Chimpanzee attraction to crystals could help us understand the fascination of our ancestors for these minerals

Hundreds of thousands of years ago, our hominin ancestors were already collecting crystals. There was something about those stones that turned them into objects of desire, even though they were of no apparent practical use. The question here is, why did they collect them? To delve deeper into the roots of this fascination, a research team led by the Ikerbasque Research Professor Juan Manuel García-Ruiz at the Donostia International Physics Centre (DIPC) conducted a series of experiments with enculturated chimpanzees at the Rainfer - Chimpatía Foundation facilities. Given the impossibility of conducting experiments on our hominin ancestors, these primates were chosen as they are the species most closely related to the Hominidae family genetically. The results show that chimpanzees are attracted to the transparency and shape of crystals, and are able to quickly distinguish them from normal rocks of similar sizes. This finding could explain why objects with properties similar to those of crystals have attracted our ancestors for nearly 800,000 years, and why they continue to arouse our interest today. Similarly, it opens up new avenues for understanding hominin behaviour, suggesting that the responses observed in front of crystals, both in hominins an non-hominin apes, may reflect a shared cognitive predisposition. So collecting crystals highlights the importance of material properties in shaping early symbolic cognitive behaviours, and could contribute to exploring their evolutionary bases.

This new study, published in the journal Frontiers in Psychology, describes the experimental development of pioneering research led by DIPC scientific staff, featuring chimpanzees rescued from illegal trafficking, exploitation and abandonment by the Chimpatía Foundation. “We demonstrated that enculturated chimpanzees are not only attracted by crystals, but are also able to clearly distinguish them from other stones with similar characteristics,” said the article's lead author, Professor García-Ruiz. “It was a pleasant surprise to discover how strong and seemingly natural the chimpanzees' attraction to crystals was, suggesting that sensitivity to this type of object may have deep evolutionary roots.”

The monolith

Modern humans diverged from chimpanzees around six or seven million years ago, so we share significant genetic and behavioural similarities with them. To find out whether the fascination for crystals is one of them, the research team designed several experiments involving presenting crystals to two groups of enculturated chimpanzees rescued by the Chimpatía Foundation and accommodated at the “Rainfer” Primate Rescue Centre. The first group of primates consisted of the chimpanzees Manuela, Guillermo, Yvan, Yaki, and Toti, while the second group included Gombe, Lulú, Pascual, and Sandy. The first experiment involved placing a large crystal —the monolith— on a platform, along with an ordinary rock of similar size. Although both objects initially attracted the chimpanzees' attention, they soon opted for the crystal and ignored the rock. After pulling it off the platform, all the chimpanzees inspected the crystal, turning and tilting it so they could observe it from different angles. After that, Yvan picked up the crystal and carried it purposefully to their sleeping quarters, where he examined it more closely.

The team observed that interest peaked immediately after exposure and declined very gradually over time. In the case of human beings, the same pattern of behaviour can be observed: as an object ceases to be novel, the attention paid to it diminishes. When the care staff attempted to retrieve the crystal from the chimpanzees' sleeping quarters, the chimpanzees resisted handing it over and it had to be exchanged for their favourite snacks: bananas and yoghurt.

A crystalline preference

A second experiment revealed that the chimpanzees could identify and select, in a matter of seconds, smaller quartz and calcite crystals (some transparent and others not), similar in size to those collected by hominins 780,000 years ago, from a pile of 20 rounded pebbles of different colours and textures. “The initial results were hugely informative and interesting, especially when Yvan chose a quartz crystal from among the pebbles to closely examine its transparency, just as the other chimpanzees did in the light from the window of their sleeping quarters or lying comfortably on their bunk beds; they were fascinated by this property,” explained García-Ruiz.

However, the crystals collected by hominins thousands of years ago and found at various archaeological sites were not transparent. To explore what other properties might have attracted the attention of hominins, the scientific team decided to add greater complexity to the experiment by including three crystals (one calcite, one quartz and one pyrite) of different sizes, brightness and transparency in the various piles of pebbles, but with one property in common: their polyhedral shape. In this case, the chimpanzees were still able to select and separate the crystals from the pebbles. Sandy, for example, gathered up the pile of pebbles and crystals and carried them to a wooden platform where he played with them. “When we were able to climb onto the platform to see what he had done, we saw two distinct groups of stones, one containing rounded pebbles and the other containing the three crystals. We were surprised by his ability to distinguish between them, separate them from the rest of the stones and sort them according to their shape by grouping those that were polyhedral separately from those that were not," as the DIPC research professor described.

Crystals in our minds

The study did not explore whether some chimpanzees were more interested in certain crystals than in others, or whether some of them were more intent on seizing them than others. Aware of this limitation, the scientific team maintained that future studies should take the personality of chimpanzees into consideration. “There are Don Quixotes and Sancho Panzas, idealists and pragmatists. Some people may be fascinated by the transparency of crystals, while others are interested in their smell and whether they are edible,” said García-Ruiz. The chimpanzees participating in the experiment are used to contact with humans and are familiar with objects that are not found in the natural world. So the same research would need to be carried out with less enculturated species, ideally with wild apes.

The combined observations of the experiments concluded that both transparency and shape are attractive properties. It may have been these same qualities that aroused the interest of early hominins in these minerals. The clouds, trees, mountains, animals, and rivers of the natural world surrounding our ancestors were informed by their curvature and branching, so few elements featured straight lines and flat surfaces. Crystals are the only natural polyhedra, that is, the only natural solids that have several flat surfaces. When early humans attempted to understand their environment, their cognitive processes may have been drawn to patterns that were different from the ones they knew.

This work opens up a new avenue for explaining our fascination for crystals and contributes to our understanding of the evolutionary roots of metaphysical and symbolic thinking, as crystals may have acted as catalysts in the conceptualisation of a transcendent ‘beyond’. Their rareness, visual appeal and geometric uniqueness may have given them special significance, serving as physical representations of ideas that transcend the immediate, tangible world. “We now know that crystals have been in our minds for at least six million years," concluded García-Ruiz.