Changing Planet

World’s Oldest Optical Illusion Found?

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Long before the famous duck-rabbit illusion (seen at right), prehistoric artists were creating mind-bending double images of their own, according to a new paper presented earlier this year at an international convention on rock art research.

The paper’s author, Duncan Caldwell has surveyed the Paleolithic art of several caves in France and discovered a recurring theme that he says can’t be simply accidental. Throughout the cave of Font-de-Gaume, and in examples from other sites as well, drawings and engravings of woolly mammoths and bison often share certain lines or other features, creating overlapping images that can be read first as one animal, then the other. Rarely, if ever, do they do the same with other animals. While images of horses, deer, extinct cattle, and even rhinos often appear in such caves, and often partially or entirely overlap each other, it is only the mammoth-bison pair that Caldwell found regularly appearing superimposed so exactly. For example in the modern drawing below of an image from Font-de-Gaume, one main body shape, underbelly, and set of legs is adorned with signs of both mammoth and bison heads at both ends. These two large, bulbous, “armor-headed herbivores” which share many physical similarities in life, seem to have had some connection for people in this region in art as well.

Early cave art researcher Henri Breuil copied this image of overlapping bison and mammoth from the walls of Font-de-Gaume in France. (Image courtesy of Duncan Caldwell)

In a particularly striking example, a small figurine has been given the details of a bison on one side and those of a mammoth on the other. The Paleolithic artist was clearly playing with the similar contours of the two animals and creating a single object that could be flipped to represent one species or the other.

The two sides of a figurine from a site near Cambrai show very different details. On one side (left), the high back leg and short front leg are characteristic of depictions of bison. On the other, the tall straight front leg and grooves depicting long hair in the midriff are typical of mammoths. (Image courtesy of Duncan Caldwell)

Nice Trick, but Is It an Illusion?

There’s a big difference between overlapping images or ambiguous profiles and a proper optical illusion however. Nigel Warburton is a senior lecturer of philosophy at The Open University and co-host of the podcast “Philosophy Bites,” which uses the duck-rabbit as its logo. For him, knowing the original context of the image is key. Speaking of the classic version, he said “If somebody had been illustrating a children’s book about rabbits, nobody would have seen it as a duck.” As he put it, “the fact that a figure can be read in two ways isn’t conclusive proof that it was intended to be read both ways.”

A classic version of the duck-rabbit illustration, contemplated by psychologist Joseph Jastrow in 1899, and philosopher Ludwig Wittgenstein in 1953. (Image via Wikimedia Commons)
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Another classic shape-shifting image is the “Old Woman-Young Woman” image, where the young woman’s neckband and ear become the old woman’s mouth and eye, respectively. (Image via Wikimedia Commons)

The duck-rabbit is different because we know that it was created not just to show both animals individually, but to call attention to the strange sensation of one replacing the other. It’s to some degree a “reflection on the nature of perception.” The mammoth-bison images clearly make use of ambiguous shapes and similarities between the animals, but that doesn’t guarantee they were intended as optical illusions.

Magic Is in the Beholder of the Eye

Perhaps the most dramatic candidate for a mammoth-bison image meeting this requirement and being an intentional illusion isn’t on a cave wall, but on a carving from a spear-thrower from the site of Canecaude. In this piece, as Caldwell sees it, it’s not that the animals share a contour or a few lines, but that just two small details allow the entire image to be read as either of the two species, and seeing one causes the other to “disappear.”

Unlike other bison-mammoths that depict two distinct but overlapping images, this carving from a spear-thrower features one image that can be seen two different ways. Above, the artifact in its natural state. Below to the right, red ovals highlight the position of the two eyes. (Images courtesy of Duncan Caldwell)
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(Image courtesy of Duncan Caldwell)

The details in question are the eyes. Caldwell describes how there is “both an upper eye, which turns the crescent beneath it into a tusk, and lower eye, beside the front leg, that transforms the same crescent which we just interpreted as a “tusk”, into a bison’s overhead horn.” Looking back and forth between the eyes then, we are able to see the entire shape transform from one animal to the other, an effect much more like the classic Gestalt shift of the duck-rabbit.

Significantly, it is hard to think of other reasons for the unusual position of the eyes. First of all, their delicately carved shapes show that they were made intentionally, and are not just accidental markings. Secondly, the details of the body of the animal, its tusk/horns, long hair, and legs are all fairly realistically represented showing the artist’s ability to make an accurate full profile view if desired. Even in the somewhat common technique in prehistoric art of using “twisted perspective” where an animal’s body will be shown in profile, but both eyes will be visible on the face in a Picasso-like manner, the eyes are generally still close together. In addition, that technique seems to be more common in flat wall art than in full sculpture, where the ability to put one eye on each side of the sculpture takes care of the perspective problem.

The possibility remains however that pieces of the complete spear-thrower that are now missing would clarify the image further, and show that the image is just a mammoth, plain and simple. For example, in a reconstruction in the book “Mammoths: Giants of the Ice Age,” authors Adrian Lister and Paul Bahn propose that the mammoth’s trunk would have looped back in the lower-left part of the carving, making the lower “eye” the curled up end of the mammoth’s trunk.

