Evolutionary Anthropology Lab

Department of Biological Sciences, Graduate School of Science, University of Tokyo

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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 from that of other animals. To answer this question, we need 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 and computational methods. Current research interests include evolution of social behavior and cultural evolution.

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Cultural evolution

Yuri Nishikawa investigated the geographic variation of folk songs in the Ryukyu Archipelago, Japan, to infer the underlying process of cultural transmission (Nishikawa and Ihara, 2022). Cultural transmission between individuals is by no means a uniform process; in fact, traits are transmitted in different ways depending on their characteristics. A prime example is found in linguistic change, where words for basic concepts are known to be less liable to borrowing from neighboring languages than other words are. Regarding folk songs, it is plausible that songs sung in different social contexts are transmitted differently. To explore this possibility, Nishikawa analyzed published scores of 1,342 traditional songs, which had been collected from all over the Ryukyu Archipelago, using the CantoCore song classification scheme to quantify the musical distance (i.e., difference) between each pair of songs. The analysis suggested that horizontal transmission between islands/regions within the archipelago had played a major role in the formation of the current geographic patterns in the Ryukyuan songs. It was also indicated that the social context of the songs had affected how they were transmitted within and between populations; in particular, "work" songs exhibited a higher degree of between-island/region diversity, suggesting a smaller inclination to diversify within populations, than "child," "ritual," and "amusement" songs. In addition, the observed pattern of variation in the work songs among islands/regions were associated with the corresponding lexical variation, whereas no connection was detected between song variation and mitochondrial DNA variation.

Cultural evolution

Takuya Takahashi developed a Markovian ancestral model to investigate the temporal and spatial dynamics of cultural evolution (Takahashi and Ihara, 2022). The model considers a network of populations connected by social pathways through which cultural diffusion occurs. Following the method of ancestral backward process, the model focuses on a cultural variant sampled in the present generation and tracks the lineage of the variant back in time to quantify the when and where of its original invention. Takahashi successfully derived analytical expressions for the equilibrium distributions of cultural age in each population and frequencies of variants originating from given populations. As a case study, the spatial pattern of dialect words within Japan was investigated (Takahashi and Ihara, 2020). It has been hypothesized that Japanese dialect variants exhibit a concentric geographic distribution centered at Kyoto, the old capital, and that this pattern can be explained by repeated inventions of new variants at Kyoto with subsequent outward diffusion. The supposed diffusion process was examined within the theoretical framework described above, using a model of population network in which new variants are created exclusively in a single cultural center. The equilibrium distribution of word ages obtained by the model replicated the observed pattern, suggesting that the emergence of concentric distributions is partially explicable by a center-periphery social structure.

Modern human behavior

Masahito Morita and collaborators analyzed data from a 2016 survey on childcare support and its effects on parents' and children's outcomes in Japan to examine in what ways childcare support may affect children's social development (Morita et al., 2021). In humans, unlike other mammalian species, children are often cared for by not only their mother, but also their father, elder siblings, grandparents, and other relatives as well as non-kin, the phenomenon called alloparenting. Studies in small-scale, non-industrialized societies have found a positive association between child survival and the presence of kin, grandmothers and older siblings in particular, although there are other studies reporting null or negative associations. In industrialized populations, child mortality is generally low, and researchers are more interested in the effect of alloparenting on other aspects of child development, such as cognitive and language abilities, educational outcomes, and emotional stability. By means of path analysis, Morita showed that childcare support in Japan does not directly improve social development of children, but does have a positive effect on parents' psychological conditions and parenting style, which in turn positively affect children's outcomes. The finding indicated a potential mechanism behind childcare support, parental characteristics, and child development in industrialized societies.

Evolution of social behavior

Social interaction between individuals has been deemed, in one way or another, to be a key factor underlying the evolutionary emergence of characteristically human traits, including extensive collaboration, prosociality, and cooperative communication. As a simplest form of social interaction, dyadic competition and cooperation are particularly well studied. However, when three or more individuals are involved, social interactions exhibit an additional facet of complexity due to coalition formation by a subset of the individuals, which may have a qualitatively different evolutionary consequence. Ihara (2020) pointed out, based on a mathematical analysis of a three-player coalition game, that an interesting property of coalition formation is that it can generate selection disfavoring exceedingly aggressive or strong individuals, given that individuals choose their allies in such a way to maximize their fitness. In other words, when a population of animals are ecologically demanded to form coalitions in order to survive and reproduce better, the prediction is that they undergo evolution toward low aggression. Specifically, Ihara argued that the reduction in size and shape of canine teeth in the Early Pliocene hominin Ardipithecus ramidus may have been driven by a heightened demand of coalition formation.

