The Piltdown Hoax: A Lesson on Confirmation Bias in Science
Scientists are supposed to be dispassionate professionals. We see them as thoughtful skeptics, seekers of truth, speaking about nature as science reveals it, even when the revelations differ from what we imagined, or from what we may have wished to be true. The truth is that scientists do have passions. They are people with dreams, ambitions, and preconceptions. They want certain things to be true, and certain things to be false, just like anybody else.
Science has evolved to compensate. At conferences, scientists ask questions of one another and debate (Figure 1), plus there is a process of formal peer-review. To be published, papers must be assessed by multiple researchers to look for bias, to assure that data support the author’s assertions, and have gone through appropriate statistical analysis. (For more information, see our Peer Review in Scientific Publishing module.) Experiments and other work must be described in sufficient detail as to be repeatable by other researchers. Finally, any specimens, physical and biological, must be examined and tested thoroughly so that all assertions are based on incontrovertible data.
Generally, the process works well for identifying unwitting errors, such as flaws in methodology or inadequate statistics, and also for exposing outright deception. But in the past, checks and balances in science were less strict, so it was possible to “cook data” and commit other fraudulent acts, thereby throwing the scientific community off-balance. Usually, people were misled just for a short time, but occasionally a hoax could remain hidden for years at a time. That’s what happened in the case of "Piltdown Man," a hoax that shook the world of science for four decades–partly, because it was so elaborate, but also because many scientists were so ripe to fall for it, especially in England, where the scheme was hatched.
The perfect storm and the perfect hoax
The place was Sussex, England, in a rural town called Piltdown. Sometime in the early 1900s, somebody took human skull fragments and an orangutan mandible (jawbone), stained all items with a variety of chemical cocktails, and buried them in a gravel pit. The unknown person also filed the teeth in the mandible and excised apish features from the bone. The perpetrator obtained various mammal bones and teeth, some authentic specimens from the Pleistocene and Pliocene epochs of geologic time, and scattered them throughout the pit to make everything appear roughly 1 million years old. Finally, the perpetrator threw in some bogus flint tools, the master plan being to create the long-sought 'missing link' between humans and apes. (For a timeline of the Piltdown discoveries and important events, see Figure 2.)
The plan worked for several reasons. To begin, the perpetrator knew what he was doing – lots of convincing "confirmatory evidence" was included with the hoax itself. Second, the idea of human evolution from ape-like ancestors was fairly new and ideas were emerging and developing rapidly. Charles Darwin had died just 30 years earlier. But most importantly, the Piltdown hoax was born in the midst of a perfect storm of events in and out of science. Notably, this involved the buildup to the First World War, which happened to overlap with discoveries of major import to paleoanthropology occurring in German lands, and elsewhere around the globe.
The people of England – including its scientists, both professional and amateur – really wanted a human ancestor that could compete with the discoveries of extinct human species in Germany. They wanted a 'missing link' on English soil and they wanted it badly – so badly that they fell into the trap of confirmation bias. This is the tendency to accept any evidence that seems to support one’s belief while rejecting all evidence that is contrary.
You can find examples of confirmation bias in everyday popular culture. Consider horoscopes, for instance, when a person is told that something will happen that day, then starts noticing occurrences that match the 'something' in one way or another, yet ignores how the same occurrences also match a different 'something' from somebody else’s horoscope. Imagine the consequences, though, when confirmation bias takes over discussions of a major issue in science. Furthermore, imagine that the bias is exacerbated by other social factors. In this case, a culture in one of Britain’s most eminent scientific institutions, where professional rivalry was fierce and where high profile success could lead to knighthood.
Punto de Comprensión
Confirmation bias is when you _____ evidence that is contrary to one's belief.
A lawyer, a zoologist, and a priest walk into a gravel pit
To get a feel for what transpired, we need to get a handle on some key players who emerged as the so-called 'Piltdown Man' entered the public scene. Charles Dawson was a solicitor around Sussex and also a passionate amateur archaeologist and collector of antiques. He wanted a place for himself in the academic world and constantly mingled with scientists. By the turn of the century, Dawson had built a reputation with some interesting finds, from ancient Chinese pottery to Roman artifacts. The latter included ceramic tile fragments that supported certain hypotheses about the Roman presence in Britain in late antiquity.
