Evolutionary Anthropology Lab
Department of Biological Sciences, Graduate School of Science, University of Tokyo
We study evolution of human behavior from both theoretical and empirical perspectives. One of the biggest questions in science is how humans have acquired the "humanness," or the set of traits that make us feel that we are qualitatively different from the rest of all animals. Although our understandings of phylogenetic and historical details of human beings have been continuously improved, it is still largely unknown why our behavior is so different, as it seems, from that of other animals. To answer this question is to specify the mechanisms by which natural selection has shaped the designs in human behavior. We hope to make some contribution to this ambitious enterprise using mathematical, computational, statistical, and experimental methods. Current research interests are in behavioral ecology of hominins and other animals, evolutionary game theory, cultural evolution, and human mate choice.
Behavioral ecology of hominins and other animals
Male provisioning of offspring and associated pair-based mating systems within multi-male multi-female groups are characteristics of human societies. Paternal care might have emerged in an early stage of hominin evolution and played a unique role in shaping the socioecological niche in which subsequent adaptation took place. We investigated mathematical models for evolution of paternal care and female monogamy to specify the conditions under which various mating systems are predicted (Wakano & Ihara, 2005; Seki et al., 2007; see also Ihara, 2002). Coalition, or coordinated aggression by multiple individuals on a target individual, is observed in primate species. Ihara (2020) evaluated the hypothesis that coalition formation in early hominins may have induced social selection favoring reduced aggression. Seki (2012) explored a hypothetical conflict between autosomal and X-linked genes for sex-specific grandmothering. Kimura & Ihara (2009) studied a model for antagonistic coevolution of male and female traits. Humans are also remarkable in their capacity for social learning (e.g., imitation). Our mathematical and computational models explored how social structure may influence the relative advantages of social learning and individual learning (e.g., trial and error) (Tamura & Ihara, 2012; Tamura et al., 2015). Tamura & Ihara (2011) developed a model for the evolution of simple communication capacities, namely, the capacity for sending a signal and the capacity for receiving it. As for empirical work, Ihara et al. (2016) tested the possibility of socially mediated estrous synchrony or asynchrony in wild anubis baboons (for a related study in chimpanzees see Matsumoto-Oda & Ihara, 2011). Nakahashi et al. (2018) developed a method to obtain "critical interbirth intervals" in hominin species, based on observed age distributions in fossil samples, and argued that the interbirth intervals of early hominins must have already been considerably shortened.
Evolutionary game theory
Evolutionary game theory has become an indispensable tool in the study of human behavior. Of particular interest is the evolution of group-wise cooperation among unrelated individuals. Group-wise cooperation is likely to have been the key adaptation for early humans expanding into the savanna environment, where predation risk was higher and food resources were scarcer. Kurokawa & Ihara (2009) developed a mathematical framework within which to analyze stochastic evolutionary dynamics of n-player games and derived the condition for the emergence of group-wise cooperation in the n-player repeated prisoners' dilemma game as a special case (see also Kurokawa et al., 2010; Kurokawa & Ihara, 2013; Kurokawa & Ihara, 2017; Kurokawa et al., 2018). Ihara (2011) investigated how cooperative behaviors may be affected by a sensitivity to cultural differences among individuals in the context of a hawk-dove game. Tamura et al. (2011) conducted individual-based simulations to study the evolution of punishing behavior from egalitarian motives. Kurokawa (2016) examined the effect of imperfect information in the two-player repeated prisoners' dilemma game considering mistakes in behavior.
