The reason why the brain is big is because...social?

►The brains of humans (left) and chimpanzees (right). By Todd Preuss, Yerkes Primate Research Center

Primates have a large brain capacity compared to most animals, and humans have the largest brain capacity in primates. Humans have three times the brain volume of their relatives. If calculated in relative size (brain/body), humans have a relative brain volume that is six times that of other embryonic mammals.

Why do humans have such a large brain capacity? This is a long-running debate in the field of evolutionary biology.

On May 24, 2018, Thursday, a study published in Nature confirmed for the first time that the ecological challenge was the main driving force for the increase of human brain volume through mathematical models, negating many human evolution complexity leads to an increase in the brain's capacity. In addition, this model also explains why human cognition tends to be more cooperative than competition [1].

"These findings are fascinating because they show that certain aspects of social complexity are more likely to be the result of human brain enlargement than cause." Mauricio González, School of Biology, St. Andrews University, UK - Dr. Forero said in a press release [2].

Over the past few decades, evolutionists have proposed multiple models to explain how human super-brain evolved. In general, the cognitive challenge is the main driving force for the evolution of the brain to larger volumes, and the specific driving forces are the following two types of models: ecologically driven and socially driven (Figure 1).

► Figure 1: Drivers of cognitive challenges. Picture source: References 1

The eco-driven model believes that cognitive challenges are mainly non-social environmental factors, such as finding food and avoiding prey. The socially driven model believes that the cognitive challenge is mainly social factors, such as cooperation with similar or competing with similar species. There is also a type of culturally driven model that considers learning from the same species and the experience of the entire group as the driving force for primates, especially humans, to cope with cognitive challenges. Evolutionists often also count such models as socially driven.

These models have many examples as evidence support. However, so far, the vast majority of evidence is only evidence of relevance. The evidence of relevance makes it difficult to draw causal conclusions. For example, there is a strong correlation between brain size and species population size. But does the animal evolve larger brain volume because of its larger population, or does it develop larger populations because of larger brain volumes?

In addition, because these models are difficult to quantify, it is not possible to accurately interpret the actual size of the brain volume. For example, why does the human brain suddenly increase so much compared to other close relatives such as apes?

Mauricio González-Forero and colleague Andy Gardner studied what factors influence the evolution of brain volume by designing a mathematical model.

The core idea of ​​this model is to quantitatively predict the evolution of brain and body volumes when individuals face ecological and social challenges through the consumption and acquisition of energy. First, the author uses a female group as a research object and divides the body mass into three parts: the brain, reproductive organs and other parts. The energy consumed by the genital organs is used to breed offspring, and the energy consumed by the brain is used to learn the ability to obtain energy (Figure 2).

► Figure 2: Model design. Image Source: References 1.

Individuals gain energy by overcoming two challenges: ecological challenges (gaining energy from nature) and social challenges (gaining energy from the population). Overcoming ecological challenges depends on the individual's own ability level, while overcoming social challenges requires ability of both themselves and the individuals in the population.

The authors also divided the social challenges into three types: the first is the “cooperative type” in which individuals themselves and individuals in the population need to cooperate to solve challenges through their respective capabilities; the second type is “interpersonal competition”. Individuals of this type need to use their own abilities (such as defeat, cheating, or other means) to obtain energy from their peers; the third is “group competition type”, that is, an individual forms a group with other individuals from another group. The group gains energy (as is often the case in human society).

By fitting these parameters to the individual's energy extraction efficiency (EEE), the researchers constructed a mathematical model. By using the existing data to train the model, the author got some interesting findings.

First, as the proportion of "cooperative" challenges increases, the individual's absolute brain volume and relative brain volume (the ratio of actual brain volume to predicted brain volume based on body volume) both show a downward trend. As competition between individuals increases, brain capacity tends to increase, while body size tends to decrease. This may be because competition weakens energy gain. When competition between groups intensifies, the absolute brain volume decreases, and the relative brain volume increases, because the body is reduced in size faster than the brain.

By modelling the published data, the researchers obtained the best interpretation model that best matches the observed data of humans (herein referred to Homo sapiens) and close relatives (such as Neanderthal, Homo erectus, etc.) (Figure 3). ). The model shows that for humans, ecological challenges contribute the most to the evolution of brain volume , reaching about 60%. The cooperative challenge contributed about 30%, and the competition between groups contributed about 10% . Contrary to intuition, the model shows that competition between individuals has little contribution to the volume of the human brain .

► Figure 3: The best explanatory model obtained by the researcher. Image Source: References 1.

“González-Forero and Gardner are on the right track,” said David Geary, an evolutionary psychologist at the University of Missouri in the United States in an interview with New Scientist.[5] However, he believes that this model is too simple to capture the complexity of social life. The current conclusion is "surprising and may be premature."

References:

1. González-Forero M, Gardner A. Inference of ecological and social drivers of human brain-size evolution. [J].Nature, 2018

2. https://www.st-andrews.ac.uk/news/archive/2018/title,2135706,en.php

3. Dunbar R I M, Shultz S. Why arethere so many explanations for primate brain evolution?[J]. Phil. Trans. R.Soc. B, 2017, 372(1727): 20160244.

4. Powell L E, Isler K, Barton RA. Re-evaluating the link between brain size and behavioural ecology inprimates[J]. Proc. R. Soc. B, 2017, 284(1865): 20171765.

5. https://www.newscientist.com/article/2169862-we-may-have-got-the-evolution-of-our-big-brains-entirely-wrong/