For an amateur, Dawson had a good amount of geological knowledge, especially with regard to Sussex, a region where Stone Age implements had been excavated in recent years, and where he traveled the roads to conduct legal work in different towns. Seeing some oddly-colored gravel near a farm led him to a pit that workers were excavating to fix a road. The gravel looked old, so Dawson asked the workers to save any bones, teeth, or other artifacts for him, should they find any in the course of the digging. According to Dawson, the workers eventually came across a fragmented, human-looking skull and gave it to him some time in 1908. The bones of the cranium – the part of the skull that held the brain – looked unusually thick, so he presumed the skull to be very ancient. He hoped it belonged to a human ancestral species, one that could rival the Germans’ famous Neanderthal man that had been known since the mid 19th century, and rival a newly discovered species in Germany called Homo heidelbergensis (Figure 3).
Dawson searched the gravel pit for more specimens and didn’t find any, but soon he met Teilhard de Chardin, a French priest-philosopher who also happened to be a trained geologist and paleontologist. In 1909, Dawson and de Chardin started searching the pit together and eventually found more pieces of the skull. In early 1911, they dug up one of the mammal fossils that the hoaxer had planted – a large tooth that de Chardin thought was from an extinct hippopotamus, but they needed confirmation from a specialist. Dawson chose Arthur Smith Woodward, an eminent geologist and head of the paleontology department at London’s Natural History Museum. Smith Woodward looked at the tooth and agreed it was from an extinct, Pleistocene hippo. This excited him enough to join Dawson and de Chardin searching the pit.
In creating the hoax, the perpetrator had buried some 40 different pieces. Dawson and his two friends had unearthed most of the mammal fossils and the flint tools by the middle of 1912. In their minds, this suggested that all of the specimens, including the skull, were several hundred thousand to a million years old. Then one day, while de Chardin was away in France, Dawson and Smith Woodward came across the orangutan mandible that, unbeknownst to them, the hoaxer had planted.
To realize that the jaw was from an ape, any anatomist would have looked at the part of the jaw that articulates with the rest of the skull. That’s because an ape jaw moves only up and down, whereas a human jaw also can move side to side, so the articulation is very different comparing apes and humans. But the articulating ends of this mandible were missing. Next, an anatomist would consider the chin region, but the one area on the chin that would have shown the mandible to be from an ape also was missing. Teeth also differ substantially between humans and apes, but the teeth on this mandible looked very worn, too warn to compare ape-human differences, plus their size looked intermediate between human and ape. (See Figure 4 for a comparison of human and ape jaws to the Piltdown Man artifact.)
Though human evolution was not his expertise, Smith Woodward ventured into this field, hypothesizing that both the skull and the mandible must be ancient – in which case, it seemed reasonable that both came from the same individual. If so, he was proposing the existence of a creature with a brain-size similar to a modern human, but with a jaw and teeth intermediate between human and ape.
Such an image was completely out of line with what we know today about the transitions between humans and our ape-like ancestors. Today, we know that our ancestors walked upright first, and evolved large brains more much later, but in 1912 most paleoanthropologists thought the opposite. They thought that the brain had begun to grow while our ancestors had still walked like apes. Going along with this was an idea that the jaw lingered in evolution, behind the brain.
So enthralled were paleoanthropologists in this idea of brain growth leading the way that they actually had succumbed to confirmation bias already on earlier occasions. Notably, they had overlooked a key finding that would have supported the image of human evolution that we have today. In Java in the 1890s, Dutch researcher Eugène Dubois had discovered Homo erectus, a species that walked fully upright and with a brain-size intermediate between modern humans and apes. In fact, its brain case was not much smaller than that of H. heidelbergensis. This should have suggested a progression from H. erectus to H . heidelbergensis to modern humans, but most researchers at the turn of the 20th century ignored Dubois. (For the modern view on the timeline of human evolution, see Figure 5.)
