In a new study, and one of the biggest yet, a team led by Arturo Alvarez-Buylla at the University of California at San Francisco completely failed to find any trace of young neurons in dozens of hippocampus samples, collected from adult humans. “If neurogenesis continues in adult humans, it’s extremely rare,” says Alvarez-Buylla. “It’s not as robust as what people have said, where you could go running and pump up the number of neurons.”
Needless to say, that’s a highly contentious claim. “There is a long history of concluding that adult neurogenesis doesn’t exist in a given species based on difficulty in identifying new neurons,” says Heather Cameron from the National Institutes of Mental Health. “This happened in rats and then in nonhuman primates, both of which are now universally acknowledged as showing adult hippocampal neurogenesis.”
Fernando Nottebohm from Rockefeller University sees things differently. He was one of the first scientists to conclusively show that adult neurogenesis occurs, by studying the brains of canaries. Alvarez-Buylla was one of his students, and Nottebohm speaks effusively about his former protégé—and his latest study. “It’s first class,” he says.
After Alvarez-Buylla left Nottebohm’s team and started his own, he showed that rodents continually add new neurons to the olfactory bulb—a region devoted to smell. But in humans, this river of olfactory neurons is finite: It’s there in infants, but dries up in adults. The same is true for the frontal lobe—the front-most part of the brain that governs our most important mental abilities. Floods of fresh neurons migrate there during early childhood, but they stop as we mature.
Next, Alvarez-Buylla turned his attention to the hippocampus—the region that’s the center of most research into adult neurogenesis. His colleagues Shawn Sorrells and Mercedes Paredes analyzed the brains of 17 adult humans who had died and donated their bodies to research. The duo searched for telltale molecules that are specifically made in young neurons, or in the stem cells that produce such neurons. To their surprise, they found nothing. “Even in our best-preserved samples, we didn’t see any evidence of neurogenesis,” Paredes says.
The same wasn’t true for children, infants, and fetuses. In 19 of these much younger brains, Sorrells and Paredes found clear signs of new neurons in the hippocampus. But even then, humans differ from even closely related animals. In macaque monkeys, neural stem cells in the hippocampus coalesce into a beautiful ribbon that pumps out new neurons. This structure breaks up in early life, and it’s mostly gone in adulthood. But in humans, the ribbon doesn’t seem to form at all.
We’re not alone in this. Another study recently suggested that whales and dolphins don’t have adult neurogenesis either. It’s tempting to point at our shared intelligence, and wonder if that has something to do with the lack of new neurons. But whales and dolphins have another trait in common with us: For mammals, we have pretty poor senses of smell. “Maybe adult neurogenesis in the hippocampus is related to smell, and smell in humans just isn’t that important,” Alvarez-Buylla says.