A Window Into Stone Age Philosophy?

If Caldwell’s analysis of the mammoth-bison phenomenon is correct, we begin to get a view not only of what prehistoric people saw, but also how they thought, and that has the potential to change our perspective and how we see other artifacts as well. That perspective shift is also what makes these ambiguity illusions so appealing in the first place. Nigel Warburton said he chose the duck-rabbit as the logo for “Philosophy Bites” because it’s also what he likes about philosophy itself. “Part of what philosophy does,” he said “is, sometimes, make you see what you already knew in a completely different light.” Perhaps someone enjoyed the same thought sitting in a cave, some 15,000 years ago.

(Updated 12/23/2010)

For More Information

Duncan Caldwell is a fellow at the Marine and Paleobiological Research Institute in Massachusetts. Follow his varied research and interests on his personal site.

Nigel Warburton is a Senior Lecturer in Philosophy at The Open University. His latest book, “Philosophy Bites,” based on the podcasts of the same name, is available from Oxford University Press.

Andrew Howley is a longtime contributor to the National Geographic blog, with a particular focus on archaeology and paleoanthropology generally, and ancient rock art in particular. In 2018 he became Communications Director at Adventure Scientists, founded by Nat Geo Explorer Gregg Treinish. Over 11 years at the National Geographic Society, Andrew worked in various ways to share the stories of NG explorers and grantees online. He also produced the Home Page of for several years, and helped manage the Society's Facebook page during its breakout year of 2010. He studied Anthropology with a focus on Archaeology from the College of William & Mary in Virginia. He has covered expeditions with NG Explorers-in-Residence Mike Fay, Enric Sala, and Lee Berger. His personal interests include painting, running, and reading about history. You can follow him on Twitter @anderhowl and on Instagram @andrewjhowley.
  • Peter Faris

    If you look at the illustration of the Canecaude spear thrower on page 119 in Mammoths of the Ice Age, by Adrian Lister and Paul Bahn, University of California Press, 1994-2007, you will find a very convincing diagram showing the so-called “lower eye” as the end of the trunk which has broken off with the shaft of the spear thrower. Too bad though, I really loved the idea of the optical illusion.

  • simon Webb

    How come the upper ‘eye’ is placed below the root of the tusk?

  • Ken Johnston

    There is a North American figure stone (Flint Ridge, Ohio) at posted May 14 which demonstrates a related motif and a compelling example of the “optical illusion” phenomena. Here, the bison’s head is also the head of a woman being depicted as riding on the neck of the proboscidean using a driving stick. In the illusion, the woman’s breasts are also the bison’s nostrils, together symbolic of the source/breath of life. The bison and the woman share an eye (left eye of bison is right eye of woman). Side two of the figure stone depicts a micro-carved skull which is also an egg in bird’s nest in macro perspective. So, the Ohio piece demonstrates two such “optical illusions” which must be an intended part of the composition and likely part of a multi-leveled symbolic system of the sculpture’s creators.

  • Duncan Caldwell

    In order to provide balance, the National Geographic article mentions the “possibility… that pieces of the complete (Canecaude) spear-thrower that are now missing would clarify the image further, and show that the image is just a mammoth, plain and simple. For example, in a reconstruction in the book ‘Mammoths: Giants of the Ice Age,’ authors Adrian Lister and Paul Bahn propose that the mammoth’s trunk would have looped back in the lower-left part of the carving, making the lower ‘eye’ the curled up end of the mammoth’s trunk.”

    This passage has left readers like Peter Faris, who left a dismissive comment here, with the impression that the two proposals
    – could be of equal weight
    – or that an interpretation based on a missing trunk section might actually trump one based on a lower eye.
    This is odd because the interpretation based on the existence of a lower eye is founded on an observable feature – the precisely carved graphic unit with apparent lids and an eyeball – while Lister and Bahn’s interpretation is no more than a stretch based on a supposed section of trunk that hasn’t left a trace – a stump, a fracture – nothing. In peer review the observationally supported analysis would pass muster while the unsupported conjecture would get the red pen.

    More importantly, a close examination of the sculpture shows that the phantom section of trunk never existed. In order for the graphic unit that I interpret as a wisent’s eye to be interpretable, instead, as a trunk tip holding something, it would have to be clasped between ridges – the residual edges of the trunk – rising from the edge of the sculpture below, but such ridges are not present.

    As if that were not enough, the edge from the base of the “trunk” to the front leg displays the same micro-denticulated silhouette as the leading edge of the back leg, where nobody would think of looking for traces of a missing trunk section and no breakage can be deduced. Instead of indicating damage, the comparable edges actually form the same pattern of parallel incisions and tiny teeth, illustrating frills of hanging fur. In the traditional reading of the Canecaude spear-thrower as a mammoth, the edge between the proboscidian’s tusk and front leg was meant to be read as the animal’s furry bib. In the sculpture’s additional reading as a bison, the same edge becomes the animal’s hairy brow.