Culture, as a pool of information transmitted non-genetically between individuals, exhibits evolutionary dynamics; that is, the cultural composition of a population changes over time due to innovation and differential transmission. This process, as an analogue to genetic evolution, is referred to as cultural evolution. Culture has changed the course of human evolution in three ways. First, culture facilitates rapid adaptation to a changing environment. Second, since cultural evolution proceeds semi-independently of genetic evolution, it may result in the spread of novel behavior not expected by a conventional model of genetic adaptation. Third, culture provides a new environment to which genetic adaptation may occur and thus a possibility of gene-culture coevolution. Ihara (2008) developed a mathematical model to describe a possible dynamics of maladaptive cultural evolution (see also Ihara & Feldman, 2004; Kendal et al., 2005). Seki & Ihara (2012) investigated the rate of cultural change considering stochastic evolutionary dynamics. Takahashi & Ihara (2019) analyzed the cultural and evolutionary dynamics involving payoff-biased social learning when cultural variants are associated with stochastic payoffs. Takahashi & Ihara (2020) developed a model to describe spatial patterns of dialect words spreading from a cental population to peripheries. Tamura (2014) documented homogamy for birthplaces in Japan, which may serve to maintain cultural variation within regions. Lee (2015) attempted to reconstruct the evolutionary history of the Koreanic language, using methods adopted from evolutionary biology (for a related work see Lee & Hasegawa, 2014). Tamura & Ihara (2017) analyzed demographic data to study the cultural evolution of the "hinoeuma" superstition in Japan.
Human mate choice
Animals do not mate at random. Mate choice directly affects the reproductive output of individuals being chosen and thus can have a profound effect on the evolution of morphology and behavior of species. Sexual selection by mate choice may have also played a significant role in shaping human behavior. Nojo et al. (2011) studied the relationship between facial resemblance and attractiveness in rural Indonesia. Nojo et al. (2012) investigated homogamy in facial characteristics among Japanese people and a possible role played by a sexual-imprinting-like mechanism. Seki et al. (2012) collected data on body stature in Japanese to examine the possibility of homogamy and sexual-imprinting-like effect for height. We also used computer simulations to examine the sexual selection hypothesis for the emergence and maintenance of phenotypic diversity among human populations (Nojo & Ihara, 2019).
Takahashi T, Ihara Y, 2020. Quantifying the spatial pattern of dialect words spreading from a central population. Journal of the Royal Society Interface 17, 20200335.
Ihara Y, Ikeya K, Nobayashi A, Kaifu Y, 2020. A demographic test of accidental versus intentional island colonization by Pleistocene humans. Journal of Human Evolution 145, 102839.
Ihara Y, 2020. A mathematical model of social selection favoring reduced aggression. Behavioral Ecology and Sociobiology 74, 91.
Morita M, 2019. Behaviours of dyads sitting outside in New York's Times Square: exploratory observation using webcam videos. Journal of Human Ergology 48, 69-81.
Takahashi T, Ihara Y, 2019. Cultural and evolutionary dynamics with best-of-k learning when payoffs are uncertain. Theoretical Population Biology 128, 27-38.
Morita M, 2019. Human behavioral ecology. In "Encyclopedia of Evolutionary Psychological Science" (Shackelford T, Weekes-Shackelford V editors). Springer, Cham.
Nojo S, Ihara Y, 2019. The effect of sexual selection on phenotypic diversification among human populations: A simulation study. Journal of Theoretical Biology 462, 1-11.
Kurokawa S, Wakano JY, Ihara Y, 2018. Evolution of group-wise cooperation: generosity, paradoxical behavior, and non-linear payoff functions. Games 9, 100.
Nakahashi W, Horiuchi S, Ihara Y, 2018. Estimating hominid life history: the critical interbirth interval. Population Ecology 60, 127-142.
Kurokawa S, Ihara Y, 2017. Evolution of group-wise cooperation: Is direct reciprocity insufficient? Journal of Theoretical Biology 415, 20-31.
Tamura K, Ihara Y, 2017. Quantifying cultural macro-evolution: a case study of the hinoeuma fertility drop. Evolution and Human Behavior 38, 117-124.
Kato A, Morita K, 2016. Forgetting in reinforcement learning links sustained dopamine signals to motivation. PLOS Computational Biology 12, e1005145.
Ihara Y, Collins DA, Oda R, Matsumoto-Oda A, 2016. Testing socially mediated estrous synchrony or asynchrony in wild baboons. Behavioral Ecology and Sociobiology 70, 1921-1930.
