At Penn Museum Conference: Summary of Richerson

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1 At Penn Museum Conference: Summary of Richerson Steven O. Kimbrough September 2007 File: PennMuseum-foils-sok.tex/pdf.

2 Two papers Conference paper: Rethinking Paleoanthropology: A World Queerer than We Supposed Incomplete. Background paper: Evolution on a Restless Planet: Were Environmental Variability and Environmental Change Major Drivers of Human Evolution? Background paper first. 1

3 Two kinds of factors set the tempo and direction of organic and cultural evolution, 1. external factors: those external to biotic evolutionary process, such as changes in the earths physical and chemical environments, and 2. internal factors: those internal to it, such as the time required for chance factors to lead lineages across adaptive valleys to new niche space.... The relative importance of these two sorts of processes is widely debated. This is about climate, an external factor. 2

4 Main thesis The climate deterioration of the last few million years resulted in high amplitude variation in climates on the scale of the classic glacial advances and retreats (21, ,000) years. These variations are too slow to favor adaptation by behavioral flexibility and culture. During the last few glacial cycles, however, the ice ages were accompanied by high amplitude fluctuations on time scales of a millennium and less. Theory suggests that these are the time scales that can favor the evolution of costly systems for adapting to variable environments by individual learning and cultural transmission. On millennial and submillennial time scales, range changes and organic evolution will be too slow to approximate adaptive equilibrium in many habitats. Evolution will thus favor big-brained creatures that can adapt rapidly by behavior means. Human culture is particularly suited to high frequency fluctuating environments because humans can use 3

5 cumulative cultural evolution to run up quite fancy adaptations to particular environments on time scales of a few decades to a few millennia. Two figures tell much of the story... 4

6 Figure 1: Richerson, Bettinger, and Boyd, August 2004: Cooling followed by increased variance. This is also Figure 3 of the conference paper. 5

7 Figure 2: Richerson, Bettinger, and Boyd, August 2004: High variance followed by warmer stability (and farming, etc.) This is also Figure 4 of the conference paper. 6

8 Variation makes for learning, culture That is, sophisticated nervous systems are mechanisms for adaptive phenotypic flexibility. Large brains are very costly organs and must pay their keep (Aiello and Wheeler 1995). Ordinary learning is an individual-level adaptation and hence is especially suited to environments that vary from generation to generation or to changes that occur within a generation. Human social learning (culture) is a complex and sophisticated system. It is suited to rapid adaptation time scales ranging from less than a generation to tens or even hundreds of generations depending upon how the cultural system is structured. A Goldilocks hypothesis: not too much, not too little. 7

9 Goldilocks Very slow environmental variation favors range changes and organic evolution. In very rapidly changing environments, each individuals environment becomes random with respect to every others and attempting to use the behavior of others as a clue to ones own adaptive behavior is useless. Over a broad range of intermediate environmental change between these extremes, social learning is favored. 8

10 There is some evidence Brain sizes increases in mammals roughly parallel the decline in mean global temperature, with much of the increase concentrated in the Pleistocene as high-frequency variation in climate increased dramatically. Claimed: cooling increased climate variance. 9

11 Human learning matters a lot The human ability to acquire complex cultural elements by imitation means that populations of humans can evolve complex adaptive traditions relatively quickly. Innovations made by one person can be added to those of others so the cultural complexity builds up over time. The runaway success of the human species owes to our ability to evolve a diverse array of technologies and social organizations suited to particular environments. Even without agriculture humans spread to most of the earth s terrestrial surface including environments far different from our tropical homeland. 10

12 So why just us? Time Preadaptations Functional constraints Cultural diversification (Admittedly speculative.) 11

13 In the end... The hypothesis that the direct cause of large brains generally and human culture in particular is increasing high-frequency environmental variation currently lacks strong empirical confirmation. It is in the spirit of using theory to search for plausible proxies for past climates. We expect that much richer data on this aspect of climate change will be forthcoming over the next decade. Now to the conference paper. 12

14 Conference Paper Internalism/externalism again. The alert reader will be struck that everyone has to be an externalist, and internalist, and a coevolutionist/niche contructionist, depending upon the scale at which the question is asked. The right questions to ask are what processes drive particular evolutionary events on what time scales? Goldilocks again. See Figure 1 of this paper. 13

15 Figure 3: Conference paper Figure 1: a=0, all culture, no learning; 14 a=1, all individual learning, no culture; V H environmental variation; R, intergenerational correlation.

16 In words... Thus, this simple contest predicts that cultural organisms should evolve in highly variable environments, but only when the variation from generation to generation is fairly high but not so high as to make it easy for selection on genes to carry the entire adaptive load. NB: This is an uncalibrated model, but: Note that this modeling exercise was complete in its essentials more than a decade before the climate variation described in the next section was discovered, so the models were not concocted to fit the data! 15

17 After more climate reports... Thus, the picture that emerges is that the plant and animal communities of last-glacial times was often far out of equilibrium with the climate of the moment. Ranges were expanding or contracting. Ill adapted species might persist because they faced no effective competition from better adapted species that had yet to expand their range to a given location.... Thus, ice age plaid ecosystems were probably highly and unpredictably dynamic. Such environments would have imposed novel constraints on hunter-gatherer adaptations. 16

18 New here: bistable hypothesis Populations so small that they lose complex tools would also have a less responsive cultural evolutionary system. Perhaps over a wide range of herbivore productivities, human population densities were bistable. A high population density equilibrium would generate a fancy technology and rapid evolutionary response to millennial and submillennial scale variation. Hence it could maintain high population density. A small population in the same conditions would have a simple toolkit and a slow response to variation and hence would remain small. Outside this middle range, an especially rich environment might allow a simple system to jump to the complex equilibrium while an especially poor one would reduce a complex population to simplicity. Perhaps in good times in good places Anatomically Modern Humans could achieve population sizes adequate to sustain more complex toolkits whereas in poorer times and places they could only sustain 17

19 simpler technologies. If environments remained poor enough long enough, a population that had achieved UP complexity might suffer a Tasmanian style loss of complexity and drop back to the MP equilibrium. Rather than reacting directly to an environmental change, a population will have a strong tendency to remain either large or small. Given a sufficiently large and persistent change, it will jump to the other equilibrium where it will again persist under environmental conditions where the other equilibrium could be sustained but can t be attained. 18

20 In conclusion In sum, the paleoclimatological and paleoecological evidence suggests that Pleistocene people faced very different challenges than did Holocene hunter-gatherers, especially the late, specialized hunter-gathers of the ethnographic sample. The paleoanthropological evidence says much the same thing. Unfortunately, neither body of evidence is sufficient to nail a description (or descriptions) of Paleolithic social life. We can fairly clearly see that MP and UP social systems were quite different. Since Anatomically Modern Humans participated in both technologies, a case can be made that cultural differences, driven by something like our bistable population hypothesis, or directly by environmental productivity, rather than genes, being responsible for the complex interdigitation of MP and UP lifeways. 19

Bio 1M: The evolution of apes. 1 Example. 2 Patterns of evolution. Similarities and differences. History

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