    Two more details even allow us to identify the bison’s sex and activity. Whereas the mammoth’s pupil is centered and “looks” forward, the downcast pupil of the lower eye (belonging to the bison) is a naturalistic representation of that animal’s downward gaze when it is charging with brandished horns. The size of the wisent’s hump completes the impression that one of the sculpture’s readings was meant to be an illustration of a huge, charging, and specifically male bison.
    National Geographic’s reference to Lister and Bahn’s book encouraged me to look for a copy. The one I got was actually the older (1994) edition, which was simply entitled “Mammoths”. What I found shocked me. Here is the full text about the spear-thrower from the ‘94 edition: “A mammoth was selected for the decorative end of a spear-thrower from Canecaude in France (top). The tusks encircle the eye, rather like musk-ox horns” (p. 98). There is not another word about the spear-thrower in the book, and yet the caption is as telltale as a Freudian slip! The authors’ realization that the crescent looked like an ungulate’s horn shows that their minds were being teased and that they came within a synapse or two of seeing the image flip from a mammoth into another large “armor-headed” herbivore: a bison or musk-ox. If only they’d put the notion of the ungulate horn together with the realization that the unexplained graphic unit under the crescent looked like a lower eye!

    But they didn’t. And when they revised their book, they apparently explained away the germ stirring in the back of their minds by imagining a ghostly section of missing trunk. The mind is a slippery thing, and sometimes explains things away all too easily. I know mine does, all the time.
    Another of the comments that have been posted here, this time by Simon Webb, pointed out the odd placement of the “mammoth’s” eye, which, as I’ve said on my website, makes the traditional reading of the sculpture as a proboscidian even stranger than the additional reading of the carving as a bison. The more anatomically correct of the two eyes is indeed the lower one belonging to the wisent, not the upper “mammoth” one, which, as Mr. Webb noted, isn’t where it would normally be – well above the tusk’s root – but lower, where we find it strangely squeezed between the crescent and edge of the sculpture. This actually provides yet another strand of evidence for the idea that the carving is both a mammoth and bison, since even an exaggerated cephalic hump on a bison isn’t quite large enough to accommodate a properly positioned proboscidian eye, forcing the brilliant sculptor to fudge the position of the pachyderm’s eyeball. The artist was pushing his (or her) impressionistic magic to the limit by placing it just outside the “tusk’s” upper edge, but obviously still succeeded in conveying the idea of a mammoth, since the image has often continued to be read that way in modern times.

    Even so, Mr. Webb’s implication is correct: if one wants to take issue with one of the two readings (and, of course, I champion both), it has to be with the idea that this sculpture represents a mammoth, rather than with the more anatomically faithful reading of it as a bison.
    Another red herring of an argument against the bison can be dismissed just as readily as the idea that the bottom eye actually represented the tip of the trunk – the notion that the top of its head is a crack post-dating the object’s manufacture. The top of the bison’s head is clearly formed both by the tusk-horn crescent, which is incontestably anthropogenic, and by the somewhat exaggerated cephalic hump that it shares with the mammoth – a compromise also used in the Font-de-Gaume repertoire.
    Although they may not rise to the level of perfect optical illusions, it is also important to note that Paleolithic carvers created visual puns that combined references to male and female sexual attributes. These puns range from a wand found at le Placard, which looks like a phallus and scrotum until one sees that it has a vulva between stumps at the base, to another wand from Dolni Vestonice with twin lumps that can be read either as breasts or testicles. Similar sexual amalgams have been suggested for the Lespugue and Savigano Venuses, most recently in an article that I wrote for the Barbier-Mueller Museum called “Supernatural Pregnancies: Common features and new ideas concerning Upper Paleolithic feminine imagery” (2010, Arts & Cultures: pages 52-75).
    Although this is neither in the Nat Geo piece nor my underlying article, the use of recognizable figure-ground illusions during the Upper Paleolithic suggests that the mind had evolved to the point where it could incorporate similar gestalt shifts in the deepest structure of languages and thought. I only wish that Steven Pinker had known of the bison-mammoth shifts when he was writing about similar ones in English in a section of “The Stuff of Thought” that he called “Flipping the Frame” (pg 42-51).
    Finally, Chandrachoodan Gopalakrishnan, who saw the report on National Geographic, informed Andrew Howley and myself that a remarkable equivalent of the Paleolithic mammoth-bison optical illusion exists in Tamil Nadu, India, where bas-reliefs in temples show bull and elephant bodies, which share the same head. The head is sculpted in such a way that it can be read first in one direction, as a bull’s, then in the other, as an elephant’s. The Paleolithic puns on bovines and proboscidians are so far apart from the Indian ones in both place and time that links between them are a tremendous long shot, but the possibility can’t be discounted entirely. First one would have to determine how old the theme is in India. Then one should probably look for antecedents to the north, since any connection between the Paleolithic and modern examples must have come across the steppes.

    In the meantime, it would be fascinating to learn more about the stories, beliefs and practices surrounding the Indian amalgams between bulls and elephants.

  • candy pig

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