When the first Homo sapiens appeared on Earth around 300,000 years ago, they weren’t the only humans roaming the land. Our ancestors were one of about nine early human species alive at the time—and one of at least 21 human species to ever exist, though the exact number is debated.
So why, scientists have asked, are Homo sapiens the only ones that are still here? After all, our relatives the Neanderthals had brains of a similar size to ours, yet they went extinct about 40,000 years ago.
“[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,” Laurent Nguyen, a neuroscientist at the University of Liège in Belgium, tells the Guardian’s Hannah Devlin.
Over the years, scientists have suggested and rejected various answers, says Chris Stringer, head of human origins research at London’s Natural History Museum, to the Guardian: “better tools, better weapons, proper language, art and symbolism, better brains.”
Now, a new study published Friday in Science may have a solution: a gene mutation in Homo sapiens that allows for the development of more neurons in the neocortex—an area of the brain involved in cognitive function.
The modern human version of the gene—called TKTL1—differs from the Neanderthal version by only one of its amino acid building blocks. This substitution is found in essentially all modern-day humans, but extinct archaic humans, Neanderthals, Denisovans and other primates all lack the mutation, per the study.
“What we found is one gene that certainly contributes to making us human,” co-author Wieland Huttner, a neuroscientist at the Max Planck Institute of Molecular Cell Biology and Genetics, tells the New York Times’ Carl Zimmer.
In the new study, researchers injected both the Neanderthal version of TKTL1 and the human version into brains of developing mice and ferrets. The animals that got the Neanderthal gene produced fewer progenitor cells, which give rise to neurons.
Next, the team knocked out the TKTL1 gene in human fetal brain tissue. This had the same result: fewer progenitor cells. In a final experiment, they used brain organoids, or lab-grown mini-brain structures created from human stem cells. When they inserted the Neanderthal version of the gene into these tissues, the same pattern occurred.
This finding “is really a breakthrough,” Brigitte Malgrange, a developmental neurobiologist at the University of Liège who was not involved in the study, tells Science’s Rodrigo Pérez Ortega. “A single amino acid change is really, really important and gives rise to incredible consequences regarding the brain.”
While having a brain with more neurons does not automatically equate to higher intelligence, the findings appear to indicate a shift in brain wiring, which may have given Homo sapiens a cognitive advantage, per the Times.
Alone, this amino acid difference probably doesn’t explain what makes our brains unique, researchers say. Previous studies have found 96 differences between the human genome and that of Neanderthals that change the structure of a protein, per the Times. Other scientists are investigating these differences.
“I don’t think it’s the end of the story,” Nguyen tells the Times. “I think more work is needed to understand what makes us human in terms of brain development.”