Catching Evolution on the Run
3-D x-ray videos of species in motion will help scientists understand how they evolved.
Scientists at Brown University are developing an imaging technology that would capture the movement of bones and muscles of animals in high-speed 3-D videos. The technology, which has been developed previously in orthopedics to study joints in humans, will be used to study the evolution of movement and anatomy in species of fish, mammals, and birds to shed light on, for example, how birds developed flight. It could also help researchers better understand movement disorders in humans.
The lack of good imaging systems has been a major obstacle for people studying movement, says Rebecca German, a biologist at Johns Hopkins School of Medicine. “When you study any kind of movement, it happens in three dimensions,” she says. “When you’re limited to looking at it in two dimensions, you’re only getting part of the story.”
Scientists have previously developed high-speed, 3-D, x-ray video systems to study the human knee and other joints, particularly after injuries or surgery. But these technologies have been used only in a limited number of clinics for specific medical applications. The Brown team, led by biologist Elizabeth Brainerd, plans to develop a similar technology for evolutionary biologists that is flexible enough to analyze different animals as they walk, swim, fly, or jump.
The approach combines two imaging technologies: computed tomography, or CT, which uses a series of x-rays taken at different orientations to reconstruct a 3-D image of the bone and tissue inside the body; and x-ray video, or cinefluouroscopy, which is able to capture events as they happen in real time, but only in two dimensions. CT is used routinely in research and in medical practice, but it is a time-intensive process and produces only static images. X-ray video is frequently used by scientists studying movement.
To image a movement such as a pig walking, the researchers first create a 3-D model of the animal’s anatomy using CT. Then they use two separate x-ray cameras to image the pig in motion from different angles. A computer matches the information from the CT model to the information in the 2-D videos to reconstruct a moving image in three dimensions. The Brown biologists have experimented with putting the two technologies together manually. Now, working with computer scientists, bioengineers, and orthopedic experts, they plan to create software to automatically align the data faster and with better accuracy to produce videos with a speed of up to 1,000 frames per second. The biologists hope to have a product that can be widely distributed to other scientists in about three years. According to German, such a technology will be a major leap forward for scientists investigating movement and “will open up whole new areas of research.”
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.