Scientists unveiled Tuesday the fossil of a lemurlike creature called "Ida" that lived 47 million years ago in what is now Germany. According to media reports, the discovery is a missing link in human evolution. The research team itself is pushing the same idea: They've just released a book about the fossil called The Link, and a documentary of the same name will air Monday on the History Channel. It seems like we're always hearing about these "missing links" in paleontology—what gives?
It's shorthand for an important evolutionary discovery. The media love the phrase missing link—news stories have used it to describe no fewer than 28 paleontological discoveries in the past decade. Scientists sometimes substitute the phrase for the more technical-sounding "transitional morphologies," which refers to anatomical structures that bear some resemblance to both older and more recent physiology. But the research community tends to frown on the use of such language, and the discoverers of Ida have been criticized in some quarters for overselling their research.
The notion of "missing links" in the fossil record predates even the theory of evolution. Charles Lyell, a mentor to Darwin, used it in 1851—eight years before the publication of On the Origin of Species—to describe an abrupt transition in the types of fossils he found in adjacent layers of sediment. In 1863, a Scottish physician named John Crawfurd uttered the phrase for the first time as a critique of evolution: He demanded the missing fossil evidence to show "how a monkey became a man."
While most paleontologists agree that the discoveries of Homo erectus in 1891 and Australopithecus africanus in 1924 answered Crawfurd's charge, his use of the phrase to describe a common undiscovered ancestor of human and nonhuman primates captured the popular imagination and still hasn't let go. (The use of the phrase to describe the new fossil has ignited debate between creationists and evolutionists, though the discovery says nothing about man's relationship to apes.)
Most media reports use missing link more generically. Sometimes the phrase implies that a fossil is the direct ancestor of two or more extant species. * Such discoveries are rarely made, however. Most fossils represent species that are morphologically similar to a predicted ancestor but not connected in a straight line to any modern species. Even if a fossil were a direct ancestor, a paleontologist couldn't be sure; all she could say is that it would be consistent with direct ancestry. (When a fossil record surrounding a species is unusually dense, as it is for humans, scientists can sometimes find predecessors with certainty.)
A missing link may also describe an intermediate anatomical form that suggests how modern organisms might have developed certain capabilities. For example, an ancient fish with proto-wrists, elbows, and shoulders might be called a missing link between sea creatures and land animals. In this sense, though, every fossil is a missing link. There's no single intermediate point between, say, opposable and nonopposable thumbs. Rather, a wide variety of fossils seem to resemble both hand structures. No one can say which version is directly related to the two. It is entirely possible that all, or none, of the fossils are steps along the way.
Paleontologists suspect that certain transitional structures might never be found because of biases in the fossil record. For example, some believe that the human ancestors who became bipedal were forest dwellers. If this were true, we may never see the hard evidence, since forests teeming with hungry scavengers and bacteria are usually bad environments for preservation.
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Explainer thanks Anna K. Behrensmeyer of the Smithsonian National Museum of Natural History, Joe Felsenstein of the University of Washington, and Ward Wheeler of the American Museum of Natural History.
Correction, May 22, 2009: The original version mistakenly referred to a fossil as the direct descendant of two or more extant species. (Return to the corrected sentence.)