World’s Oldest Optical Illusion Found?

Thumbnail image for duck-rabbit.jpg

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.

bisonmammoth14a.jpg
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.

bisonmammoth14c.jpg
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.”

duck-rabbit.jpg
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)
Thumbnail image for Thumbnail image for Youngoldwoman.jpg
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)
Thumbnail image for bisonmammoth14b.jpg
(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.

Changing Planet

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 nationalgeographic.com 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.