Three-dimensional printing makes it possible to reproduce australopith bones that no longer exist, having been lost to decay millennia ago. But what about the bones of species that paleontologists have never uncovered? The study of human evolution relies on the fossil record, but the vast majority of ancient skulls and bones faded into dust rather than becoming fossilized for posterity. Digital modeling can raise them—or at least their facsimiles—from the ashes.
Of course, digital modeling alone is not sufficient: Other tools come into play. Perhaps the most important is the use of an evolutionary tree, a type of "family tree" for species. The primate evolutionary tree stretches back about 65 million years—but how exactly did researchers reconstruct it? To understand an this tree, it helps to start with a simple family tree.
Children's DNA comes directly from their parents, and because siblings share the same DNA source, their genes are very similar. Their maternal grandparents contributed to their DNA through their mother, and to their cousin Bob's DNA via their aunt. Because they share a common DNA source with Bob, their DNA will also be similar—but not as similar as that shared by the siblings, because the DNA source that Bob and his cousins have in common is one generation further away than the DNA source that the siblings share. In general, the more generational (or geologic, in the reference of evolution) time that separates cousins from a common ancestor, the more differences will be seen in their DNA. The offspring of one of the siblings and Bob's child would be second cousins, one generation further removed from the common source of DNA, and so they would be even less closely related to each other.
Researchers can use this genetic information when they look at the DNA sequences of modern primates. For example, the DNA of humans and that of chimpanzees is more similar than that of humans and gorillas. Relatively speaking, if humans and chimps are first cousins, then gorillas are our second cousins. Applying the family tree insight, this distinction indicates that humans and chimps share a grandparent, whereas humans, chimps and gorillas have the same great-grandparent. The last common ancestor of humans and chimps lived more recently than the last common ancestor of humans, chimps and gorillas.
Comparing the DNA of different primates thus allows scientists to visualize the course of primate evolution. Orangutan DNA differs even more from human DNA, indicating that the last common ancestor of orangutans and humans lived even longer ago than the last common ancestor of humans and gorillas. As the DNA of more and more species is compared, a tree emerges. But DNA is not the only method of comparing species.
"Morphometrics" is the study of an organism’s form, and Delson specializes in a subfield called geometric morphometrics to describe his specimens. This method records certain "landmark" coordinates on a skull's surface, creating a three-dimensional frame that encodes information about the skull's shape and size. By comparing the 3-D frames of various species and specimens, Delson and his colleagues can quantify how closely related they are.