For years the standard line had been that the human brain contains 100 billion (10^11) neurons. But when a Brazilian neuroscientist, Dr Suzana Herculano-Houzel, started asking where that number came from, she couldn't find a good answer. So she decided to do an explicit count. She pureed the brains of five deceased male adults and then counted the number of neuronal nucleii in each vat of brain soup. It turns out the the correct number seems to be 86 billion neurons (8.6*10^10).
How'd we loose 14 billion neurons? "Even though it may sound like a small difference", says Herculano-Houzel, "the 14bn neurons amount to pretty much the number of neurons that a baboon brain has or almost half the number of neurons in the gorilla brain. So that's a pretty large difference actually."