In 1912, Charles Dawson, an amateur British archaeologist, told Arthur Woodward of the British Museum about a fragment of skull found in the Piltdown quarry in Sussex, England. Soon, Dawson, Woodward and a third man, Pierre Teilhard de Chardin, returned to the quarry and found more skull fragments, a jaw bone and an elephant molar. By the end of the year they presented their findings to the British scientific community. These were the remains of an ancient human ancestor, they said, one that shared features with both apes and men. That is, the "missing link."
The British paleontological community was enthralled by such a find on British soil. Others weren't quite so enthusiastic, and many were skeptical. But after Dawson discovered a second skull, Piltdown II, the finds seemed more plausible.
Over the next decades, however, as more hominids were found around the world---australopithecines, Peking man, Homo erectus---Piltdown Man got less and less attention; it didn't fit into the growing collection of human ancestors, either in shape or geography.
In 1925, geologist F. H. Edmonds found that Dawson's dating of the gravels in which the fossils were found was in error. They were younger than Dawson had claimed. In 1947, a new test for fluorine content was applied to the fossils and established that they had a relatively recent origin. And then in 1953, scientists finally exposed Piltdown Man as a hoax, composed of pieces of medieval-era human skull, an orangutan jaw, and a couple of genuine fossils from the Mediterranean region.
The bones had been treated with an iron solution, and the teeth filed to fit or to show wear. A "canine" tooth included in the lot had been filled with sand and was patched with gum.
That the hoax wasn't exposed earlier is rather amazing, but the forgery was a good one and the initial analysis was pretty bad, even for its time.
The perpetrator of the hoax has never been found, though there are theories aplenty. Charles Dawson would seem to be the prime suspect, but there is little evidence he did it, and he died in 1916 without leaving a convenient deathbed confession. Other suspects over the years have included various acquaintances of Dawson, museum curators, Pieree Teilhard de Chardin, the guy they hired to do the digging, and Sir Arthur Conan Doyle, who was Dawson's neighbor. (Conan Doyle's The Lost World supposedly describes the hoax.)
What makes finding who did it even harder is that there is no obvious motive for such a forgery, particularly one that consisted of finds made over a period of years. Perhaps the forger or forgers just thought it was funny, an April Fool's joke for the ages.
(Find more details, including a timeline and references, at the Piltdown Man web site.)