A 47-million-year-old fossil of a newly discovered primate species has been trumpeted in the media as "the missing link" in human evolution. Nicknamed "Ida," the fossil is remarkably well-preserved, but paleontologists have scoffed at the "missing link" claim: it's not even clear if Ida is a close relative of us anthropoids, and in any case, the whole metaphor of "the missing link" only really works in the outdated model of evolution as a linear chain or ladder. But all the hoopla surrounding Ida inspired Nature editor Henry Gee to ask (via Twitter), how long have people been using the expression "the missing link"?

The Oxford English Dictionary notes that as early as 1851, geologist Charles Lyell used the plural form "missing links" in a general way in the third edition of Elements of Geology: "A break in the chain implying no doubt many missing links in the series of geological monuments which we may some day be able to supply." Also in 1851, in an address to the Geological Society of London (of which he was president), Lyell compared the fossil record of humans to that of "elephantine quadrupeds":

As the living Indian elephant is more intelligent than the African species, it may possibly also be superior to all the extinct proboscidians of the Sewalik group; but if so, how could it supply even one of those missing links in the chain of successive development of which we stand in need?

At the time, Lyell rejected "the doctrine of successive development" to account for the rise of new species, particularly for humans.  He went on to say, "For the superiority of man, as compared to the irrational mammalia, is one of kind rather than of degree, consisting in a rational and moral nature." But Lyell eventually came around to the thinking of Charles Darwin, who was working on his landmark book, The Origin of Species.

Soon after the publication of The Origin of Species in 1859, the expression "missing link(s)" began to be used by Darwin's critics. In 1860, William Hopkins wrote a long article for Fraser's Magazine on Darwin's theories, in which he posed this question:

It may be said perhaps that man may have existed long before the term indicated by any known records of his existence, and that he began his earthly career under a character much more approximate to that of the ape than that which he has now acquired by the progressive advancement of his faculties. But then, where are the missing links in the chain of of intellectual and moral being?

By this time Lyell had become a supporter of evolutionary theory, and he responded to Hopkins in his 1863 book The Geological Evidences of the Antiquity of Man. According to Janet Browne's biography Charles Darwin: The Power of Place,  Lyell's use of "missing links" in that book (as in his subheading,"Occasional Discovery of the missing Links in a Fossil State"), "lodged permanently in the public mind."

But many other writers both pro- and anti-Darwin were using "missing links" — and more notably, the singular "missing link" — to refer to human evolution even before the 1863 publication of The Antiquity of Man, so it wasn't merely Lyell who popularized it. The expression was often used to criticize biologist T.H. Huxley, who was such a strong supporter of evolutionary theory that he was known as "Darwin's Bulldog." For instance, the Scottish journal The Caledonian Mercury responded to a Huxley lecture in its January 11, 1862 issue:

Until the existence of some animal was discovered which should supply the missing link between man and the gorilla, there was a great gap even in Mr. Darwin's theory of the origin of species.

And an editorial about Huxley in the London Times of Oct. 7, 1862 stated:

It is conceivable, though improbable in the highest degree, that scientific research may discover what has been presumptuously called 'the missing link' between the human skeleton and the skeleton of the highest class of apes; but what will have been gained by such a revelation?

Similar disparaging references to "the missing link" can be found in reviews of Huxley's 1863 book Evidence as to Man's Place in Nature, though Huxley himself apparently didn't use the term until the following year in an essay about Neanderthals.

By the mid-1860s, the notion of "the missing link" came to be established in the public consciousness. And it was further cemented when Darwin followed up Origin of Species with The Descent of Man in 1871, applying his theories to human evolution. Darwin's critics used the "missing link" expression to describe gaps in the fossil record, at a time when the only early hominid fossils that had been discovered were Neanderthals (who some suspected were simply humans). Later discoveries, such as Homo erectus (discovered in Java in 1891) and Australopithecus africanus (discovered in Africa in 1924), were routinely hailed as "the missing link." The term persisted, despite the fact that evolutionary scientists didn't think that a single "link" was required to prove a connection from earlier hominids to Homo sapiens.

Now in the 21st century the paleontological picture has become even more complex, and yet the simplistic "missing link" metaphor still makes the headlines. Sometimes a popular expression can become so firmly rooted in our language that it stays with us despite the fact that it conceals far more than it reveals.

(Thanks to my brother Carl Zimmer for suggesting the topic. Carl is a noted science writer, and if you want to read an excellent dissection of the Ida hype, check out the post "Science Held Hostage" on his blog The Loom.)

(Update: Welcome, readers of Nature and The Loom.)