Allman's thoughts about von Economo cells are still evolving. As new data comes in, he discards initial concepts and integrates others. Unlike the stereotypical cautious scientist, he doesn't hesitate to put forward bold hypotheses based on a few observations. The theory that von Economo neurons underlie social cognition is audacious. And it's tempting to seize upon the cells as a simple explanation for the basis of our species' complex social nature. But Allman knows that's a stretch.
His theory has its skeptics. Anthropologist Terrence Deacon, of the University of California at Berkeley, questions whether the neurons are truly a different type of brain cell or are simply a variation that arises in large brains. He says that the differences in our brains that make us human are more likely to have arisen from large-scale changes than from subtle changes in neuron shape. "I don't think it's a very big part of the story," he says of Allman's idea. Yet, he adds, when it comes to understanding the human brain, "so long as we recognize that we have so little to go on, under those circumstances all hypotheses should be entertained."
Point taken. But it's hard not to be seduced by Allman's theory when some of the most compelling evidence comes not from the animal pathology lab but from the medical clinic.
William Seeley, a neurologist at the University of California at San Francisco, studies a poorly understood neurodegenerative disease called frontotemporal dementia. Patients suffer a breakdown in their character, losing social graces and empathy, turning insensitive, erratic and irresponsible. Marriages and careers implode. Many patients seem to lack physical self-awareness: when diagnosed with other illnesses, they deny having any problems. Brain imaging studies of patients with the dementia have uncovered damage to frontal areas of the brain.
In 2004, Seeley heard Allman lecture about von Economo neurons. As Allman clicked through his PowerPoint slides, Seeley saw that the cells were clustered in the same brain regions that the dementia targeted, the ACC and FI. "It was kind of like, Eureka," Seeley recalls. He thought the cells might help researchers figure out why those areas were vulnerable to destruction. "Also, I thought, what an interesting way to learn something about human nature. Maybe the deficits that patients develop might be in things that are uniquely human. So there was a big rush of ideas."
Afterward, over coffee, Seeley and Allman agreed to team up to find out whether von Economo neurons were damaged in people with frontotemporal dementia. Analyzing brains from deceased patients, the scientists discovered that, in fact, about 70 percent of von Economo neurons in the ACC had been destroyed, whereas neighboring brain cells were largely unaffected. "It is very clear that the original target of the disease is these cells, and when you destroy these cells you get the whole breakdown of social functioning," says Allman. "That's a really astounding result that speaks to the function of the cells about as clearly as anything can."
This unusual neural system seems to underlie a lot of what makes us human. But the fact that elephants and whales apparently share the same neural hardware opens the mind to a tilt in perspective: our brains may be more similar to those of other smart, social animals than we thought.
Ingfei Chen lives in Santa Cruz, California.
Photographer Aaron Huey lives in Seattle.