Evolution of human intelligence linked to mental illness vulnerability
Researchers have traced genetic variants in the human genome to reveal that advances in cognitive abilities around 500,000 years ago were soon followed by mutations increasing susceptibility to psychiatric disorders. This suggests a trade-off in brain evolution. The study, published in Cerebral Cortex, analyzed 33,000 genetic variants to build an evolutionary timeline of brain-related traits.
By combining genome-wide association studies with analyses of mutation ages, Ilan Libedinsky and colleagues at the Center for Neurogenomics and Cognitive Research in Amsterdam created a timeline of genetic changes spanning millions of years of human evolution. Humans diverged from chimpanzees and bonobos more than 5 million years ago, with brain size tripling since then and accelerating over the past 2 million years.
The team examined 33,000 variants linked to traits including brain structure, cognition, psychiatric conditions, eye shape, and cancer. Most emerged between 3 million and 4,000 years ago, with a surge in the past 60,000 years, coinciding with Homo sapiens' migration out of Africa.
Variants for advanced cognitive abilities appeared relatively recently. Those tied to fluid intelligence—logical problem-solving in novel situations—emerged around 500,000 years ago, about 90,000 years after cancer-related variants and nearly 300,000 years after metabolic ones. Psychiatric disorder variants followed closely at 475,000 years ago.
A similar pattern occurred around 300,000 years ago with cortex shape variants for higher-order cognition, and in the past 50,000 years with language variants trailed by those for alcohol addiction and depression. “Mutations related to the very basic structure of the nervous system come a little bit before the mutations for cognition or intelligence, which makes sense, since you have to develop your brain first for higher intelligence to emerge,” says Libedinsky. “And then the mutation for intelligence comes before psychiatric disorders, which also makes sense. First you need to be intelligent and have language before you can have dysfunctions on these capabilities.”
Some variants linked to alcohol consumption and mood disorders may stem from Neanderthal interbreeding, aligning with fossil evidence. Why these psychiatric predispositions persist is unclear, but their modest effects might offer advantages in certain contexts, Libedinsky notes.
“This kind of work is exciting because it allows scientists to revisit longstanding questions in human evolution, testing hypotheses in a concrete way using real-world data gleaned from our genomes,” says Simon Fisher at the Max Planck Institute for Psycholinguistics in Nijmegen. However, the study only covers variable genetic sites in modern humans, missing fixed ancient changes.