Indeed, Dubois’ finding was ignored to such an extent that Smith Woodward and Dawson may not even have known about it. Or at least they were not thinking about it as they considered the mandible freshly excavated, the skull from four years earlier, the flint tools, and the mammal bones that really were authentically ancient. In their minds, they had the 'missing link' and they were eager to show the world that the link was English – so eager that they failed to run certain tests that were well within Smith Woodward’s capability. He should have tested the skull and mandible for remnant organic material, for instance, by measuring the nitrogen content. This would not have revealed the precise age of the bones, but it would have shown at least that they were not hundreds of thousands of years old, thereby exposing the hoax. He also could have looked at the teeth with a magnifying glass, which would have shown the teeth had been filed. Avoiding such testing, he was essentially ignoring the possibility that bones from different time periods could have ended up in the same gravel pit, either by accident or by intention. This is another way that confirmation bias plays out.
Punto de Comprensión
At the time of the Piltdown Man discovery, paleoanthropologists thought our ancestors _____ first and then _____.
A patriotic missing link
The public learned of the find on December 18, 1912, when Dawson and Smith Woodward presented the skull and mandible and other specimens from the gravel pit in a ceremony at the British Geological Society. Reporters were there, along with photographers, and the story took off. The new species was nicknamed "the first Englishman" – and for a very political reason.
Britain and Germany were massing arms and propaganda and war was imminent. With Neanderthal and H. heidelbergensis bones in Germany and elsewhere on the continent, the English wanted a piece of the action. The Piltdown hypothesis was patriotic, and so the Piltdown discoverers were considered national heroes, especially Dawson since he had made the initial find of the skull. Consequently, the ancient 'species' was given the scientific name Eoanthropus dawsoni.
There were some skeptics right from the beginning. The absence of the articulating end of the mandible was a problem. At the Geological Association meeting, one anatomist and one dentist noted that the mandible was too apish and the cranium and face too human to belong to the same individual. Also, the wearing on the teeth did not make sense, neither in a human or ape, the dentist said. Nobody took the dentist seriously, but within several months critics were pointing out how a canine tooth was missing from the mandible, and that it might settle the case – if only one could be found.
It didn’t take so long. In late summer of 1913, Teilhard de Chardin, now back from France, found a canine tooth in the Piltdown gravel pit. Its size and shape matched the characteristics predicted for it, and now the skeptical voices were mostly silenced. But the magnitude of the confirmation bias had yet to show itself in full force.
An anonymous confession?
Perhaps worse than ignoring the oddly worn down teeth and the discovery of H. erectus, Dawson’s team, Smith Woodward in particular, failed to grasp the meaning of an object discovered under a hedge near the gravel pit in 1915, one of the last specimens to turn up at the Piltdown site. Carved from elephant bone in a pattern that could only be achieved with metal instruments, it had the form of a flattened, wide stick, narrowing on one side, presumably where it was supposed to be held. Pretty much anyone who looks at it in modern times draws the same conclusion: it looks like a cricket bat (Figure 6).
Somebody – either the hoaxer or someone who suspected the hoax and wanted to expose it – created the object as a kind of punchline. The missing link was not only the first Englishman but also the first cricket player. During the debunking of the hoax in the early 1950s, there was chatter in the Museum regarding who could have perpetrated the hoax in the first place, and regarding who was suspicious that there might be a hoax. In both cases, the name Martin Hinton figured prominently. Hinton was a zoologist and fossil authenticator at the museum. He was also a notorious practical joker who had a falling out with Smith Woodward over a money issue, so there’s a considerable amount of speculation that the cricket bat was Hinton’s work. Letters written by Hinton and American colleague suggest that Hinton suspected the Piltdown site was a scam and Dawson to be a fraud. Seeing his rival, Smith Woodward, put such effort into Piltdown and was building fame from it (Smith Woodward would eventually be knighted for his scientific contributions and continue digging at the Piltdown site for another 20 years), perhaps Hinton planted the Stone Age cricket bat to blow the lid on the hoax, thereby rendering Smith Woodward the ultimate embarrassment.