Sexual selection

Saori Nojo investigated on the sexual selection hypothesis for human phenotypic diversity. The level of genetic differentiation among human populations is generally low as compared with those in great apes; nonetheless, there is a considerable amount of geographic variations in human phenotypes, such as skin pigmentation and craniofacial morphology. Furthermore, studies have pointed out that the extent of human inter-population phenotypic diversity is greater than what is explicable by genetic drift alone. The sexual selection hypothesis posits that the excess of phenotypic diversity is partially attributable to variations in preference for mating partners. Nojo explored this hypothesis using a computer simulation, in which the inter-population diversity of a quantitative trait is compared with selectively-neutral genetic loci (Nojo and Ihara, 2019). It was revealed that phenotypic diversification between populations connected by gene flow can be enhanced by people's preference for typical characteristics of their own populations or an arbitrary preference that is culturally transmitted within populations. As an empirical evaluation, an experiment was performed with participants from Okinawa Islanders and Mainland Japanese, a pair of neighboring populations, to study their preferences of faces (Nojo et al., 2022). It yielded partial support for the existence of such preferences as assumed in the simulation. These findings suggest that some of the human phenotypic variations may be understood as a product of cultural, rather than ecological, diversification.

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Denton KK, Kendal JR, Ihara Y, Feldman MW, 2023. Cultural niche construction with application to fertility control: A model for education and social transmission of contraceptive use. Theoretical Population Biology 153, 1-14.
doi:10.1016/j.tpb.2023.06.001

Takahashi T, Ihara Y, 2023. Spatial evolution of human cultures inferred through Bayesian phylogenetic analysis. Journal of the Royal Society Interface 20, 20220543.
doi:10.1098/rsif.2022.0543

Nishikawa Y, Ihara Y, 2022. Cultural transmission of traditional songs in the Ryukyu Archipelago. PLoS ONE 17, e0270354.
doi:10.1371/journal.pone.0270354

Nojo S, Kimura R, Ihara Y, 2022. An empirical investigation on the sexual selection hypothesis of human phenotypic diversity: A test in Okinawa and Mainland Japan. Letters on Evolutionary Behavioral Science 13, 21-27.
doi:10.5178/lebs.2022.94

Takahashi T, Ihara Y, 2022. Application of a Markovian ancestral model to the temporal and spatial dynamics of cultural evolution on a population network. Theoretical Population Biology 143, 14-29.
doi:10.1016/j.tpb.2021.10.003

Kobayashi H, Kobori O, Ihara Y, Yaguchi H, Yasuda T, 2022. Understanding ostensive behavior in making inferences of referential intentions. Letters on Evolutionary Behavioral Science 13, 1-5.
doi:10.5178/lebs.2022.90

Nishikawa Y, Ishida T, 2021. Genetic lineage of the Amami islanders inferred from classical genetic markers. Meta Gene 30, 100956.
doi:10.1016/j.mgene.2021.100956

Morita M, Saito A, Nozaki M, Ihara Y, 2021. Childcare support and child social development in Japan: investigating the mediating role of parental psychological condition and parenting style. Philosophical Transactions of the Royal Society B 376, 20200025.
doi:10.1098/rstb.2020.0025

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.
doi:10.1098/rsif.2020.0335

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.
doi:10.1016/j.jhevol.2020.102839

Ihara Y, 2020. A mathematical model of social selection favoring reduced aggression. Behavioral Ecology and Sociobiology 74, 91.
doi:10.1007/s00265-020-02875-4

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.
doi:10.11183/jhe.48_2.69

Takahashi T, Ihara Y, 2019. Cultural and evolutionary dynamics with best-of-k learning when payoffs are uncertain. Theoretical Population Biology 128, 27-38.
doi:10.1016/j.tpb.2019.05.004

Morita M, 2019. Human behavioral ecology. In "Encyclopedia of Evolutionary Psychological Science" (Shackelford T, Weekes-Shackelford V editors). Springer, Cham.
doi:10.1007/978-3-319-16999-6_1380-1

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.
doi:10.1016/j.jtbi.2018.10.058

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Ihara Y. Historical and ecological perspectives of the evolution of the human language faculty. Joint Conference on Language Evolution, Kanazawa, 5-8 September 2022.

