Commentary on Wynn
Abstract: 60 words
Main Text: 1379 words
References: 726 words
Total Text: 2165 words
Evolutionary theories of human cognition should refer to specific times in the primate or hominid past. Though alternative accounts of tool manufacture are possible (frontal lobe function), Wynn demonstrates the power of archaeology to guide cognitive theories. Many cognitive abilities evolved not in the "Pleistocene hunter-gatherer" context, but earlier, in the context of other patterns of social organization and foraging.
Wynn’s target article on cognitive archaeology brings a much-needed perspective to research on the evolution of human cognitive mechanisms. Theories in evolutionary psychology often present hypotheses about adaptive pressures that shaped psychological abilities without referring them to specific times in the hominid past. Wynn adds precision to the definition of the "environment of evolutionary adaptedness" (EEA) for spatial cognition and tool manufacture, and questions whether our early Pleistocene ancestors were at all adapted to a hunter-gatherer lifeway. Whether or not Wynn’s theory of the cognitive skills required for tool manufacture is correct, his work represents an often-missed step in developing evolutionary theories of cognition. Below, I outline the steps involved in developing such theories, and discuss Wynn’s contributions within that framework.
Cosmides and Tooby (1987, 1992) have outlined the usefulness of
Marr’s (1982) computational theory approach to developing theories
of cognition. There are several steps involved:
1) Specify the adaptive function of the computation, i.e., what
is it that having this cognitive ability allows us to do?
2) Identify the time period during which that adaptive
problem existed.
3) Identify the EEA, the relevant selection pressures that
prevailed during that time.
4) Propose a set of processes and representations that could
serve the identified function. These must be powerful enough
to solve the problem.
5) Make predictions about patterns of behavior the proposed
computations would generate.
6) Devise tests between alternative theories that could
explain the same pattern and one’s own computational theory.
Though not subscribing to this framework, Wynn emphasizes the power of
using archaeology for Steps 2-3; in his words, defining
the timing and context of developments
in human cognition. Archaeology can also make contributions
to the other steps.
For all its emphasis on evolutionary forces, evolutionary psychology seldom discusses the archaeological record of hominid evolution. Wynn shows us why archaeology is necessary. Evolutionary psychologists refer frequently to the EEA for humans, usually characterized as the selection pressures acting on "Pleistocene hunter-gatherers" 2,000,000-10,000 years ago, who are modelled as being like current hunter-gatherers.1 However, the definition of the EEA for a particular adaptation is the set of selection pressures that occurred while that adaptation was evolving (Tooby & Cosmides, 1992); thus not all cognitive mechanisms have the same EEA. Developing a computational theory of the adaptive function of a mental process requires specifying the conditions that prevailed while it was evolving. Knowing those conditions depends on archaeology.
Wynn never uses the term "EEA" but does define the time frames for particular adaptations in spatial cognition, which is crucial for identifying the relevant selection pressures. The period of adaptation for an ability pre-dates appearance of the fully- developed ability. Thus, if the spatial skills required for making Mode 1 stone tools are present in other apes, then the EEA for these skills includes conditions present for Miocene apes. (However, the recent finding that crows spontaneously impose shape on tools raises questions about whether this skill is unique to primates (Weir, Chappell & Kacelnik, 2002).) The EEA for imposing bilateral symmetry in toolmaking comprises those selection pressures acting on Homo habilis and erectus ~2.5-1.5 million years ago, from when flaked stone tools first appeared to when clear evidence of symmetry appeared. The EEA for imposing more elaborate forms of symmetry includes the changing selection pressures acting on Homo erectus and archaic Homo sapiens ~1.5-0.5 million years ago. Homo erectus occupied a wider variety of habitats than earlier hominids -- Africa, Asia, Europe (Vekua et al., 2002) -- and foraged but did not hunt large game. The major selection pressures acting on Homo erectus seem to have been those of foragers moving into new habitats with unfamiliar food resources. Archaic Homo sapiens, in contrast, were big game hunters, and faced somewhat different selection pressures.