Kurokawa S, 2016. Does imperfect information always disturb the evolution of reciprocity? Letters on Evolutionary Behavioral Science 7, 14-16.
Lee S, 2015. A sketch of language history in the Korean peninsula. PLOS ONE 10, e0128448.
Ihara Y, 2015. Human mate choice. pp.335-339 in "International Encyclopedia of the Social & Behavioral Sciences 2nd edition" (Wright JD editor-in-chief) 11. Elsevier, Oxford.
Tamura K, Kobayashi Y, Ihara Y, 2015. Evolution of individual versus social learning on social networks. Journal of the Royal Society Interface 12, 20141285.
Tokumasu Y. How is sexual difference constructed, and what does it construct? ALE (Adolescence x Language Evolution). The University of Tokyo, Tokyo, 5 July 2019.
Morita M. Exploring socioecological foundations for the evolution of language: observation of communication in a natural setting using webcam videos. The 14th Annual Conference of European Human Behaviour and Evolution Association. Toulouse, 23-26 April 2019.
Ihara Y. When and why language emerged. Tokyo Lectures in Evolinguistics 2019. The University of Tokyo, Tokyo, 11-13 March 2019.
Ihara Y. Cultural phylogeny and diffusion. The (co-)evolution of genes, languages, and music from data analyses to theoretical models. Yokohama City University Kihara Institute for Biological Research, Yokohama, 17 Jul 2018.
Nakahashi W, Horiuchi S, Ihara Y. Hominid interbirth interval and evolution of paternal care. 2018 Annual Meeting of the Society for Mathematical Biology & the Japanese Society for Mathematical Biology. The University of Sydney, Sydney, 8-12 July 2018.
Ihara Y, Kaifu Y. Dispersal to islands by the Pleistocene humans: evaluating alternative scenarios. The 1st AsiaEvo Conference. Sheraton Dameisha Resort, Shenzhen, 18-20 April 2018.
Ihara Y. Evolution of physical weakness by social selection through choice of collaborative partners. Kyoto Conference on Evolinguistics. Kyoto University, Kyoto, 11-12 November 2017.
Tamura K, Ihara Y. Quantifying cultural macro-evolution: A case study of the hinoeuma fertility drop. Inaugural Cultural Evolution Society Conference. Max-Planck Institute for the Science of Human History, Jena, 13-15 September 2017.
Ihara Y. An introduction to mathematical modeling in evolutionary archaeology. Perspectives on Prehistoric Cultural Evolution: From Archaeology to Behavioral Experiment, AP Shinagawa, Tokyo, 7-8 August 2017.
For all students
Prospective students are required to take an entrance exam (given in the Japanese language) to Department of Biological Sciences (see here).
For overseas students
For qualified students with overseas education, the score of the GRE Subject Tests may be accepted in lieu of the entrance exam. Department of Biological Sciences requires the GRE subject score of Biology (see here). The University of Tokyo provides international students with access to a number of financial aid options (see here).
Kotaro Aizawa, Kenichi Aoki, Emily Emmott, Naoki Hatanaka, Mika Igarashi, Ayaka Kato, Mariko Kimura, Yutaka Kobayashi, Satoshi Komori, Shun Kurokawa, Mai Kuroshima, Sean Lee, Yuki Mizusaki, Saori Nojo, Kento Orihara, Motohide Seki, Satoshi Sekiguchi, Kohei Tamura, Satoshi Tamura, Tasuku Toyama, Mariko Tsumaki, Shusuke Yamashita, Taro Yoshida
PhD in 2019
Saori Nojo (National Institute of Technology, Kisarazu College)
PhD in 2013
Shun Kurokawa (University of Tokyo)
Kohei Tamura (Tohoku University)
PhD in 2012
Motohide Seki (Kyushu University)
Evolutionary Anthropology Lab
Recent and forthcoming events
9 April 2020
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29 November 2019
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