When a new primate fossil is uncovered, DNA testing is rarely possible: There is often no tissue available to test. Instead, dating methods that examine the decay of radioactive elements such as radiocarbon can reveal how long ago a specimen lived, and this time window, along with morphometrics—comparing the remains' shape with other fossils and known species—can help researchers arrive at its position on the evolutionary tree. But although these fossils fit some nodes of the evolutionary tree, others remain blank—the primates that were the direct ancestors of some branches died and decayed without entering the fossil record. When it comes to reconstructing the skulls of these ancestral primates, digital modeling rises to the fore.
Delson's geometric morphometrics can be used to infer and create a 3-D coordinate system for every skull on the tree. Based on these shapes, he, or rather, his computer program, can work backward to surmise what an ancestral skull looked like. He uses the evolutionary tree to determine how similar the older skull should be to modern primates' skulls: It will hew more to the shape of more closely related skulls, although more distant descendants can also contribute information about the skull's contours. Once the program determines which skulls will be used to find the final product, and how much importance each contributing skull carries, it can combine their morphometric frames to deduce the frame of the ancestor's skull—and from the frame, what the actual skull would have looked like. Finally, Delson can plug the digital reconstruction into the 3-D printer to produce a fossil replica of a skull that no longer exists.
How does a 3-D printer work? Read on: “3-D Printing Gets Ahead: How Does a Printer Make a Fossil?”