ピルトダウン頭骨：人類進化の捏造事件

フッ素法

骨が地中に埋もれると、土中の水分や地下水に含まれる微量のフッ素が骨組織内に入り込み、フッ素の含有量が増えていく。したがって、骨に含まれているフッ素の量を測定すればその古さを推定することができる。水分中のフッ素量は地域によって異なるため、遠く隔たった土地から出土した骨の比較や絶対年代の測定はできないが、同一地点や近接した地域から出たものの相対的な古さを知るには優れた方法である。

この手法は19世紀になってヨーロッパで断続的に行なわれるようになり、第二次世界大戦後にロンドン自然史博物館のケネス・オークリー（Kenneth Oakley）によって確立された。

捏造の発覚

1949年、ケネス・オークリーがフッ素法でピルトダウン人頭骨の年代測定を行い、骨が5万年前のものであることを明らかにした。これによりピルトダウン人が類人猿と現生人類のミッシングリンクであることが否定された^[5][6]。 加えて1953年にはオークリー率いるオックスフォード大学の研究者らによるいっそう精密な年代測定と調査・分析が行われ、その結果、下顎骨はオランウータンのものであり、臼歯の咬面は人類のそれに似せて整形されていたこと、古く見えるよう薬品と思われるものにより石器などとともに着色されていたこと、伴出した獣骨は他の地域の産であることなどが突き止められた。 類人猿の下顎骨は人骨とは決して接合できないものであるが、捏造犯は接合部分を巧妙に除去して矛盾を隠し、着色を施して偽装したとのことである。

こうして40年近くにわたって古人類学界を混乱させたピルトダウン人は捏造された化石であると断定され、事件は一応の決着を見た。ただし、最初にドーソンからもたらされた頭頂骨と側頭骨は、後期更新世に由来する化石の現生人類（クロマニョン人の類）と考えられている。

 Identity of the forger

The identity of the Piltdown forger remains unknown, but suspects have included Dawson, Pierre Teilhard de Chardin, Arthur Keith, Martin A. C. Hinton, Horace de Vere Cole and Arthur Conan Doyle.^[13][14]

The focus on Charles Dawson as the main forger is supported by the accumulation of evidence regarding other archaeological hoaxes he perpetrated in the decade or two before the Piltdown discovery. The archaeologist Miles Russell of Bournemouth University analysed Dawson’s antiquarian collection, and determined that at least 38 of his specimens were fakes.^[15][16] Among these were the teeth of a reptile/mammal hybrid, Plagiaulax dawsoni, “found” in 1891 (and whose teeth had been filed down in the same way that the teeth of Piltdown Man were to be some 20 years later); the so-called “shadow figures” on the walls of Hastings Castle; a unique hafted stone axe; the Bexhill boat (a hybrid seafaring vessel); the Pevensey bricks (allegedly the latest datable “finds” from Roman Britain); the contents of the Lavant Caves (a fraudulent “flint mine”); the Beauport Park“Roman” statuette (a hybrid iron object); the Bulverhythe Hammer (shaped with an iron knife in the same way as the Piltdown elephant bone implement would later be); a fraudulent “Chinese” bronze vase; the Brighton “Toad in the Hole” (a toad entombed within a flint nodule); the English Channel sea serpent; the Uckfield Horseshoe (another hybrid iron object) and the Lewes Prick Spur. Of his antiquarian publications, most demonstrate evidence of plagiarism or at least naive referencing, Russell wrote: “Piltdown was not a ‘one-off’ hoax, more the culmination of a life’s work.”^[17] In addition, Harry Morris, an acquaintance of Dawson, had come into possession of one of the flints obtained by Dawson at the Piltdown gravel pit. He suspected that it had been artificially aged – “stained by C. Dawson with intent to defraud”. He remained deeply suspicious of Dawson for many years to come, though he never sought to discredit him publicly, possibly because it would have been an argument against the eolith theory, which Morris strongly supported.^[18]

Professor Adrian Lister of the UK’s Natural History Museum has said that “some people have suggested” that there may also have been a second ‘fraudster’ seeking to use outrageous fraud in the hope of anonymously exposing the original frauds. This was a theory first proposed by Miles Russell.^[19] He has explained that the piece nicknamed the ‘cricket bat’ (a fossilised elephant bone) was such a crudely forged ‘early tool’ that it may have been planted to cast doubt upon the other finds, the ‘Earliest Englishman’ in effect being recovered with the earliest evidence for the game of cricket. This seems to have been part of a wider attempt, by disaffected members of the Sussex archaeological community, to expose Dawson’s activities, other examples being the obviously fraudulent ‘Maresfield Map’, the ‘Ashburnham Dial’, and the ‘Piltdown Palaeolith’.^[20][21]Nevertheless, the ‘cricket bat’ was accepted at the time, even though it aroused the suspicions of some and ultimately helped lead to the eventual recognition of the fraud decades later.^[22]

In 2016, the results^[23] of an eight-year review^[24] of the forgery were released, identifying Dawson’s modus operandi. Multiple specimens demonstrated the same consistent preparation: application of the stain, packing of crevices with local gravel, and fixation of teeth and gravel with dentist’s putty. Analysis of shape and trace DNA showed that teeth from both sites belonged to the same orangutan.^[24] The consistent method and common source indicated the work of one person on all the specimens, and Dawson was the only one associated with Piltdown II. The authors did not rule out the possibility that someone else provided the false fossils to Dawson, but ruled out several other suspects, including Teilhard de Chardin and Doyle, based on the skill and knowledge demonstrated by the forgeries, which closely reflected ideas fashionable in biology at the time.

Teilhard de Chardin had travelled to regions of Africa where one of the anomalous finds originated, and resided in the Wealden area from the date of the earliest finds. Hinton left a trunk in storage at the Natural History Museum in London that in 1970 was found to contain animal bones and teeth carved and stained in a manner similar to the carving and staining on the Piltdown finds. Phillip Tobias implicated Arthur Keith in helping Dawson by detailing the history of the investigation of the hoax, dismissing other theories, and listing inconsistencies in Keith’s statements and actions.^[25] Other investigations suggest that the hoax involved accomplices rather than a single forger.^[26]