There was just one problem: He was a fake.

In 1953, scientists at the British Natural History Museum and University of Oxford reported that the Piltdown fossil was actually a hodgepodge of human and orangutan bones, none of them more than 720 years old. The remains had been meticulously worn down with a file and stained with iron and acid to give the appearance of age. Dental putty was used to hold the teeth in place.

The scientists called the fake "extraordinarily skillful," and the hoax "so entirely unscrupulous and inexplicable as to find no parallel in the history of paleontological discovery."

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But their investigation couldn't resolve one question: Who would have done such a thing, and why?

Writing in the journal Royal Society Open Science, a new team of investigators says it has an answer. The Piltdown forgery was the work of one man — solicitor and amateur archaeologist Charles Dawson, who first "uncovered" the remains.

"Whether Dawson acted alone is uncertain, but his hunger for acclaim may have driven him to risk his reputation and misdirect the course of anthropology for decades," the researchers write. "The Piltdown hoax stands as a cautionary tale to scientists not to be led by preconceived ideas, but to use scientific integrity and rigor in the face of novel discoveries."

The first mention of the skull came in February 1912, when Dawson sent a letter to his friend Sir Arthur Smith Woodward, head of geology at the British Museum, about an exciting new skull he'd uncovered on his land near the town of Piltdown. Just five years earlier, German scientists had uncovered the mandible of a 600,000 year old Homo heidelbergensis — the ancestor of Neanderthals and modern humans. This specimen "will rival H. heidelbergensis in solidity," Dawson promised.

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Later "excavations" at two Piltdown sites revealed a jaw bone, teeth, stone tools and a piece of carved fossil bone deemed a "cricket bat." They also cast a cloud of suspicion over everyone who took part. A volunteer in Woodward's department, Martin Hinton, seemed a likely suspect, especially after researchers at the Natural History Museum discovered a trunk of stained bones he'd left in storage there. Some skeptics eyed Teilhard de Chardin, a French Jesuit priest and prominent paleontologist, who discovered a canine tooth that figured prominently in the skull's identification. Even Sir Arthur Conan Doyle, the famed creator of Sherlock Holmes, was implicated; he was a member of the same archaeological society as Dawson and, for various complicated reasons, may have wanted to put one over on the scientific establishment by faking the hominid bones.

Whoever committed the forgery, the consequences were long-lasting. The belief that modern humans evolved in Britain persisted for another 40 years — it was so ingrained that many scientists dismissed a real archaic human fossil, the Taung Child, when it was uncovered in South Africa in 1924. And the hoax weakened the public's trust in science. Even today, creationists point to Piltdown Man to justify their suspicion of evolution.

To figure out who was responsible, paleoanthropologist Isabelle De Groote and more than a dozen other researchers reexamined the Piltdown skull and attempted to retrace how it was made. Their techniques — DNA sequencing, spectroscopic analysis — weren't available to the scientists who exposed the hoax in 1953 and indeed may have seemed even more improbable to Dawson's turn-of-the-century colleagues than the idea of Piltdown Man himself.

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"Understanding what was used to fake the fossil that misled scientists for four decades and how they were manufactured may bring us closer to identifying whether there were one or more hoaxers, and why they would have risked their reputation to fool the scientific community," De Groote and her team write.

They started their search with the orangutan mandible. DNA sequencing indicated that the jaw and all the teeth came from the skull of one orangutan, even a tooth Dawson claimed to have found at a second Piltdown site several kilometers away. It's likely that the skull was purchased at a curiosity shop and broken into pieces by the forger. In addition, tiny cavities in the teeth were stuffed with pebbles and covered with putty to make them heavy — indicating that the forger knew fossil bones weigh more than recent ones.

Studies of the human remains were less successful; De Groote's team was unable to extract material from the bone for identification and dating. The researchers believe that at least two, and possibly three, skulls were used to make the cranial "fossil." Though the bones are thicker than a standard skull, they fall within the range of human variation, and their thickness is probably why the forger opted to use them.

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But the overall modus operandi of the forger was skillful and incredibly consistent, and only one of the 20 or so people who have been implicated in the hoax could have achieved the whole thing: Charles Dawson.

"The story originated with him," the authors write. "Nothing was ever found at the site when Dawson was not there, he is the only known person directly associated with the supposed finds at the second Piltdown site, the exact whereabouts of which he never revealed, and no further significant fossils, mammal or human, were discovered in the localities after his death in 1916."

Dawson was an experienced fossil hunter (and faker — a number of his other finds ultimately turned out to be hoaxes) who had friends in the paleontology community and a thorough understanding of what a "missing link" fossil ought to look like. He would have had the means to acquire the necessary human and orangutan remains and the knowledge to ensure that they were "discovered" in the right way. He also seemed to continue to modify his forgery in response to fellow scientists' analysis of the remains.

"When a jaw and the skull bones were announced, there was a big discussion at the Geological Society about what the canine in such an animal would look like," De Groote told the BBC. "And, ta-da — six or seven months later, a canine shows up and it looks exactly like what they had predicted."

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A look at Dawson's letters revealed why an apparently successful solicitor and respected amateur scientist would attempt such an audacious hoax. By age 45 he'd written or co-authored more than 50 scientific articles but was still waiting for recognition as an archaeologist. In 1909, he wrote to Smith Woodward, "I have been waiting for the big ‘find’ which never seems to come along." He dreamed of being elected a fellow of the Royal Society but was never nominated — until he announced the Piltdown discovery.

The study authors' chief criticism is reserved not for Dawson but for the scientists who believed him. Piltdown Man met turn-of-the-century researchers' preconceived notions for what an archaic human fossil would look like — so they were far less skeptical than they ought to have been.

"Solving the Piltdown hoax is still important now," De Groote and her colleagues write. "It stands as a cautionary tale to scientists not to see what they want to see, but to remain objective and to subject even their own findings to the strongest scientific scrutiny."