For more than 40 years, researchers have disputed whether eating habits or social interactions led to the large brains of humans and other primates. Now, a research team in Lund, Sweden, can show that our giant brains result from both diet and sociality.

One feature that separates humans from all other vertebrates is our large brains. It’s not just humans who can boast voluminous organs for controlling the nervous system. Even apes have relatively large brains.

In a new study published in the scientific journal Systematic Biology, a Swedish-British research team examined how primates’ brains have evolved. All to understand the mechanisms that have led to modern humans (Homo sapiens) and other primates having such large and complex brains.

The researchers collected information from previous studies on brain size, body mass, and other variables such as diet and activity. They then used advanced statistical methods to analyse the material.

“Our results show that it was a combination of diet and sociality. Shifting from simple to more complex levels of sociality resulted in relatively larger brains. In comparison, the transition to a more leaf-based diet led to relatively smaller brains,” says Masahito Tsuboi, a biology researcher at Lund University who conducted the study with colleague Mark Grabowski at Liverpool John Moore University.

Given the lively debate about which of these factors played the dominant role, the study provides novel insights into our understanding of how primates’ brains evolved into what they are today.

“In some cases, our model prediction was extremely precise. With knowledge of body mass, diet, and sociality, we could predict brain size across 32 species with 98 per cent accuracy. This means that, even though we don’t know the brain mass of a South American monkey, we don’t need to measure it. It adds evidence that diet and sociality were important drives of brain size evolution in primates,” says Masahito Tsuboi.

By revealing the driving forces behind the development of primates’ brain size, the study provides a valuable stepping stone for future researchers on the origin of humans, or the evolution of primates in general, regarding important variables to focus on.

“It’s a question that fascinates the whole world. Nobel laureate Svante Pääbo’s work on the ancient human genome is currently hotly debated. We are all interested in where we come from because understanding our origins will guide our paths towards the future,” says Masahito Tsuboi.

The study is published in Systematic Biology: “Both Diet and Sociality Affect Primate Brain-Size Evolution.”