The fossil remains of early humans are exceptionally rare. Scientists trying to reconstruct the evolutionary history of our species often have to draw long, dotted lines between a few key fossils.
By Paul Rincon
So introducing a bogus ancestor into our family tree can throw the entire study of human evolution off course.
Piltdown reconstruction: The face of a fraud
This is exactly what happened with the Piltdown skull, which was exposed as an elaborate hoax exactly 50 years ago this month.
Its discovery had generated frenzied excitement. Piltdown man was argued to be 500,000 years old and therefore an irrefutable "missing link" between humans and apes.
Only one fossil of such great antiquity was accepted by British scientists of the day - the Heidelberg jaw found in 1907. But Piltdown, named after the Sussex village where it was discovered, was more complete - and English to boot.
Plaudits were heaped on the amateur geologist Charles Dawson and his friend Arthur Smith Woodward, keeper of geology at the British (now Natural History) Museum, who had unearthed the fossil fragments together in the years 1911-15.
Piltdown had a large, human-like braincase, but its jaw was ape-like, fitting predictions about how our ancestors looked. Bones from a beaver, rhino and hippo were also found, along with supposed stone tools known as eoliths.
Thousands of text books would have to be revised because of the hoax, said the Daily Mirror
In 1914, a curious elephant bone implement was found under a hedge at Piltdown. One unidentified wag suggested that it looked like a cricket bat.
In fact, Piltdown man was a modern forgery and not even entirely male. The jaw belonged to a female orang-utan. The skull was human. Much of the material had been stained brown to make it look fossilised.
"The cricket bat was a joke - though Dawson and Woodward obviously didn't get it," says Dr Andy Currant, a vertebrate palaeontologist at the Natural History Museum in London.
Piltdown was accepted as genuine until 1953, when scientists from Oxford University and the British Museum used chemical testing to prove it was a fake.
The high forehead and heavy jaw of Piltdown had reinforced misconceptions that human brains grew large at an early stage in our species' evolution. In 1925, a genuine fossil ancestor from South Africa was dismissed in England because it didn't look like Piltdown.
The hoaxers made other anatomical gaffes. They filed down molars in the jaw to remove obvious orang-utan dental traits, but were blissfully ignorant of the way human teeth wear down.
"Human teeth wear more on the buccal [cheek] side of the crown and not as much on the lingual [tongue] side," says Professor Tim White of the University of California, Berkeley.
Where the hoaxers obtained their specimens is a mystery. One possible trail leads to the Natural History Museum. In 1911, the British Museum bought a collection of animal remains from Borneo. An original inventory appears to list the lower jaw of an orang-utan as missing.
Radiocarbon dating showed the human skull from Piltdown was less than 1,000 years old. Its unusual thickness suggests the owner suffered from Paget's disease, a hereditary thickening of bone.
A similar skull reportedly disappeared in the 1900s from Hastings Museum, an institution with which Charles Dawson had strong connections.
Dawson has long been prime suspect as the forger. But a clever piece of scientific detective work has implicated another character in the saga.
In 1976, an old canvas trunk belonging to Martin Hinton, a volunteer in Smith Woodward's geology department at the time of Piltdown, was found in the Natural History Museum.
It contained mammal bones and teeth stained a similar mahogany brown as the Piltdown material and carved like the cricket bat.
Palaeontologist Brian Gardiner has subjected the Piltdown bones and the Hinton items to a technique called flame atomic absorption.
PILTDOWN MAN IN TIME
1911 - first skull fossils found
1912 - discoveries publicised
1914 - cricket bat surfaces
1915 - Charles Dawson dies
1949 - Piltdown ages queried
1953 - Fossil fakes unmasked
The chemical signature of the Piltdown material matches Hinton's bones and teeth, suggesting they were stained using the same methods.
Gardiner believes this lays the blame squarely at Hinton's door. But not everyone is convinced.
The continuing fascination with Piltdown, 50 years after it was exposed, stems partly from its status as an unsolved case.
The list of suspects is long and constantly expanding. One investigator has even accused Sir Arthur Conan Doyle - creator of Sherlock Holmes - of conceiving the hoax.
Professor Chris Stringer, palaeoanthropologist at the Natural History Museum and Andy Currant believe Charles Dawson was the main culprit, planting everything except perhaps the cricket bat.
Dawson was no stranger to archaeological forgeries. He exhibited bizarre phoney fossil toads and almost certainly forged two Roman tiles with rare inscriptions.
"Ninety-nine per cent of the evidence points towards Dawson. But Hinton might have been behind the cricket bat," says Currant.
The joke: The "first Englishman" had a "cricket bat"
"Whoever planted the cricket bat wasn't part of the original hoax and had a different message, namely: 'We're on to you and we're going to mess your site up,'" says Stringer.
This may have prompted the original forger to plant more human bones at a site nearby called Piltdown II. Dawson discovered these in 1915.
"Piltdown II was an attempt by the original forger to throw people off the original site. It was a reaction to the discovery of odd material they hadn't planted," adds Currant.
Whether Hinton planted all the material, or just some, he had a motive. He quarrelled with Smith Woodward over payment he said he was owed for an academic contract. He may have wanted to humiliate his boss as an act of revenge.
But Smith Woodward's arrogance and aloofness had made him many enemies in the British Museum, raising the possibility that others assisted Hinton in his vendetta.
At a dinner party in 1975, Kenneth Oakley, one of the team that exposed Piltdown in 1953, allegedly named Charles Chatwin as a conspirator. Chatwin was an assistant for Smith Woodward in the geology department at the time of Piltdown.
If the hoaxers could see the fuss still generated by their handiwork, they would no doubt be amused.
"Piltdown is a piece of nonsense which has used up a phenomenal amount of good time," says Currant.
"I'd like to see the 50th anniversary commemorated by the crushing of all the material and the burning of the Piltdown archive."
Piltdown Man: The Context And Exposure Of A Scientific Forgery is an exhibition that runs at the Natural History Museum from 25 November.