Ecological dominance, social competition, and coalitionary arms races: Why humans evolved extraordinary intelligence
Introduction
Humans have an unusual array of characteristics that distinguish us from other species. Our cognitive abilities are most remarkable. Hominin brain size increased more than 250% in less than 3 million years. Much of this increase occurred in the past 500 thousand years and disproportionately affected the size (Ruff, Trinkaus, & Holliday, 1997) and the organization of the neocortex (Adolphs, 2003, Deacon, 1997a, Holloway, 1996, Rilling & Insel, 1999, Semendeferi et al., 2001). The behavioral changes were even more impressive, especially in the last few thousand generations (e.g., Klein, 1999; Mithin, 1996). The Upper Paleolithic/Late Stone Age “creative explosion,” continuing a long tradition, has generated an unparalleled expansion of information and individual expression, albeit within the restraints of collective meaning (Caspari & Lee, 2004, Henshilwood & Marean, 2003, McBrearty & Brooks, 2000). Notwithstanding the impressive cognitive adaptations of other species such as chimpanzees and dolphins (e.g., de Waal & Tyack, 2003, Mann & Sargeant, 2003, Premack & Woodruff, 1978), the products of human minds stand out as one of life's most impressive features.
Many hypotheses have been proposed concerning the selective advantages of cognitive change during human evolutionary history. Most explanations involve ecological problem solving, such as tool use (e.g., Darwin, 1871, Gibson & Ingold, 1993, Washburn, 1959, Wynn, 1988), hunting (e.g., Dart, 1925; Hill, 1982, Washburn & Lancaster, 1968), scavenging (e.g., Blumenschine & Cavallo, 1992), foraging (e.g., Isaac, 1978; Kaplan, Hill, Lancaster, & Hurtado, 2000), extended life history (e.g.,van Schaik, & Deaner, 2003), food processing (e.g., Wrangham, Jones, Laden, Pilbeam, & Conklin-Brittain, 1999), and savanna (e.g., Laporte & Zihlman, 1983) or unstable (Potts, 1998, Vrba, 1995) environments. None has achieved complete or general acceptance, even when combined in synthetic models and causally linked to social dynamics.
Common problems for these models include difficulties with explaining why humans evolved such extraordinary cognitive competencies (e.g., awareness of the self as a unique and social being; Tulving, 2002), considering that many other species hunt, occupy savanna habitats, have long lifetimes, endured the same climatic fluctuations, and so forth. Additional problems arise from the lack of clear domain-specific adaptations for the above scenarios, with the possible exception of tool construction (Hodges, Spatt, & Patterson, 1999) and folk biology (e.g., mentally representing the essence of hunted species; Atran, 1998). Even these adaptations, however, pale in comparison with the human cognitive abilities of consciousness, language, self-awareness, and theory of mind (TOM). These competencies do not appear to be adaptations for tracking prey or collecting fruit, nor spurious outcomes of neurogenesis or other developmental processes (but see Finlay et al., 2001, Williams, 1966). All these models, moreover, have difficulties accounting for the diversity of culture into seemingly nonutilitarian areas, such as art (Coe, 2003) and religion (Boyer, 2001).
One possibility is that an advance in linguistic abilities, such as abstract symbolic representation, was the Rubicon for a dramatic origin of cultural abilities (e.g., Washburn, 1978, White, 1959). Some have suggested that a sudden genetic change might underlie this transition (e.g., Calvin & Bickerton, 2000, Klein & Edgar, 2002; cf. Enard et al., 2002). It is uncertain what benefit such a saltational mutation event might have for the initial individual in which it occurred, for there would not be anyone else to talk or “culture” with. Complex adaptations, and cultural abilities surely qualify as such, are products of long directional selection with successful intermediary stages (Dawkins, 1986, Mayr, 1982), including language (Nowak, Komarova, & Niyogi, 2001). Our close relatives, great apes, exhibit behavioral variations and traditions involving social learning that suggests a more gradual transition (de Waal, 2001, McGrew, 2003, Stanford, 2001, van Schaik et al., 2003, Whiten et al., 1999, Wrangham et al., 1994). The fossil record indicates a continuous, albeit rapid, pattern of increase in cranial capacity among hominins (e.g., Lee & Wolpoff, 2003, Lewin & Foley, 2004, Ruff et al., 1997). Although apparently abrupt artifact changes are suggestive of significant transitions, the hypothesis that the Upper Paleolithic creative explosion was caused by a neurological “hopeful monster” remains implausible.
Sexual selection is another recent explanation consistent with several characteristics of hominin cognitive evolution (Darwin, 1871, Miller, 2000). The main idea is that mate choice by hominin females for increasingly intelligent males was an important selective pressure acting on cognitive abilities. As Darwin (1871) speculated, “mental endowment” of the human was analogous to the “ornamental plumage” of the peacock. Although we agree that mate choice is likely to have been a significant force, it is unclear why hominins were the only taxon in which sexual selection favored such elaborate mental displays. The finding that men and women have a different pattern of specific cognitive abilities suggests that different features of sexual selection, including female–female competition, might have contributed to human cognitive evolution, but the lack of sex differences in overall levels of general intelligence is inconsistent with the female choice hypothesis (Geary, 1998). Although a chance genetic event is possible, “perhaps there was a mutation affecting their sexual preferences” (Miller, 2000, p. 71); there are additional factors associated with hominin evolution that suggest that a more comprehensive scenario is likely.
