I got a press question about the term "missing link" the other day. For obvious reasons. The question arose, where did the term come from?
I thought to turn to my Oxford English Dictionary, and here's part of the entry on "missing link."
1851 C. LYELL Elem. Geol. xvii. 220 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. 1862 Caledonian Mercury 11 Jan. 7/6 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. 1864 T. H. HUXLEY Further Remarks Human Remains Neanderthal in Nat. Hist. Rev. (Electronic ed.), It by no means follows that he should have supposed the philosopher to be the ‘missing link’. 1873 Jrnl. Anthropol. Inst. 2 445 If, indeed, Lavater did not thus..supply the missing link which has so frequently been said to be wanting in order to connect the two species together. 1875 B. JOWETT tr. Plato Dialogues (ed. 2) IV. 154 The metaphysical imagination was incapable of supplying the missing link between words and things. 1897 C. M. CAMPBELL Deilie Jock i. 29 I've heard talk o' some missing link, atween men and puggies. 1904 Collier's 7 May 18/4 (advt.) O'Sullivan Rubber Heels are now described as the missing link between wings and shoes. Their buoyancy is due to the elasticity of new rubber. 1930 D. H. LAWRENCE Assorted Articles 102 One woman..wrote to me out of the blue: ‘You, who are a mixture of the missing-link and the chimpanzee, etc.’and told me my name stank.
The entry is longer, and goes as far into the recent past as Pat Shipman and Alan Walker, but I only wanted to bridge the gap between the earliest occurrence and D. H. Lawrence, which seems quite properly literary of me to do. I also like the Plato commentary. "Missing link" between Platonic forms and things is almost as diaphanous a concept as the missing link in a genealogical chain.
Probably Huxley would have been more influential here than Lyell in the mainstreaming of "missing link", but the term seems to have been in the common vernacular by the time Darwin was writing.
For more on "missing links" and A. sediba, I can recommend Carl Zimmer's piece in slate: "Yet Another 'Missing Link'. I have considerable sympathy for his argument, although I'm not as opposed to the term, "missing link," as are many biologists. I'm a bit of a reactionary: Most biologists seem intent on making every fossil into an extinct side-branch of evolution. Evolution is like a tree, but in some senses the chain metaphor works -- there really must have been genealogical links between us and our ancestors.
"Missing link" is bad because it's hackneyed, and because it's usually impossible to test if a particular fossil actually was a genealogical ancestor. But I for one am not all that interested in filling in the record of extinct side branches of evolution. We're trying to understand the links on the chain connecting us to other creatures, even if we can't say we've found them.