coevolution has been a dominant process underlying human evolution perhaps ever since the initial divergence of hominins from their last shared ancestor with the great apes. Looking forward, Richerson and Boyd see great promise for new genomic tools to help clarify geneculture coevolution in several ways: by providing better marker-based assessments of human paleodemography; detecting genomic footprints of selection and thereby revealing exactly where and when selection took place in the human genome; and yielding mechanistic insights into the structures and functions of particular genes that have been under natural or social selection.



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