When Bruce Lahn was a child in China, his dad bought him a set of books called 10,000 Questions. They were science question-and-answer books, and Lahn spent hours looking at them, even before he could read very well. "A lot of people know what they want to do from very early on," he says. "I think I fit in that category."

The son of two physicists, Lahn (whose Chinese name is Lan Tian) started studying genetics at Beijing University in the late 1980s. Though he enjoyed the logic of physics and math, he felt the biological sciences held greater potential for future discoveries. He also was fascinated by how genes shape behavior, particularly human behavior.

Part of that fascination came from noticing the differences among cultures. "My mom went to the United States in the early 1980s to visit and sent back photos," he remembers. "When I was a little kid, we'd been told that in the West, people lived under cruel capitalism with not enough food to eat. I was stunned when I realized that compared with the people in those photos, we lived in hovels and didn't have nice clothes to wear. I started wondering why some societies don't develop the way others do. What are the reasons behind the differences in societies and in human behavior?"

At Beijing University in the mid 1980s, Lahn participated in the student movement. Driven by his curiosity for the outside world and the desire for a different kind of education, he transferred to Harvard University to complete his undergraduate studies in biology.

While working toward his Ph.D. in David Page's lab at the Massachusetts Institute of Technology, Lahn was asked to investigate a rare defect in the human Y chromosome. He had a larger vision. Lahn and his adviser eventually cloned about half of the genes on the Y chromosome and showed how the Y chromosome evolved from a chromosome that was once very similar to the X chromosome. (Males have one X and one Y chromosome; females have two Xs.)

After earning his doctoral degree, Lahn changed course. Spurred by his interest in human behavior, he began studying the genetics of human brain evolution. How the human brain evolved, he says, "is a big, interesting question—not just to scientists but to everyone. And at the time, it was a novel field—in fact, it was hardly a field at all."

Though many believe humans are no longer evolving, Lahn's work has suggested that the human brain is still changing. He's also shown that genes involved in the regulation of brain size during development might also be involved in the expansion of the brain during evolution.

Lahn's studies have focused in part on two genes, ASPM (abnormal spindle-like microcephaly associated) and microcephalin. Both have critical roles in ensuring the development of a large brain in humans. Lahn showed that one form of ASPM appeared in humans as recently as about 6,000 years ago, but it is now found in 30 percent of the world's population. One form of microcephalin appeared about 35,000 years ago and is now found in about 70 percent of humans. ASPM and microcephalin spread rapidly because they somehow improve survival or reproduction, allowing more copies of the genes to be passed on. This work suggests that the human brain may still be experiencing the effects of evolution.

In 2006, Lahn and colleagues published evidence that the advantageous form of the microcephalin gene appears to come from interbreeding between early humans and a now-extinct Homo species, such as the Neanderthal. His data showed that interbreeding likely was rare, and that the new form of the microcephalin gene could have come from a single instance. Before Lahn's research, competing theories held that Homo sapiens either never interbred with other Homo species, or that extensive interbreeding took place. This study offered up a third option—that interbreeding did occur, albeit rarely, and that it may have made our species more genetically fit.

Lahn still maintains an interest in human evolution and "what makes us human," but his emerging research interest is in stem cell biology. His lab is focusing on the molecular basis of "stemness." What makes a stem cell a stem cell at the molecular level? What molecular changes take place when stem cells differentiate into specialized cell types such as skin cells, liver cells, or muscle cells?

"My motivation for doing science is to contribute to not just an immediate benefit, but also to a broader understanding that may influence how we look at human life and think about our role on the planet," Lahn says. In the future, he imagines himself finding ways to translate science into policy.

Lahn's interest in discovery and diversity extends to his activities outside the lab. "I love traveling or any activity that exposes me to something different," he says. "People can live in such different ways—if you don't see that, you can't really imagine them and appreciate the diversity of the world, both natural and cultural. I also believe that seeing a lot of different things is the best way to keep an open mind, and I think that's critical, not just for science but as a person."