Neanderthals have long been portrayed as our dim-witted, thuggish cousins. Now groundbreaking research has – while not confirmed the stereotype – revealed striking differences in the brain development of modern humans and Neanderthals.

The study involved inserting a Neanderthal brain gene into mice, ferrets and “mini brain” structures called organoids, grown in the lab from human stem cells. The experiments revealed that the Neanderthal version of the gene was linked to slower creation of neurons in the brain’s cortex during development, which scientists said could explain superior cognitive abilities in modern humans.

“Making more neurons sets the basis for higher cognitive function,” said Wieland Huttner, who led the work at the Max-Planck-Institute of Molecular Cell Biology and Genetics. “We think this is the first compelling evidence that modern humans were cognitively better than Neanderthals.”

Modern humans and Neanderthals split into separate lineages about 400,000 years ago, with our ancestors remaining in Africa and the Neanderthals moving north into Europe. About 60,000 years ago, a mass migration of modern humans out of Africa brought the two species face-to-face once more and they interbred – modern humans of non-African heritage carry 1-4% of Neanderthal DNA. By 30,000 years ago, though, our ancient cousins had vanished as a distinct species and the question of how we out-competed Neanderthals has remained a mystery.

“One concrete fact is that wherever homo sapiens went they would basically out-compete other species there. It’s a bit weird,” said Prof Laurent Nguyen, of the University of Liège, who was not involved in the latest research. “These guys [Neanderthals] were in Europe a long time before us and would have been adapted to their environment including pathogens. The big question is why we would be able to out-compete them.”

Some argue that our ancestors had an intellectual edge, but until recently there has been no way to scientifically test the hypothesis. This changed in the last decade when scientists successfully sequenced Neanderthal DNA from a fossilised finger found in a Siberian cave, paving the way for new insights into how Neanderthal biology differed from our own.

The latest experiments focus on a gene, called TKTL1, involved in neuronal production in the developing brain. The Neanderthal version of the gene differs by one letter from the human version. When inserted into mice, scientists found that the Neanderthal variant led to the production of fewer neurons, particularly in the frontal lobe of the brain, where most cognitive functions reside. The scientists also tested the influence of the gene in ferrets and blobs of lab-grown tissue, called organoids, that replicate the basic structures of the developing brain.

“This shows us that even though we do not know how many neurons the Neanderthal brain had, we can assume that modern humans have more neurons in the frontal lobe of the brain, where [the gene’s] activity is highest, than Neanderthals,” said Anneline Pinson, first author of the study.

Chris Stringer, head of human origins research at the Natural History Museum in London, described the work as “pioneering”, saying that it started to address one of the central puzzles of human evolution – why, with all the past diversity of humans, we are now the only ones left.

“Ideas have come and gone – better tools, better weapons, proper language, art and symbolism, better brains,” Stringer said. “At last, this provides a clue as to why our brains might have outperformed those of Neanderthals.”

More neurons does not automatically equate to a smarter type of human, although it does dictate the brain’s basic computing capacity. Human brains contain about twice the number of neurons as the brains of chimpanzees and bonobos.

Nguyen said the latest work is far from definitive proof of modern humans’ superior intellect, but demonstrates that Neanderthals had meaningful differences in brain development. “This is an exciting story,” he added.

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