The Costs of Our Ancestors’ Evolving Intelligence

A timeline tracking genetic alterations spanning millions of years of human evolution indicates that variants linked to elevated intelligence appeared most rapidly around 500,000 years ago, succeeded by mutations that heighten the risk of mental illness.

The findings point to a “trade-off” between intellect and mental health issues in brain evolution, according to Ilan Libedinsky from the Center for Neurogenomics and Cognitive Research in Amsterdam, Netherlands.

“The genetic changes linked to mental disorders clearly involve regions of the genome associated with intelligence, indicating a significant overlap,” says Libedinsky. “[The progress in cognitive abilities] might have made our brains more susceptible to mental health issues.”

Humans branched away from our closest relatives, chimpanzees and bonobos, over 5 million years ago, with brain size tripling since then, exhibiting the fastest growth rate in the last 2 million years.

While fossils enable the examination of shifts in brain size and shape, they provide limited insights into the brain’s functional capacities.

Recently, genome-wide association studies have explored the DNA of diverse populations to identify mutations associated with traits like intelligence, brain size, height, and various diseases. Concurrently, other research teams are investigating specific mutation characteristics that imply age, facilitating the estimation of when those variants emerged.

Libedinsky and his team are pioneers in merging these methodologies to form an evolutionary chronology of genetics linked to the human brain.

“There’s no evidence that our ancestors were conscious of their behaviors or mental health issues; we can’t trace them in the paleontological record,” he notes. “We aimed to see if our genome could serve as a kind of ‘time machine’ to uncover this information.”

The research team analyzed the evolutionary roots of 33,000 genetic mutations identified in modern humans, linked to various traits such as brain structure, cognition measures, mental illnesses, and health-related characteristics like eye shape and cancer. While most genetic variations exhibit only a weak tie to traits, Libedinsky emphasizes that “these links offer a valuable starting point but are far from conclusive.”

The study revealed that most genetic variants emerged roughly between 3 million and 4,000 years ago, with a notable surge of new variants arising over the past 60,000 years. Homo sapiensexperienced significant migration out of Africa.

According to Libedinsky, mutations linked to higher cognitive skills evolved relatively recently compared to other traits. For instance, those associated with fluid intelligence (logical problem-solving in new situations) surfaced on average around 500,000 years ago, about 90,000 years after mutations related to cancer and 300,000 years later than mutations connected to metabolic functions. Following closely were the intelligence-related variants and those related to psychiatric disorders, appearing on average around 475,000 years ago.

This trend initiated approximately 300,000 years ago, continuing with the rise of numerous variants influencing cortical shape (the brain’s outer layer crucial for higher-level cognition). In the last 50,000 years, several variants associated with language have evolved, followed by variants linked to alcoholism and depression.

“Mutations influencing the fundamental structures of the nervous system emerged slightly earlier than those influencing cognition and intelligence, which is logical since a developed brain is necessary for advanced intelligence,” Libedinsky states. “Additionally, it makes sense that intelligence mutations precede mental health disorders, as these capabilities must exist before dysfunction occurs.”

These timelines align with evidence indicating that Homo sapiens obtained certain variants linked to alcohol use and mood disorders through interbreeding with Neanderthals, he added.

It remains uncertain why evolution has not eradicated variants that predispose individuals to mental health issues; however, Libedinsky suggests that their mild effects could be advantageous in certain situations.

“This area of research is thrilling because it enables scientists to revisit enduring questions in human evolution and empirically test hypotheses utilizing actual genomic data,” says Simon Fisher from the Max Planck Institute for Psycholinguistics in Nijmegen, Netherlands.

Nonetheless, this research can only assess genetic sites that vary among contemporary humans, potentially overlooking ancient, now widely shared changes pivotal to human evolution. Fisher emphasizes that developing tools to probe “fixed” genetic regions could lead to deeper understanding of our unique human characteristics.