https://www.nytimes.com/2000/01/04/science/a-decade-of-discovery-yields-a-shock-about-the-brain.html

The new cells seen in the rat brains appear at a higher rate after challenges like intense training, injury or an infection, Dr. Raastad said. Within a few years, researchers found the cells in adult mice, guinea pigs, rabbits and monkeys. In the mid-1980's, other researchers found irrefutable evidence that new cells were born in the brains of adult canaries learning new songs and chickadees that were remembering where they had stashed their winter seeds. But researchers still did not believe that new cells were created in human brains, Dr. Raastad said.

In 1997, Dr. Elizabeth Gould, an assistant professor of neuroscience at Princeton, and colleagues showed that neurogenesis, or the birth of new cells, occurred in the hippocampuses of tree shrews and marmoset monkeys. But Dr. Rakic and others said this was not possible in humans.

In 1998, Dr. Gage demonstrated that the number of brain cells in the hippocampuses of mice raised in stimulating environments increased by 15 percent -- and that the cells were born in the ventricle zone.