Evolutionary biologists focus on movement as a way to understand why different anatomical features evolved. Studying skeletal motion is particularly important since bones in the fossil record are one of the main sources of information about the past. "To determine how those extinct animals might have moved, we need a very precise understanding of how living animals move," says Brainerd.

One of the biggest puzzles for evolutionary biologists is understanding how a complex structure like a bird's wing evolved. Wings would have appeared gradually, so they must have had some usefulness long before they could function in flight. "What's the use of half a wing?" asks Brainerd. One recent observation, she says, is that birds tend to flap their wings when they run uphill. A current theory, which her team plans to investigate, is that wings provide a downward force when a bird is running up a hill or a tree to avoid a predator. That gives its feet better traction, much as the spoiler on a sports car helps the wheels grip the road. By understanding how a bird's shoulder joint behaves as it runs uphill flapping its wings, the team can then search the fossil record for clues that early birds made similar movements.

The technology could also have medical applications. For instance, German studies the evolution and mechanics of chewing and swallowing, which are hindered in people who have had strokes or who have nerve damage or certain diseases. "When the tongue moves food in the mouth while chewing or swallowing, it's an asymmetrical movement," she says. The type of 3-D x-ray image being developed at Brown would give the kind of complete picture not yet possible with other methods.