Wynn wrestles with a difficult problem in doing Steps 1 and 4, above. Steps 1 and 4 are related: knowing the function of this new spatial ability would clarify the necessary representations and processes. Wynn identifies the ability that is of interest: imposing form and symmetry on created objects. However, what adaptive problem does this ability solve? What is the function of imposing symmetry? It is unclear why it was more adaptive to make symmetrical than asymmetrical tools. There is a link between Step 1 and Steps 2-3: knowing the context and selection pressures acting during a period of time allows one to specify adaptive function. However, Wynn does not take full advantage of the power of archaeology here. He has done an excellent job of describing the relevant context, yet he does not refer the question of adaptive function to the specific context of Homo erectus or archaic Homo sapiens. Instead, he considers and rejects adaptive explanations based on preference for symmetry, mate value, and navigation, none of which are problems specific to those time periods. Focusing on the evolutionary context of those species would strengthen his analysis here.
Archaeology can also contribute to Steps 5-6, comparing the evolutionarily-derived theory to alternative accounts. One alternative theory to Wynn’s is that imposition of symmetry depended not on new cognitive abilities, but on manual dexterity absent before 1.5 million years ago. Analysis of muscle attachments on hands and wrists of fossil skeletons could illuminate this. Another possibility is that the necessary spatial skills were already present, but using them for innovations in tool use required greater frontal lobe capacities. I believe domain-general frontal executive functions would be more likely candidates than the more general "associative abilities" Wynn discusses, as unspecified associative abilities fail the solvability criterion of Step 4. Anticipating a future need for a tool (Suddendorf & Corballis, 1997) planning, and working memory might be the crucial cognitive skills. Here, archaeology and neuroscience together can supply answers. Semendeferi and colleagues showed that parietal cortex, seat of our spatial skills, is not proportionately larger in humans than in other primates relative to body size (2000), whereas the frontal pole, involved in executive function, is disproportionately larger in humans (2001). Changes in skull morphology that significantly distinguish our species -- a domed skull and a less retracted face -- allowed more room for the frontal lobes (Lieberman et al., 2002). These two sources of data imply that selection was for frontal lobe abilities, not spatial skills. Analysis of hominid endocasts to determine the extent of key sulci and gyri could also shed light on the relative size of parietal and frontal lobes (Falk, 1987).
One of Wynn’s most significant contributions is clarifying the evidence that a hunter-gatherer lifestyle did not emerge until ~200,000 years ago, that our ancestors were not like modern hunter-gatherers. What were they like? Wynn’s conclusion that Homo erectus did not live in groups because they did not have speech is odd given the many group-living social primates who lack speech. Like archaic Homo sapiens, Homo erectus could have lived in groups, even if those groups lived differently than modern hunter-gatherers. Both species were social foragers facing different adaptive problems.
One conclusion to draw from the recency of hunter-gatherers is that the hunter-gatherer way of life is the result, not the cause, of evolution in human psychological mechanisms. Between the emergence of a hunter-gatherer lifestyle ~200,000 years ago and the spread of anatomically modern humans out of Africa 80,000 years ago (cf. Thorne et al., 1999, Capelli et al., 2001), only 120,000 years, or 6,300-8,000 generations2 elapsed. The claim that humans have a large number of psychological adaptations with special design features for anything like the modern hunter-gatherer lifestyle is difficult to reconcile with these numbers.
I hope that collaborations between archaeologists and cognitive psychologists will become more common. The type of task analysis Wynn does for hominid toolmaking over time should be taken as a model for Steps 2-3 in characterizing a psychological mechanism. Archaeology can help define adaptive functions for certain abilities by identifying the relevant time and selection pressures. Archaeology can also rid evolutionary psychology of vague assertions about "Pleistocene hunter-gatherers." Spatial cognition, cooperation3, living in small groups, hierarchy negotiation are all adaptive problems that should be referred not to "our hunter-gatherer ancestors" but to earlier time periods, with other patterns of social organization and foraging.
Knowing one’s ancestors is centrally important in the mythology of hunter-gatherers all over the world. If evolutionary psychologists really want to take a lesson from hunter-gatherers, we had better start talking to our ancestors. Wynn has shown us one way to do so.
1. E.g., Cosmides & Tooby, 1987; Cosmides, 1989; Cosmides & Tooby, 1992; Ellis, 1992; Wright, 1994; Buss, 1999; Silverman et al., 2000; Kurzban, Tooby & Cosmides, 2001.
2. This assumes generation times ranging from 15-19 years of age (Smith & Tompkins, 1995; Bogin & Smith, 1996; Dean et al., 2001; Christopher Dean, personal communication, 4/12/02).
3. Stone et al. (2002) define the EEA for social exchange as at least as long ago as the Miocene.
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