But if that was the intention, it backfired. Missing the punchline entirely, Smith Woodward and Dawson published a scientific paper hypothesizing the specimen to be some kind of bone tool invented by E. dawsoni for some unknown Stone Age use. It may seem incredible, but that’s how strong the confirmation bias was for Piltdown Man and by this time the First World War was in full swing, so anything that helped Piltdown helped the image of Britain.
Piltdown Man reigns through the 1920s
In 1916, Dawson died, but prior to his death, he told Woodward about a second skull he’d discovered recently at a site about 3 kilometers from the gravel pit. There was no mandible to go with rest of the skull this time, but there was a tooth, which resembled the teeth of the other skull. Of course, it did, for it too had been filed down. Smith Woodward presented the second skull to the world in 1917. This pretty much silenced any remaining murmurs of skepticism, despite the fact that no one knew the exact location of Dawson’s supposed second dig site. The new species was now widely accepted, and this went on through the 1920s, even in the face of new discovery, this time out of Africa.
In 1924, Raymond Dart, an Australian working in South Africa discovered a skull from a species that he eventually called Australopithecus africanus. It had a small braincase like an ape and a jaw and teeth less apelike and more like a human. That was inconsistent with the Piltdown picture, as was the idea of humanity emerging in Africa. Dart was utterly ignored by all of the big shots in paleoanthropology. For the next fifteen years or so, A. africanus was not even mentioned in paleoanthropology textbooks and Dart was unable to get papers accepted for publication.
In addition to discouraging attention from the work of Dart and Dubois, the hoax altered the careers of its supporters, Smith Woodward being an extreme case in point. Throughout the 1920s, most of his research was connected with Piltdown Man right up to retirement and beyond, for he then bought a house in the area and kept digging for more than two decades after the last Piltdown specimens had been excavated, always hoping to find more. And shortly before he died in 1948, Smith Woodward had written a book affirming E. dawsoni as the missing link. (See Figure 7 for a depiction of Dawson and Smith Woodward.)
Punto de Comprensión
The A. africanus fossil discovered in 1924, with a small braincase and more human jaw and teeth, proved to scientists at the time that the Piltdown Man was a hoax.
Piltdown Man's collapse
Raymond Dart’s find was the first in a long line of discoveries that by the 1940s would show an evolution of human ancestors in which the brain remained small as the jaw became more human-like. During the 30s and 40s, more fossils of H. erectus were also unearthed to go along with Dubois’ Java man. So a picture was emerging in which human ancestors walked upright, lost their ape-like jaws and teeth, and only later developed larger brains. This made Piltdown Man look increasingly like a side branch in the human family that just didn’t fit. As Dubois and Dart were vindicated, the world of paleoanthropology was growing skeptical of Piltdown by the 1940s, at least outside of Great Britain.
For the British people it took a little longer. In 1949, Kenneth Oakley and L.E. Parsons, both researchers at the Museum of Natural History, tested the original Piltdown skull and the mandible for fluorine content. This revealed that the mandible was not from the same time period as the other skull bones and that none of the bones were hundreds of thousands of years old. Other tests revealed the filing of the teeth, the staining of all of the buried specimens, and even the geographic origin of one piece – an elephant tooth with very high radioactivity, proving that it came from a site in Tunisia. The Hoax was debunked officially in 1953 on a BBC television show and in 1959 carbon dating technology had reached the point where the bones could be dated more precisely. The skull was from medieval times and the mandible was slightly more recent.
Assessing the evidence
Recent analysis of mitochondrial DNA (mtDNA) extracted from the Piltdown bones have yielded some fascinating information about the hoax. It shows, for instance, that the orangutan bone material came from a single orangutan and that the skull pieces used for the two skulls came from either two or three human individuals. Modern study also has revealed that the forger allowed putty to set too quickly, causing small cracks, and other details that all converged on one conclusion: there was only one perpetrator and that perpetrator was an amateur, not a trained conservator.