Morita M, Nishikawa Y, Tokumasu Y, Ihara Y. A three-phase scheme for the evolution of human music: A review from an anthropological perspective. Joint Conference on Language Evolution, Kanazawa, 5-8 September 2022.

Nishikawa Y, Ihara Y. Cultural transmission of traditional songs in the Ryukyu Archipelago. Joint Conference on Language Evolution, Kanazawa, 5-8 September 2022.

Takahashi T, Onohara A, Ihara Y. Inferring the temporal and spatial evolution of accent systems in Japanese dialects: A phylogeographic approach. Joint Conference on Language Evolution, Kanazawa, 5-8 September 2022.

Onohara A, Takahashi T, Ihara Y. Conditions for Japanese lexical-pitch accents to change from a complex to a simpler state. EHBEA 2022, Leipzig (online), 19-22 April 2022.

Takahashi T, Onohara A, Ihara Y. Bayesian analysis of human dialects in the presence of horizontal transmission: application of coalescent and phylogenic models. Biology22, Basel, 17-18 February 2022.

Nishikawa Y, Ihara Y. The role of social context in the cultural evolution of traditional Ryukyuan songs. Cultural Evolution Society Conference 2021, Sapporo (online), 9-11 June 2021.

Takahashi T, Ihara Y. Simulating the diffusion of Japanese dialects through a network model. Cultural Evolution Society Conference 2021, Sapporo (online), 9-11 June 2021.

Takahashi T, Ihara Y. Spatial evolution of Japanese dialects explained by a network model of cultural evolution. European Human Behaviour and Evolution Association 15th Conference, Krakow (online), 24-27 March 2021.

Nishikawa Y, Ihara Y. The role of social context in cultural evolution of traditional Ryukyuan songs. European Human Behaviour and Evolution Association 15th Conference, Krakow (online), 24-27 March 2021.

Tokumasu Y, Kondo S, Okanoya K, Ihara Y. Roles of low-pitched voice in Japan. European Human Behaviour and Evolution Association 15th Conference, Krakow (online), 24-27 March 2021.

Morita M, Saito A, Nozaki M, Ihara Y. Parental psychological condition and parenting behaviour mediate the associations between childcare support and child social development in Japan. European Human Behaviour and Evolution Association 15th Conference, Krakow (online), 24-27 March 2021.

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.

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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 GRE Tests may be accepted in lieu of the entrance exam. Department of Biological Sciences requires the GRE General Test score (see here). University of Tokyo provides international students with access to a number of financial aid options (see here).

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Kotaro Aizawa, Kenichi Aoki, Emily Emmott, Yukari Hanada, Akira Handa, Naoki Hatanaka, Soichiro Hirata, Mika Igarashi, Ayaka Kato, Mariko Kimura, Yutaka Kobayashi, Satoshi Komori, Yuto Kosugi, Shun Kurokawa, Mai Kuroshima, Sean Lee, Leonardo Daichi Matsumura, Yuki Mizusaki, Yuri Nishikawa, Saori Nojo, Ayaka Onohara, Kento Orihara, Ryotaro Otake, Mizuki Sato, Motohide Seki, Satoshi Sekiguchi, Akari Shimada, Takuya Takahashi, Kohei Tamura, Satoshi Tamura, Yudai Tokumasu, Tasuku Toyama, Mariko Tsumaki, Shusuke Yamashita, Akito Yokoi, Taro Yoshida

PhD in 2021

Takuya Takahashi (University of Zurich)

PhD in 2019

Saori Nojo (National Institute of Technology, Kisarazu College)

PhD in 2013

Shun Kurokawa (Japan Advanced Institute of Science and Technology)

Kohei Tamura (Tohoku University)

PhD in 2012

Motohide Seki (Kyushu University)

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Evolutionary Anthropology Lab

Department of Biological Sciences
Graduate School of Science
University of Tokyo
Hongo 7-3-1, Bunkyoku
Tokyo 113-0033, Japan

Recent and Forthcoming Events

21 March 2024
MScs were awarded to Soichiro Hirata for his thesis "Evolutionary models of maladaptive rite-of-passage cultures," and to Yukiko Kawanishi for her thesis "The coexistence of resource, foragers, and producers."

22 December 2023
Phylogenetic Analysis Workshop (Intelligence and Cultural Evolution Theory) was held in Tokyo (Organizers: K. Kubo, E. Nakamura, H. Matsumae, and K. Tamura).

20 June 2023
The second printing of the book "In Pursuit of Human Nature" (edited by Y. Ihara, M. Umezaki, and M. Yoneda) was released.
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