Model Man of Heidelberg who could be a direct ancestor Homo sapiens
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A timeline of genetic changes over millions of years of human evolution shows that variants associated with higher intelligence emerged most rapidly around 500,000 years ago, followed by mutations that made us more prone to mental illness.
The results suggest a “trade-off” in brain evolution between intelligence and psychiatric problems, he says. Ilan Libedinsky at the Center for Neurogenomics and Cognitive Research in Amsterdam, the Netherlands.
“Mutations associated with mental disorders apparently affect a part of the genome that is also associated with intelligence. So there is a coincidence here,” says Libedinsky. “[The advances in cognition] “This may have come at the cost of making our brains more vulnerable to mental illness.”
Humans split from our closest living relatives – chimpanzees and bonobos – more than 5 million years ago, and since then our brains have tripled in size, with the fastest growth occurring in the last 2 million years.
Although fossils allow scientists to study such changes in brain size and shape, they cannot tell much about what those brains were capable of.
Recently, however, genome-wide association studies have examined the DNA of many people to determine which mutations correlate with traits such as intelligence, brain size, height, and various types of diseases. Meanwhile, other teams were analyzing specific aspects of the mutations that hint at their age, providing estimates of when these variants first appeared.
Libedinsky and his colleagues combined both methods together for the first time to create a timeline of the evolution of genetics associated with the human brain.
“We have no trace of our ancestors' knowledge of their behavior and mental problems – you can't find them in the paleontological record,” he says. “We wanted to see if we could build something like a ‘time machine’ with our genome to figure this out.”
The team examined the evolutionary origins of 33,000 genetic variants found in modern humans that are associated with a wide range of traits, including brain structure and various measures of cognition and mental states, as well as physical and health-related features such as eye shape and cancer. Most of these genetic mutations are only weakly associated with a trait, Libedinsky said. “References can be useful starting points, but they are far from deterministic.”
They found that most of these genetic variants arose between about 3 million and 4,000 years ago, with an explosion of new ones occurring in the last 60,000 years—around the same time. Homo sapiens made a major migration out of Africa.
Variants associated with more advanced cognitive abilities have emerged relatively recently compared to variants for other traits, Libedinsky says. For example, those associated with fluid intelligence—essentially logical problem solving in new situations—appeared on average about 500,000 years ago. This is about 90,000 years later than variants associated with cancer and nearly 300,000 years later than variants associated with metabolic functions and disorders. These variants associated with intelligence were followed by variants associated with psychiatric problems, on average about 475,000 years ago.
This trend repeated itself about 300,000 years ago, when many variants emerged that affected the shape of the cerebral cortex, the outer layer of the brain responsible for higher-order cognition. Over the past 50,000 years, many variants related to language have emerged, followed by variants related to alcohol addiction and depression.
“Mutations related to the most basic structure of the nervous system occur slightly before mutations in cognition or intelligence, which makes sense because you have to develop your brain first for higher intelligence to emerge,” Libedinsky says. “And then the mutation of intelligence precedes mental disorders, which also makes sense. You need to be smart and have language first before you can have dysfunction in those abilities.”
The dates also coincide with evidence suggesting that Homo sapiens have acquired some variants associated with alcohol use and mood disorders from crossing with Neanderthalshe adds.
It's not clear why evolution hasn't weeded out variants that predispose to mental illness, but it may be because the effects are modest and may provide benefits in some contexts, Libedinsky says.
“This kind of work is exciting because it allows scientists to return to long-standing questions about human evolution, specifically testing hypotheses using real data obtained from our genomes,” says Simon Fisher at the Max Planck Institute for Psycholinguistics in Nijmegen, the Netherlands.
Even so, these kinds of studies can only examine genetic regions that still vary among living people, meaning they miss older, now universal variations that could be key to our evolution, Fisher adds. Developing tools to study “fixed” regions could provide deeper insight into what really makes us human, he says.
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