Human intelligence evolved at a cost - the risk of mental illness

A sweeping analysis of our DNA suggests that the genetic ingredients for human intelligence arrived in bursts – and vulnerability to mental illness often followed close behind.

The pattern points to a possible evolutionary trade-off: as the brain gained new cognitive powers, it may also have become more fragile.

Reconstructing evolution through DNA

“We don’t have any trace of the cognition of our ancestors with regard to their behavior and their mental issues,” said Ilan Libedinsky, from the Center for Neurogenomics and Cognitive Research in Amsterdam.

“We wanted to see if we could build some sort of ‘time machine’ with our genome to figure this out.”

Libedinsky’s team stitched together two powerful lines of modern research. Genome-wide association studies (GWAS) comb through the DNA of large human cohorts to flag variants weakly linked with traits.

These traits range from height and cancer risk to memory, language, and psychiatric conditions. In parallel, new methods can estimate when many of those variants first arose by reading molecular “age” signals left behind by mutation processes.

By pairing trait associations with age estimates, the group reconstructed a long arc of brain-related genetics across millions of years of human evolution.

Brain traits evolved in waves

Most variants tied to human traits appeared between about 3 million and 4,000 years ago, with a notable surge over the past 60,000 years – around the time Homo sapiens dispersed widely out of Africa.

Within that broad window, variants associated with brain and mind showed a striking sequence.

Mutations linked to foundational nervous system structure tended to emerge first. Only later did variants tied to higher cognition – such as fluid intelligence and our capacity for novel problem-solving – become common.

On average, intelligence-associated variants dated to roughly 500,000 years ago, Libedinsky said. That is about 90,000 years later than the typical dates for cancer-related variants and nearly 300,000 years after many variants affecting metabolism.

Brilliance and mental illness intertwined

Then came a twist: variants weakly linked to psychiatric vulnerability clustered just behind the cognitive wave.

“Mutations related to psychiatric disorders apparently involve part of the genome that also involves intelligence. So there’s an overlap there,” Libedinsky told New Scientist.

“[The advances in cognition] may have come at the price of making our brains more vulnerable to mental disorders.”

The same cadence appears again at finer scales. Starting around 300,000 years ago, many variants influencing the shape of the cortex arrived. In the past 50,000 years, a host of language-related variants rose to prominence, followed closely by variants associated with 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,” Libedinsky shared with New Scientist.

“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.”

Shared roots of mind and illness

The findings echo a longstanding idea in evolutionary neuroscience: the features that make human cognition so flexible may also raise the risk of it misfiring.

A larger, more plastic cortex supports language, planning, and abstract thought. But the same plasticity could heighten susceptibility to mood disorders, psychosis, or addiction in certain genetic and environmental contexts.

Crucially, the study does not claim these variants “cause” intelligence or illness. Individual genetic effects are tiny and probabilistic, and the associated traits are shaped by thousands of variants, plus environment and culture.

Still, the timing suggests overlapping genetic neighborhoods – regions of the genome that influence both cognitive gains and psychiatric risk – were repeatedly brought under selection as hominin brains evolved.

Neanderthal echoes in our DNA

The dates line up with another key chapter in our past: encounters with Neanderthals. Genetic evidence suggests modern humans picked up some variants tied to alcohol consumption and mood through interbreeding.

Libedinsky noted that those signals dovetail with the late-arriving clusters in the team’s timeline. Yet the approach has built-in blind spots. It can only analyze variants that still vary among living humans.

Mutations that swept through ancient populations and became fixed – now shared by nearly everyone – remain invisible to GWAS-based methods. Those “settled” changes might include some of the most defining shifts in the human brain.

“This kind of work allows scientists to revisit longstanding questions in human evolution, testing hypotheses in a concrete way using real-world data gleaned from our genomes,” Simon Fisher of the Max Planck Institute for Psycholinguistics told New Scientist.

But, he added, truly cracking “what makes us human” will require new tools capable of interrogating those fixed regions as well.

Evolution didn’t purge mental illness risk

If certain variants nudge risk for depression or schizophrenia, why weren’t they weeded out? One possibility is that their effects are modest and context-dependent, blunted in small bands of hunter-gatherers and amplified in modern environments.

Another mechanism is pleiotropy. The same genetic regions may confer adaptive traits – enhanced cognition, complex language, and exploratory behavior – whose benefits exceed their population-level costs.

In addition, some “risk” alleles may have offered advantages in specific ecological or social niches, helping them persist.

Whatever the mechanism, the new timeline highlights a delicate balancing act. As our ancestors’ brains expanded and reorganized, the genetic underpinnings of cognition appear to have advanced in stages, with mental illness vulnerability never far behind.

A living record of a restless brain

Fossils can show us the swelling domes of Homo brains over the past 2 million years, but they cannot tell us how those brains thought, felt, or failed.

Modern DNA, parsed with care, offers a complementary narrative. It’s not a simple march toward genius, but a tangled ascent in which gains in reasoning, language, and learning arrived in waves – shadowed by new ways for the mind to go awry.

As Libedinsky puts it, the genome is a kind of time machine. It does not let us watch our ancestors think. But it does let us glimpse when the scaffolding for thought was assembled – and when its weak points appeared.

The study is published in the journal Cerebral Cortex.

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