Darwin closed The Descent of Man by noting two fundamental aspects of the human condition that at face value might seem contradictory: “man with all his noble qualities, with sympathy which feels for the most debased, with benevolence which extends not only to other men but to the humblest living creature, with his god-like intellect—… still bears in his bodily frame the indelible stamp of his lowly origin.” Ever since that time, philosophers as well as biologists have sought to reconcile these two sides of human nature, at times emphasizing our biological similarities and close evolutionary ties to other primate species, and at other times accentuating the features that seem to separate hom*o sapiens from the remainder of the biological world. Indeed, some have argued that Darwin might better have entitled his treatise The Ascent of Man. Among the characteristics that might be deemed uniquely human are extensive tool use, complex symbolic language, self-awareness, deathawareness, moral sensibilities, and a process of cultural evolution that, while necessarily rooted in biology, goes well beyond standard biological evolution per se. Following the reasoning and terminology of the French philosopher Teilhard de Chardin, Theodosius Dobzhansky (1967) argued that two transcendent events have occurred to date in the Earth’s history: the ancient emergence of life, which initiated the biosphere and enabled biological evolution; and the recent emergence of intelligence in hom*o sapiens, which initiated the noosphere (“thinking arena”) and enabled cultural evolution. In Part III of this volume, leading academicians with backgrounds ranging from genetics to linguistics and the 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.
Culture and cultural evolution are greatly facilitated by another uniquely human characteristic: complex grammatical language, which allows people to share acquired knowledge, negotiate agreements, and otherwise interact readily in social contexts. The net result is that our ancestors were able to colonize a previously unoccupied “cognitive niche,” one hallmark of which is enhanced survival due to environmental manipulation through cause-and-effect reasoning and social cooperation. But even if the evolution of general intelligence and the capacity for language are explicable in terms of the physical and social selective advantages they afforded our ancestors, the question remains as to why our evolved cognitive capabilities extend also to the kinds of abstract reasoning sometimes displayed in, for example, science, philosophy, law, government, and commerce. In Chapter 13, Steven Pinker reviews the history of speculation about the emergence of abstract intelligence, ranging from standard evolutionary scenarios for how physical and social evolution might have favored bigger brains, to supernatural causation (as was invoked by Alfred Russel Wallace, the codiscoverer of natural selection). Pinker then develops a somewhat different perspective on abstract intelligence that builds on a longstanding observation in linguistics: people often extend word constructions based on concrete scenarios to more abstract concepts, by analogy. Under Pinker’s scenario of “metaphorical abstraction,” cognitive schemas and social emotions that were important in promoting the capacity for language and adapting humans to the cognitive niche eventually became assembled into increasingly complex mental structures that have been co-opted to perform abstract mental functions they had not originally evolved to promote directly.
Language is again the topic of discussion in Chapter 14, where Terrence Deacon recounts a long history of oft-tortuous speculation about how a social capability that appears to be as complex and variable in expression as language might have arisen and come to occupy such a central position in human evolution. The basic problem, as Deacon and some others have seen it, is somewhat akin to explaining the emergence of other extravagantly complicated traits that in their initial evolutionary stages are not necessarily of clear utility to their bearers in the struggle for existence; indeed, one well-known modern linguist has argued that language competence did not evolve by standard natural selection because its rudiments would not likely have facilitated effective communication. In The Descent of Man, Darwin at one point resorted to the concept of sexual selection to explain the emergence of language, suggesting that human vocal complexity and the mental capacity it reflects might have evolved in part as a means to attract mates. On the other hand, even a prelinguistic symbolic communication or protolanguage could probably have contributed to a novel cognitive niche (see Chapter 13) that in turn imposed novel selective demands on the proto-human brain and vocal apparatus for more effective communication. In any event, to add another perspective to the deliberations, Deacon suggests that a relaxation (rather than an accentuation) of selective pressures at the organismal level may have been the source of many of the complex and synergistic features of the uniquely human capacity for language.
High intelligence, cognition, and the capacity for reasoning that the human brain enables are so central to the human condition as to be inseparable from what makes us uniquely human. They are also highly adaptive features without which human culture could only be rudimentary at best. But is reasoning a single all-purpose procedure of the human mind, or, alternatively, is it an amalgam of special-purpose (i.e., “domain-specific”) operations each having evolved in response to a specific suite of adaptive challenges posed by particular social or physical environments that were encountered routinely by our ancestors? The former hypothesis is sometimes referred to as the “blank-slate” theory of cognition in traditional psychology whereas the latter hypothesis tends to be favored by many evolutionary psychologists who envision the evolved architecture of the human mind to include multiple cognitive specializations each molded by natural selection to solve a particular adaptive problem. In Chapter 15, Leda Cosmides, Clark Barrett, and John Tooby review the history of these and other ideas about the nature of the neurocognitive system and human intelligence. Based in part on the results of psychological tests designed to distinguish experimentally between blank-slate and domain-specific operations of human cognition, the authors conclude that the human mind probably contains a multitude of different adaptive specializations for reasoning. One of the most salient of these specialized adaptations, the authors argue, is the hypertrophied human capacity to detect cheaters in social contracts.
Morality is a uniquely human attribute, to which Darwin attached a special significance: “I … subscribe to the judgment of those writers who maintain that of all the differences between man and the lower animals the moral sense or conscience is by far the most important.” In the final chapter of these proceedings, Francisco Ayala makes a fundamental distinction between the capacity for ethics (i.e., the human capacity for a moral sense) and the expression of moral norms that can vary from one human society to another. The former, Ayala argues, is an inevitable byproduct of the biological evolution of high intelligence, which itself arose from selection pressures for other fitness-enhancing capabilities such as bipedalism and tool use; whereas the latter, by contrast, are products of cultural evolution rather than biological evolution. This distinction between morality and moral norms generally parallels the obvious distinction between the capacity for creative language and the particular languages that happen to be spoken by particular societies. In developing this line of argumentation, Ayala invokes the distinction between an adaptation (something targeted quite directly by natural selection—in this case, higher intelligence) and an exaptation (something that arises by being co-opted to serve a positive role other than its original selection-promoted function). Ayala’s distinction between ethics and moral norms is helpful but it nevertheless leaves open important questions regarding whether and to what extent particular moral norms (as well as a general moral sensibility) are genuinely adaptive for the human groups that display them (as opposed to being nonadaptive or perhaps even maladaptive on some occasions). Such questions no doubt will continue to intrigue sociobiologists and philosophers alike.
11. How Grandmother Effects Plus Individual Variation in Frailty Shape Fertility and Mortality: Guidance from Human-Chimpanzee Comparisons
12. Gene–Culture Coevolution in the Age of Genomics
13. The Cognitive Niche: Coevolution of Intelligence, Sociality, and Language
14. A Role for Relaxed Selection in the Evolution of the Language Capacity
15. Adaptive Specializations, Social Exchange, and the Evolution of Human Intelligence
16. The Difference of Being Human: Morality
