Scientists showed that new nerve cells were created in certain areas of the adult rat brain. They didn't apply to humans. Then, in 1985, scientists reported that they hadn't been able to find evidence of nerve cell creation in rhesus monkeys. This lent support to the belief that creation or regeneration of nerve cells didn't occur in higher species. Then Gage and colleagues reported the creation of new nerve cells in certain brain areas of adult marmoset monkeys.
Marmosets are small, long-tailed monkeys.
A number of scientists, therefore, continued to defend the dogma that the human brain cannot repair itself by creating new cells. And that lost nerve cells are never replaced. Gage and colleagues proved that new nerve cells are created in the adult human brain. In effect BrdU's a marker of new cells. And that's what happened.