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Brain growth under social pressure: mathematical modelling of brain growth when individuals face social challenges

Periodic Reporting for period 1 - SocialBrain (Brain growth under social pressure: mathematical modelling of brain growth when individuals face social challenges)

Reporting period: 2016-10-01 to 2018-09-30

Growing and maintaining a large brain entails substantial energetic costs. A large brain can evolve if costs are compensated by benefits from associated cognitive abilities. Leading hypotheses for brain evolution consider benefits arising from the solution of ecological and social problems. However, progress has been hindered by the unavailability of mathematical theory generating testable hypotheses from known causes. Here I developed testable mathematical models that yield quantitative predictions for brain mass through ontogeny when individuals evolve under social pressures. The overall objective was to assess the relative role of the social and ecological hypotheses in brain evolution, particularly in humans. I found that a combination of ecological and social problems can lead to the evolution of human-sized brains, where ecological problems contribute by increasing brain size while social problems contribute by decreasing brain size. These results contrast with previous understanding according to which social problems drive human brain expansion. Importantly, the models developed here offer a previously unavailable tool to study quantitatively the causes of brain evolution.
I formulated mathematical models using elements of metabolic theory and life history theory, and analysed them using methods from optimal control and differential game theory. In semester 1, I formulated the models; in semester 2, I obtained numerical solutions; in semester 3, I produced a manuscript with the results; in semester 4, the paper was published in Nature and I applied for longer term fellowships to exploit the novel research line introduced in this fellowship. In semester 4, I was interviewed by multiple news agencies across the world. Details of my activities are below:

Research publications:
- González-Forero M, Gardner A. (2018) Inference of ecological and social drivers of human brain-size evolution. Nature 557, 554-557.

Outreach publications:
- González-Forero M. (2018) At Nature Ecology and Evolution Community: https://natureecoevocommunity.nature.com/users/92985-mauricio-gonzalez-forero/posts/33491-the-human-brain-from-ecology-and-seemingly-culture
- González-Forero M. (2018) At The Conversation: https://theconversation.com/why-do-humans-have-such-large-brains-our-study-suggests-ecology-was-thedriving-force-96873

Training activities:
Workshops attended:
- Mean Field Games, Sapienza University (2017).
- Numerical Methods for Optimal Control Problems, Sapienza University (2017).

Mentoring received:
- Have been actively mentored by my host PI, Andy Gardner, in publishing in the highest impact journals, writing applications for fellowships and grants, and promoting my work.

2-way transfer of knowledge to the host institution:
- Have given multiple talks in the University, exposing local researchers to cutting edge mathematical modelling techniques.
- Have engaged with members of the University, which has led to two new possible future collaborations.

Conference talks given:
- The Institute of Behavioural and Neural Sciences Mini Symposium in St Andrews (2016).
- The European Human Behaviour and Evolution Association Conference in Paris (2017).
- The Models in Population Dynamics, Ecology, and Evolution Conference in Leicester (2018).

Other dissemination activities:
Invited talks given:
- Centre for Biological Diversity, University of St Andrews (2016).
- Department of Zoology, University of Cambridge (2017).
- Department of Human Behavior, Ecology and Culture, Max Planck Institute for Evolutionary Anthropology (2017).
- Stochastic Models for the Inference of Life Evolution group, Collège de France (2018).
- Department of Anthropology, Durham University (2019).
- Upcoming: School of Mathematics, University of St Andrews (2019).

Online dissemination:
- Actively promoted my work online via my website (mgforero.info) and twitter account (@MauricioGForero).

Selected media coverage:
2019:
- Discover Magazine: http://discovermagazine.com/2019/jan/human-origins
2018:
- Associated Press: https://apnews.com/7fe4e81cbb114ee2adf42c6e49f19b45
- LA Times: http://www.latimes.com/science/sciencenow/la-sci-sn-human-brain-evolution-20180523-story.html
- Vox: https://www.vox.com/science-and-health/2018/5/23/17377200/human-brain-size-evolution-nature
- New Scientist: https://www.newscientist.com/article/2169862-we-may-have-got-the-evolution-of-our-big-brains-entirely-wrong/
- TBS eFM Radio: http://cdn.podbbang.com/data1/tbsadm/thism180529002.mp3

Networking initiatives:
- Co-organizer with P. Avila, of the University of Lausanne, of the symposium “Life history evolution: bridging theory and data” at the Congress of the European Society for Evolutionary Biology (ESEB) in Turku, Finland, 2019.
- Visited R. Dunbar at Oxford University (December 2018). Dunbar is the main proponent of the dominant hypothesis to explain brain evolution, which my results refute. My visit sought to identify Dunbar’s concerns on my work, which he referred to as very promising.
- Was invited to contribute a paper to the collection “Links between cognition and fitness” in the journal Frontiers in Ecology and Evolution.

Next career step
Previous understanding indicated that social problems drove human brain expansion. This understanding has been primarily supported by comparative correlational analyses between brain size and group size across mammal species. However, correlational approaches have limited ability to identify causality, and there has been a lack of research tools to assess evolutionary causality for human brain size. My models offer a novel tool that enables causal assessments for the evolution of taxonomically unique traits. Using this causal approach, I found that, under conditions that lead to the evolution of human-sized brains, social problems diminish brain size suggesting that empirically observed correlations between brain size and group size may not reflect that social problems drive brain expansion. More broadly, the work I carried out here lays the foundation of a new approach that enables to study evolutionary causality for brain size from observational data of a single species. This represents a methodological breakthrough and may have wide implications to substantially expand our understanding of taxonomically unique brains, such as humans’.
Inferred scenario for Homo brain expansion. From González-Forero & Gardner Nature 557, 554-557