The hoax inspired the development of various scientific tests that came into use for authenticating paleoanthropological specimens from the 1960s onward. Thus, in a real sense, Piltdown did have a silver lining in terms of its long-term effects on science. Since exposure of the hoax in the 1950s, however, the investigation has focused more on who could have perpetrated the deception.
One resident of Sussex who visited the Piltdown site on several occasions at the height of the hoax and who dealt with evidence on a regular basis was Sir Arthur Conan Doyle, the famous author who created Sherlock Homes. Despite credentials as a medical doctor, Conan Doyle was ostracized by the science community in England, because he had taken up pseudoscience. As part of a new pop culture movement, he attended séances and promoted his belief in communication with spirits of dead people. In connection with this, he engaged a practice called 'spirit photography' and circulated what he alleged were 'spirit photographs'. Professional scientists scoffed at the idea, explaining that the pictures were merely double exposures. Thus, they said that Conan Doyle was not scientific, that he didn’t grasp the concept of repeatability in science, that he didn’t understand how evidence worked.
Since Sherlock Homes was all about evidence and deduction, some historians have speculated that Conan Doyle was so insulted that maybe he orchestrated the Piltdown hoax to trap scientists, to show that they didn’t know about evidence. It’s an intriguing hypothesis, perhaps more so today, given the bone analysis showing that the perpetrator was an amateur. A problem with the Conan Doyle hypothesis, however, is that he never actually revealed the hoax. He never came out and said,"Aha! You fell for the bait."
Over the years, there has been speculation about de Chardin and even Smith Woodward, but both would have known how to set the putty. Moreover, Smith Woodward spent the rest of his life looking for more Piltdown specimens, so he really looks more like the victim rather than the perpetrator.
For Charles Dawson, the evidence is circumstantial, but there’s a great deal of it, especially when considering his other discoveries. Though known as the greatest amateur archaeologist during his lifetime, once the Piltdown hoax was exposed in 1953, Dawson’s Roman tiles came under scrutiny with chemical tests. It turns out they were forged. So were his Chinese pottery pieces. Of more than forty artifacts that highlighted Dawson’s amateur scientific career, most were either forged or included some aspect of deceitful tampering.
Finally, there is Martin Hinton, the fossil authenticator whom we discussed earlier in connection with the Stone Age "cricket bat." During a renovation of the Natural History Museum in the mid-20th century, a couple of researchers found a case of bones in a room where Hinton had worked. The bones had been stained, then cut, apparently to see how deep the stain penetrated. Given Hinton’s reputation as a practical joker, given the suspicions about Piltdown that he expressed (not just in the early years in his letters, but later during Kenneth Oakley’s investigations in the 1940s and 50s), the evidence does support the idea of Hinton as the cricket bat maker. But the case of the bones is harder to interpret. Was he experimenting with the stains in preparation to carry out the hoax or was he experimenting after he grew suspicious merely to see how it was done? The modern evidence regarding the amateurish nature of the forgery weighs against Hinton as the hoax perpetrator, but leaves him high on the list for carving the cricket bat.
The Piltdown Man was once hailed as the "missing link" in the evolution of apes to humans. However, the discovery at Piltdown - human skull fragments, ancient mammal bones, and archaic tools - was an elaborate hoax. The deception took a long time to be revealed due to errors by the discoverers. They succumbed to confirmation bias by accepting any evidence that supported their discovery and rejecting any contradictory evidence.
Confirmation bias is the tendency to accept any evidence that seems to support one’s belief while rejecting all evidence that is contrary.
The 1912 discovery of Piltdown Man was widely hailed as the discovery of the "missing link" between humans and apes, but was really an elaborate deception.
The hoax perpetrator buried ancient animal bones, flint tools, and both human and ape skull fragments in the English town Piltdown.
The discoverers of the Piltdown Man were so enthusiastic about their findings that they ignored contradictory evidence and failed to carefully test the bones.
By the 1940s the scientific community were skeptical of the Piltdown discovery and it was officially declared a hoax in 1953.