Animal corpses rarely defy the dictate of "ashes to ashes, dust to dust" to become fossils—and even if they do, they don't remain sturdy for long. By the time paleontologists get their hands on ancient remains, the fossils are incredibly fragile. So for decades, researchers have tended to do their close analyses with replicas instead. But in the past decade, 3-D printing has enabled a new solution: printing out copies of skulls and bones.

"My research is in the evolution of higher primates," says paleoanthropologist Eric Delson, who uses digital models and an Objet Eden260 printer housed at Lehman College in New York City to produce models of primates' skeletons. 3-D printing allows him to make accurate replicas without damaging the originals, and to generate larger versions of fossils and even reconstructions of lost bones. In fact, the digital models that 3-D printers use as templates make it possible to re-create the remains of long-lost ancestral primates, putting classical model-making methods to shame by printing calculated re-creations of bones that failed to form fossils.

Typically, paleontologists make models by covering an ancient skull or fragment with liquid rubber (latex or silicon) to create a mold. These molds, however, sometimes develop inaccurate bulges if the material clings too tightly to the fossil surface or bubbles if the sticky material dries outside of a vacuum environment. Making the molds is a time consuming process, as researchers and technicians must work slowly and carefully to avoid damaging fragile skeletons. Despite the labor, even a successful mold quickly becomes useless after producing about six to 12 replicas, or casts; over time, the wear and tear on it prevents casting of a high-fidelity match of the fossil's shape.

"Those casts are fine," Delson says, "but they require equally specialized people, equipment and care that you don't damage [the fossils]."

To reproduce a fossil using 3-D technology, on the other hand, you don't even need to touch the original skull. A CT or laser scanner can capture a skull's surface without covering it in goo. Laser scanners bounce beams of light off the skull, which reflect back to a sensor on the scanner, dividing the surface up into about a million points that the scanner can reconstruct into a surface. Computed tomography (CT) scans, on the other hand, substitute x-rays for light rays, dividing a shape up into cross sections without ever physically contacting it.

Because 3-D printing involves the use of a digital template, the procedure can create more than just replicas: It can blow up or shrink down the copies it reproduces. "The ability of the 3-D printer to modify size is one of the great things about it for both research and teaching purposes," Delson says. A small stone tablet etched with cuneiform letters might cause difficulties for researchers who want to read it and study the depth of the incised letters. But scanning the tablet produces a malleable digital incarnation that can be expanded to twice its original size to help researchers better understand how it was produced.

Digital models also make reconstruction easier. Take the remains of a famous pre–Homo sapiens hominin discovered in Ethiopia in the 1970s, Australopithecus afarensis, nicknamed Lucy: although dozens of Lucy's bones have been excavated, about 60 percent of the skeleton is missing. To model the complete fossil, researchers have had to reconstruct the missing pieces. If they unearthed the right hand but not the left, for instance, the right limb can be scanned, and the resulting digital template can be flipped into a mirror image, such that a plastic model of the left hand can be printed. The remains of other australopiths in the area also have become digital templates: the vertebrae of a larger individual might be the wrong size for Lucy's frame, but the digital scan can be shrunk to fit, then printed out and fit in place. Digital printing and 3-D models allow researchers to create more accurate replicas than traditional methods like sculpting would.

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