Uniquely Human

We noted above that defining humans is a parlor game with roots going back to the Greek philosophers, but during the past decades, this exercise has acquired a more solid foundation based on comparative analyses. But the difficulties of adequately characterizing humans as a species and distinguishing humans from other primate taxa are often overlooked. The overwhelming majority of humans now live in settled societies, surrounded by written texts or even more modern media technology, in very large societies that have multilayered organizations and with numerous institutions. However, all of this is very recent, with the oldest elements not even 12,000 years old. The few remaining hunter-gatherers have life styles that are most similar to those in which humans lived for most of their history, including natural levels of fertility (Hawkes 2006). In addition, we can assume that human universals reflect our most ancient human roots. This relies on the argument that if humans display a common trait across our vast range of social and environmental conditions, this common trait must also have been in the early modern humans that evolved in Africa, and then populated the world. Especially, where the two sets of traits overlap, we can have some confidence in their deep roots. Finally, important insights into human nature have emanated recently from cross-cultural experimental studies of human economic choices (e.g., Fehr and Rockenbach 2004; Henrich et al. 2005).

These developments have made it possible to update and organize the existing lists of the derived traits of humans relative to the reconstructed traits of the last common ancestor. Whole books have been devoted to the subject (e.g., Antweiler 2007), and this is not the place to develop a detailed list. But we want to do two things here: first, to clarify the procedure and, second, to present a selected summary of the major differences between ourselves and other primates.

A summary of derived features requires that we not only know the differences but also their polarity. A difference between two sister taxa can be due to a change in one, a change in the other, or a divergence in both. Polarity in mind and behavior is usually fairly straightforward (the issue is much less obvious at the genomic level), but to make sure, it is useful to compare humans with the genus Pan and with the other great apes, as well. Fortunately, in spite of their remarkably variable social organization and subsistence, the great apes, as a group, are rather homogeneous with respect to cognition (Deaner et al. 2006; Burkart et al. in press), brain size (Schoenemann 2006), and life history (Robson et al. 2006). This homogeneity implies that they are rather conservative, making it easier to infer polarity.

A summary of the nonmorphological and nonphysiological features that are derived in humans relative to the great apes and not discussed in subsequent chapters (see also Flinn et al. 2005; Richerson and Boyd 2005; Burkart et al. in press) would include the following unusual features:

• Cumulative material culture and social institutions and rituals, all critically dependent on language. Culture is perhaps our preeminent adaptation.

• Unusual subsistence ecology, involving skill-intensive hunting and gathering and extremely intense cooperation, also in between-group conflict (Ridley 1996; Kaplan et al. 2000; Gurven 2004).

• Slower development, longer lifespan, accompanied by higher female reproductive rates and midlife menopause (Robson et al. 2006) and extensive allomater-nal help (Hrdy 2009).

• Unusual cognitive abilities, including language, long-term planning, causal understanding, and episodic memory. These abilities build on shared intention-ality, i.e., the ability to participate with others in collaborative activities with shared goals and intentions (Tomasello and Moll, this volume), which also involves language-based teaching.

So, there is a gap, and it would be foolish to deny it. A mere extrapolation of any of the great ape phenomena is very unlikely to explain the dramatic differences, and it would seem that what we are looking for is a set of selective pressures not encountered by the other great apes or a confluence of capacities, ecological circumstances, and a certain amount of serendipity that set our ancestors on a different path than other great apes. This could be a completely novel set of pressures encountered by no species before, such as pressures emanating from cultural evolution (see Silk and Boyd, this volume; McElreath, this volume). However, that still begs the question why cultural evolution became so much stronger in humans than in the other apes. Thus, we should also look for selective pressures that are novel for the apes but convergently present among other primates, other mammals, or among birds, and which may have precipitated the operation of the truly unique cultural selection. It seems unlikely that we will ever settle on a single account of how we became such an unusual species. But we now have a much richer body of theory and comparative data that allow us to develop more complete and compelling hypotheses that we can critically evaluate.

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