How social structures emerge: Computer simulations uncover universality in cultural anthropology observations

What rules shaped humanity's original social networks? Researchers in Japan developed new mathematical models to understand what conditions produced traditional community structures and conventions around the world, including taboos about incest.

Illustration of communities joining through a marriage [Credit: © Caitlin Devor,
University of Tokyo, CC BY 4.0]

"We think this is the first time cultural anthropology and computer simulations have met in a single research study," said Professor Kunihiko Kaneko, an expert in theoretical biology and physics from the University of Tokyo Research Center for Complex Systems Biology.

Researchers used statistical physics and computer models common in evolutionary biology to explain the origin of common community structures documented by cultural anthropologists around the world.

The earliest social networks were tightly knit cultural groups made of multiple biologically related families. That single group would then develop relationships with other cultural groups in their local area.

In the 1960s, cultural anthropologists documented social networks of indigenous communities and identified two kinship structures common around the world. In areas with hunter-gatherer communities, anthropologists documented direct-exchange kinship structures where women from two communities change places when they marry. In areas with agrarian farming communities, kinship structures of generalized exchange developed where women move between multiple communities to marry.

"Anthropologists have documented kinship structures all over the world, but it still remains unclear how those structures emerged and why they have common properties," said Kenji Itao, a first year master's degree student in Kaneko's laboratory, whose interdisciplinary interests in physics, math and anthropology motivated this research study.

Experts in anthropology consider the incest taboo to be an extremely common social rule affecting kinship structures. The ancient incest taboo focused on social closeness, rather than genetic or blood relationships, meaning it was taboo to marry anyone born into the same cultural group.

Graphical representations of simulated social networks. Researchers used computer simulations based on principles
of statistical physics and evolutionary biology to model how human societies would form under different conditions.
Direct-exchange (left), generalized-exchange (center), and restricted-exchange (right) social structure
models observed by cultural anthropologists in the 1960s are represented graphically
[Credit: Kenji Itao, University of Tokyo, CC BY-SA 4.0]

Itao and Kaneko designed a mathematical model and computer simulation to test what external factors might cause generations of biologically related families to organize into communities with incest taboos and direct or generalized exchange of brides.

"Traditionally, it is more common for women to move to a new community when they marry, but we did not include any gender differences in this computer simulation," explained Itao.

Simulated family groups with shared traits and desires naturally grouped together into distinct cultural groups. However, the traits the group possessed were different from the traits they desired in marriage partners, meaning they did not desire spouses similar to themselves. This is the underlying cause of the traditional community-based incest taboo suggested by the study.

When the computer simulation pushed communities to cooperate, generalized exchange kinship structures arose. The simulation demonstrated different kinship structures, including the direct exchange basic structure, emerge depending on the strength of conflict to find brides and the necessity of cooperation with specific other communities.

"It is rewarding to see that the combination of statistical physics and evolution theory, together with computer simulations, will be relevant to identify universal properties that affect human societies," said Kaneko.

The current computer model is simple and only included factors of conflict and cooperation affecting marriage, but researchers hope to continue developing the model to also consider economic factors that might cause communities to separate into different classes. With these additions, the theory can hopefully be extended to explore different communities in the modern, global society.

"I would be glad if perhaps our results can give field anthropologists a hint about universal structures that might explain what they observe in new studies," said Itao.

The study is published in the Proceedings of the National Academy of Sciences.

Source: University of Tokyo [January 21, 2020]

The little auks that lived in the Pacific

Findings from a 700,000-year-old fossil bone indicate that a close relative of the most abundant seabird species in the North Atlantic, the modern dovekie, or 'little auk', used to thrive in the Pacific Ocean and Japan.

Current distribution of the modern dovekie across the Atlantic [Credit: Kyoto University/
Junya Watanabe & Justin Ammendolia]

Seabirds are top predators in the marine ecosystem, and their distributions are shaped by numerous environmental factors in the ocean. As such, extensive scientific inquiries have been conducted on how seabirds respond to fluctuating oceanic environments in the ecologic and geologic timescales.

"The North Pacific has been one of the most intently investigated regions, but the fossil record of seabirds in the Pleistocene Epoch, about 2.6 to 0.01 million years ago, has been scarce," explains first author Junya Watanabe of Kyoto University's School of Science. "This has led to a frustrating lack of information in this critical time period concerning the origin of modern seabird communities."

In recent years, Watanabe and his team had been investigating seabird fossils from several locations in Chiba and Tokyo prefectures, gaining new insight on the Pleistocene seabird community in the region.

The group had been successful in identifying 17 fossils representing at least 9 species of birds: three species of ducks, a loon, an albatross, a shearwater, a cormorant, an extinct penguin-like seabird called mancalline auk, and a dovekie. Most of these species can be found in the region today; however, the presence of a dovekie was completely unexpected. Watanabe explains his findings published in Journal of Vertebrate Paleontology.

"At first it confused us that the fossil didn't match any of the Pacific auks, but once we compared it with Atlantic ones, the similarity with the modern dovekie was apparent. It is not clear whether the present fossil is from the same species or a very close cousin, but we are positive it at least comes from the same lineage."

The dovekies we know today are mostly restricted to the North Atlantic and Arctic oceans, with their rare sightings in Japan considered accidental visits. Given the unlikeliness of such accidental visitors to be preserved as fossils, the new findings suggest that dovekies were once fairly common in Japan and the Pacific.

He continues, "Now the question is why dovekies are so rare in the North Pacific today, it's almost paradoxical given their abundance in the North Atlantic. That question remains unexplained, at least until recovery and investigation of further fossil materials."

Interestingly, local decline and extinction events in the past are common in many seabird groups. Deciphering possible causes of such events requires integration of knowledge from various disciplines, including paleontology, paleoclimatology, oceanography and seabird ecology. Watanabe and his team see this as a challenging but rewarding endeavour.

Source: Kyoto University [January 21, 2020]