A new analysis supports the previously overlooked "brain lag" hypothesis—the idea that, in some primate lineages, the evolution of larger body size preceded the evolution of larger brain size—while also building on that hypothesis by suggesting that some lineages' brain sizes then continued to grow beyond an expected baseline. Robin Dunbar of the University of Oxford presents these findings in the open access journal PLOS One on July 1, 2026.
Late in the evolution of humans, the brain underwent drastic increases in overall size, as well as in size relative to the body. The brain lag hypothesis suggests that, for humans and other large-brained primates, body size generally increased first, and brain sizes took time to catch up later.
However, a 1999 analysis of evolutionary timelines of primate brain and body sizes found no statistical evidence to support that idea. The analysis relied primarily on anatomical traits and timelines derived from dating of primate fossils. Since then, molecular genetic techniques have made more accurate evolutionary timelines available, and alternative statistical analysis techniques have emerged.
Using those advancements, Dunbar revisited the same dataset used in the 1999 analysis. He found evidence that, for some primate lineages, including our own, brain size did indeed reliably lag behind body size before "catching back up" to expected baseline relationships between body and brain size for different lineages.
However, instead of just catching up, some lineages' brain sizes then significantly overshot the expected baseline, entering a potentially higher cognitive level. This finding represents an expansion on the original brain lag hypothesis.
Dunbar also suggests that his new findings are in line with the idea that large-brained primates evolved to form large social groups as a protective strategy against predators, which selected for larger brain sizes since social ties require complex cognitive demands. This might have been facilitated by energy savings from shifting to diets focused more on energy-dense fruits, seeds, and nuts, instead of foliage.
However, that "social brain" hypothesis has been challenged by other researchers, and more research will be needed to continue to clarify the evolutionary story behind this study's new findings.
Dunbar adds: "Monkeys, apes, and of course humans switched from brawn to brain over time as their main way of protecting themselves against predators."
Author interview: https://plos.io/4xRnHPl
In your coverage, please use this URL to provide access to the freely available article in PLOS One: https://plos.io/4wq85kn
Citation: Dunbar RIM (2026) Evolutionary lags in the primate brain size/body size relationship revisited. PLoS One 21(7): e0351073. https://doi.org/10.1371/journal.pone.0351073
Author countries: UK.
Funding: The author was funded by a grant from the European Research Council (grant number 295663). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
