CT spans evolution from Lucy to Homo sapiens

Science is helping to unravel mysteries of humankind's evolutionary history thanks to researchers at the University of Texas at Austin and the Ethiopian government. The Texas team completed the first high-resolution CT scan of the famous Lucy fossil, a human ancestor who lived 3.2 million years ago.

The Ethiopian government entrusted Lucy, a distinct species of human ancestor known as Australopithecus afarensis, to UT Austin's John Kappelman, a professor of anthropology, and his team, and Dr. Richard Ketcham, an associate professor of geological sciences and director of the High-Resolution X-Ray Computed Tomography Facility at UT Austin.

For 10 days, the scientists scanned all 80 pieces of the 1-meter-tall Lucy. The skeleton is only about 40% intact, but that still makes Lucy the most complete adult of her species found thus far. Custom-built foam mounts held the specimens securely in the scanner. The specimen is now safely archived in digital format.

“We should learn a lot of new things about Lucy, such as how she moved and how she chewed. This will be an important addition to our knowledge about our evolutionary history, how we got from ‘there' to ‘here,'” Ketcham said.

CT is an important technology for paleontology because it nondestructively provides details of internal structures and 3D morphological data and supplies a digital archive of specimens that can be sent out to a wide range of researchers and others, he said.

To scan the fossil, Ketcham and his team used an ultrahigh-resolution Xradia MicroCT, which could be thought of as an industrial CT. The machine is optimized for inanimate objects rather than patients, according to Ketcham.

“Patients move and cannot take too much radiation; industrial CT takes advantage of the fact that we don't have to worry about these things by using higher energy x-rays, small focal spots-which necessarily have low intensity-and longer acquisition times,” he said.

The Xradia microCT is optimized to look at small things, items less than 4 cm in diameter and particularly less than 5 mm in diameter, and it can achieve resolutions down to approximately 1.5 microns.

“Our set of instruments was uniquely well suited to Lucy because they do a good job of imaging dense material, and we have different machines that are optimized for looking at different-sized elements,” he said.

Of primary interest for Ketcham is trabecular bone, which contains behavioral signals of how various joints were used. The bone constantly dissolves and reforms throughout life, and it adopts a configuration to better withstand the forces exerted on it, he said.

“For example, there is a big difference in the metacarpals of a knuckle walker versus a tree swinger,” he said. “Other things we can see using CT data are the thickness of cortical bone, detailed tooth and enamel morphology, and paths of major blood vessels through bone.”

Lucy came to the U.S. as part of a world premiere exhibit organized by the Houston Museum of Natural Science. After being scanned in Austin, Lucy moved to the Pacific Science Center in Seattle, where she was on display through March.