Missing link
"Missing link" is the term used to indicate a hypothesized missing stage in the evolution of a species. The theory of gradualism suggests that each species has evolved from another in a step-by-step manner. However, punctuated equilibrium says that species go for long periods of time with little change and evolve rapidly when a change in the environment occurs. According to both approaches, however, fossils of these links will tend to be rare because of the difficulty and rarity of fossilization. Where such a transitional form is absent, it is referred to colloquially as a "missing link".
Many evolutionists consider the challenge to find missing links to be a Catch-22. This is because each missing link that is found creates two more new missing links. Thus the challenge can never be satisfied.
Often the term missing link refers to the long-sought fossil that would show a species intermediate between humans and apes. However, this is a misnomer as humans, are, in fact, a species of great ape. Many candidates have been proposed, such as Piltdown Man (since shown to be a fake) and Lucy. All serious scientists believe that Lucy (a fossil found in East Africa) was an ape-like human ancestor called an australopithecus three million years ago. A huge number of fossils have been found of various species of extinct hominids, and genetic studies have been used to accurately find the relationships between these, humans, and other great apes.