But while several studies show that humans boast a similar brain size to Neanderthals, almost no research has assessed whether we produce similar amounts of neurons as our closest cousins. That is, until now.
According to a new study in Science, researchers identified a variation in one of the proteins in our brains that increases the number of brain progenitor cells. This suggests that our species produced more neurons during our development than Neanderthals did during theirs about 130,000 to 40,000 years ago. Occurring primarily in the neocortex, our increased neural output brings scientists one step closer to understanding why our species is so different.
Building brains with protein
Modern humans and Neanderthals have relatively large brains and strikingly similar neocortices, which are the large areas of the brain that govern complex functions such as sensory perception, attention, and memory. But whether this similar size implies similar neuronal production remains enigmatic.
Researchers are studying protein variations in the brains of both modern humans and Neanderthals to solve this mystery.
They focused on the TKTL1 protein, which is present in the frontal lobes of the neocortices of both modern humans and Neanderthals, with only one variation in their amino acid building block sequences. To be specific, the only difference between the two is that the TKTL1 protein contains an occurrence of arginine at a specific location in its modern human sequence and a lysine at the same location in its Neanderthal sequence.
According to the researchers, even this slight change in the amino acid sequence of the protein caused a significant increase in the number of basal radial glial cells, a type of brain progenitor cell present in the frontal lobe of the modern human brain. Since these cells are responsible for the production of new neurons, their increase in turn leads to a much greater number of neuron cells in the brains of modern humans.
Protein vs protein
The first evidence for these findings comes from an unlikely source. Conducting a series of experiments on mouse embryos, the researchers introduced the two versions of the TKTL1 protein into the frontal lobes of the brains of mouse embryos. The team then measured the resulting amount of basal radial glial progenitor cells in the neocortices of mice, noting that the cells increased with the advanced human variant of the protein, leading to higher neuron production.
Hoping to solidify these findings further, the team then tested the two variants on human brain organoids, artificial tissues made from stem cells to mimic the human brain. Using these tissues, they replaced the modern human variant with the Neanderthal variant to see if the switch would affect the human brain.
“We found that with the Neanderthal type of amino acid in TKTL1, fewer basal radial glial cells were produced than with the modern human type and, as a consequence, fewer neurons as well,” says Annelyn Pinson, study author and researcher at the Institute for Molecular cell biology and genetics Max Planck, in a press release. “This shows us that although we don’t 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 TKTL1 activity is highest, than Neanderthals.”
The study solidly demonstrates one of the biggest differences between the brains of modern humans and the brains of Neanderthals. But according to the research team, the study also serves a broader purpose, potentially showing what makes our species unique.
“It is tempting to speculate that this promoted modern human cognitive abilities related to the frontal lobe,” concluded Wieland Hüttner, lead author of the study and another researcher at the Max Planck Institute for Molecular Cell Biology and Genetics, in a press release.