A different approach to the problem of the evolution of human cognition involves the consideration of the brain as a “social tool” (Alexander, 1971, Alexander, 1989, Brothers, 1990, Byrne & Whiten, 1988, Dunbar, 1998, Humphrey, 1976, Jolly, 1966, Jolly, 1999). This hypothesis suggests that many human psychological adaptations function primarily to contend with social relationships, with ecological constraints (e.g., hunting or extractive foraging) being a more secondary source of recent evolutionary change. It appears that some human cognitive competencies, such as TOM and language, are most readily understood in terms of social selection pressures, although cognitive competencies for interacting with the physical (e.g., navigating) and biological worlds are evident as well (Geary & Huffman, 2002). The primary mental chess game, however, was with other intelligent hominin competitors and cooperators, not with fruits, tools, prey, or snow. Human social relationships are complex and variable. Predicting future moves of a social competitor–cooperator, and appropriate countermoves, amplified by networks of multiple relationships, shifting coalitions, and deception, make social success a difficult undertaking (Alexander, 1987, Alexander, 1990b, Axelrod & Hamilton, 1981, Byrne & Corp, 2004, Daly & Wilson, 1988a, Daly & Wilson, 1988b, de Waal, 1982, de Waal, 2002, Stanford, 2001).
Indeed, the potential variety of human social puzzles is apparently infinite; no two social situations are precisely identical, nor are any two individuals ever in exactly the same social environment. Moreover, social relationships can change rapidly, requiring quick modification of strategy. Variability in these dynamics creates conditions that should favor the evolution of brain and cognitive systems above and beyond more traditional modular systems (Fodor, 1983, Tooby & Cosmides, 1995). These systems have been cast in terms of general intelligence, domain-general abilities, or executive functions that are capable of integrating and co-opting information processed by more restricted, domain-specific mechanisms (Adolphs, 2003, Blakemore et al., 2004, Geary, 2005, Preuss, 2004) and using mental simulations, or “scenario building” (Alexander, 1989), to construct and rehearse potential responses to changing social conditions. These complex cognitive processes would be more capable of contending with, and producing, novelties of cultural change and individual-specific differences (Flinn, 1997, Flinn, 2004, Tomasello, 1999).
The social tool hypothesis initially encountered the same problems as the physical environment hypotheses did. The uniqueness issue was especially difficult. Comparative analyses indicated that group size and proxy measures for brain size (e.g., cranial capacity, neocortex ratios) were associated in a wide range of taxa, including primates (e.g., Kudo & Dunbar, 2001, Pawlowski et al., 1998, van Schaik & Deaner, 2003). A major problem, however, remained unresolved: Given that hominin group size was unlikely to have been larger than that of their close relatives (the other hominoids), what was qualitatively different about the hominin social environment? Why did hominins, in particular, form more socially complex groups, hence creating an environment in which more sophisticated forms of social cognition (e.g., TOM) and general intelligence would have been more strongly favored by natural selection than in related species? Why were coalitions more important, and more cognitively taxing, for our hominin ancestors than for any other species in the history of life? Why did hominins evolve special abilities such as “understanding other persons as intentional agents” (Tomasello, 1999, p. 526)? The critical missing pieces of the puzzle were provided by Alexander, 1989, Alexander, 1990a in two seminal essays.
Section snippets
The ecological dominance–social competition model
“… humans had in some unique fashion become so ecologically dominant that they in effect became their own principal hostile force of nature, explicitly in regard to evolutionary changes in the human psyche and social behavior…the real challenge in the human environment throughout history that affected the evolution of the intellect was not climate, weather, food shortages, or parasites—not even predators. Rather, it was the necessity of dealing continually with our fellow humans in social
The hominin fossil record
The temporal sequence of change in hominin anatomy, as documented in the fossil record, is the single source of data on the order of acquisition of key human traits. For example, the first substantial increases in hominin brain size, and perhaps reorganization, occurred with the appearance of the genus Homo roughly 2 mya (see Lee & Wolpoff, 2003). The fossil record reveals that encephalization is not causally linked with bipedality or stone tool use (Darwin, 1871) because encephalization
Brain
The human brain is roughly two to three times larger than that of both our closest relatives and the earliest fossil hominins and comes at a cost of 20% of our metabolic resources (Armstrong, 1990). Given this, it is unlikely that the human brain would have evolved without an extraordinary functional payoff (Dunbar, 1998). But the differences between human and nonhuman brains are not in size and calorie consumption alone. In addition to the much more complex patterns of cerebral convolutions (
Concluding remarks
The EDSC model proposes that hominins uniquely evolved sophisticated brains because they increasingly became “their own principal hostile force of nature” (Alexander, 1989, p. 469) via increased inter- and intragroup competition and cooperation. Sophisticated social–cognitive and linguistic capacities were favored because such skills allowed individuals to better anticipate and influence social interactions with other increasingly sophisticated humans. This “runaway” directional selection
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