The term missing link refers to an idea derived in a fairly obvious manner from the "Great Chain of Being": a concept, much beloved of medieval scholars and theologians that traces its roots back to Aristotle. According to this notion of an inherent organismic Scala Naturae, all living creatures are ranked (or occupy positions) from "lower" to "higher," with humans, the crowning glory of creation, at the top (though between humans and God lie angels, archangels, and other spiritual beings). This archaic terminology still survives in some areas of science today, with vertebrates, for instance, continuing to be classified by many as "lower" (fish, amphibians, reptiles) versus "higher" (birds, mammals).
The metaphor of a chain of being, with its lowest members connected to the highest by an insensible gradation of intermediate forms, naturally adapts to the notion of a series of links. Thus in pre-Darwinian times it was widely held that every species must represent a link in this chain, just as it occupied its own preordained place in nature; some mid-nineteenth century naturalists, though, working in a world whose immensity and diversity were already becoming recognized, vaguely envisaged the eventual discovery of a complete set of living intermediates. With the advent of the notion of evolution by natural selection propounded by Charles Darwin (1809882) and his younger contemporary Alfred Russel Wallace (1823913) in 1858, and influentially enlarged upon by the former in On the Origin of Species in 1859, interpretation of the fossil record, already long known, assumed a new dimension. Earlier western scholars had tended to the view that the ancient fossil organisms, often very different from those familiar in the modern world, were best interpreted as victims of the Noachian floodr, in view of the evidently complex stratigraphy involved, as witnesses to a series of "catastrophes" for which biblical authority was sought. With the introduction of the concept of evolution, an alternative explanation was at hand: that extinct organisms represent stages along the route through time from ancestral forms to the modern biota.
From this point it was but a short leap to the notion of ancient "missing links" in the chain. In popular lore the most famous of these lies between apes and humans, and many fossils have been acclaimed in this intermediate role. Darwin had emphasized in his Descent of Man (1871) that today's humans and apes are descended from nothing like an existing ape or monkey, but from a common ancestor (which, by definition, can be neither). Still, the half-human, half-ape image, of a form caught in the act of clawing its way up toward humanity, seized the public imagination, and even that of scientists. Among the latter the influential Ernst Haeckel (1834919) went so far as to name this hypothetical form Pithecanthropus alalus (the "ape-man lacking speech").
It is useless for paleoanthropologists to protest that if a form is missing, it cannot be a link, and that if it is to be a link, it cannot be missing. It is well known now that it is far more accurate to speak in terms of a ramifying bush of ancestors and descendants than of links in a chain. But it will be a long time before we are able to exorcise the evocative "missing link" from our vocabularies.
See also EVOLUTION, HUMAN
Darwin, Charles. The Origin of Species (1859). New York: Bantam Classic, 1999.
Darwin, Charles. The Descent of Man (1871). Amherst, N.Y.: Prometheus, 1997.