News release

From: Frontiers

Chimps’ love for crystals could help us understand our own ancestors’ fascination with these stones

Ancestors of modern humans collected crystals for which they had no apparent use. A new chimp study could help researchers understand the roots of this infatuation.

For hundreds of thousands of years, our hominin ancestors collected crystals. Something about these stones made them desirable, even when they weren’t used for anything. But then why collect them at all? To learn more about the roots of this fascination, scientists in Spain ran experiments with individuals from the species most closely related genetically to hominins and humans: chimpanzees. Chimps were drawn to crystals’ transparency and shape and could quickly distinguish them from normal rocks. These findings could show why objects with crystal-like properties have intrigued our ancestors for almost 800,000 years – and continue to pique our interest today.

Crystals have repeatedly been found at archaeological sites alongside Homo remains. Evidence shows hominins have been collecting these stones for as long as 780,000 years. Yet, we know that our ancestors did not use them as weapons, tools, or even jewelry. So why did they collect them at all?

Now, in a new Frontiers in Psychology study, scientists in Spain investigated which characteristics of crystals may have made them so fascinating to our ancestors. They designed experiments with chimpanzees – one of the two great ape species most closely related to modern humans – to identify the physical properties of crystals that may have attracted early hominins.

“We show that enculturated chimpanzees can distinguish crystals from other stones,” said lead author Prof Juan Manuel García-Ruiz, an Ikerbasque Research Professor on crystallography at the Donostia International Physics Center in San Sebastián. “We were pleasantly surprised by how strong and seemingly natural the chimpanzees’ attraction to crystals was. This suggests that sensitivity to such objects may have deep evolutionary roots.”

The Monolith

Modern humans diverged from chimps between six and seven million years ago, so we share substantial genetic and behavioral similarities. To find out if fascination with crystals is one of them, the researchers provided two groups of enculturated chimpanzees (Manuela, Guillermo, Yvan, Yaki, and Toti in group one and Gombe, Lulú, Pascual, and Sandy in group two) from the Rainfer Foundation with access to crystals.

In the first experiment, a large crystal – the monolith – was placed on a platform, along with a normal rock of similar size. While initially both objects caught the chimps’ attention, soon the crystal was preferred and the rock disregarded. Once they had removed it from the platform, all chimps inspected the crystal, rotating and tilting it so they could view it from specific angles. Yvan then picked up the crystal and decisively carried it to the dormitories.

Interest was strongest early after exposure and declined very gradually over time, the team observed. The same pattern is found in humans as the novelty of an objects fades. When caretakers tried to retrieve the crystal from the chimps’ enclosure, they had to exchange it for favored snacks: bananas and yogurt.

A crystal-clear preference

A second experiment showed that the chimps could identify and select smaller quartz crystals – similar in size to those collected by hominids – from a pile of 20 rounded pebbles within seconds. When pyrite and calcite crystals, which have different shapes than quartz crystals, were added to the pile, chimps still were able to pick out crystal-type stones. “The chimpanzees began to study the crystals’ transparency with extreme curiosity, holding them up to eye level and looking through them,” García-Ruiz said. Chimps repeatedly examined the crystals for hours.

Sandy, for example, carried pebbles and crystals in her mouth to a wooden platform where she separated them. “She separated the three crystal types, which themselves differed in transparency, symmetry, and luster, from all the pebbles. This ability to recognize crystals despite their differences amazed us,” García-Ruiz said. Chimps also do not usually use their mouths to carry objects, so this behavior could mean they were hiding them a behavior consistent with treating the crystals as valuable, the team pointed out.

Crystals in our minds

The study did not examine if some chimps were more interested or laid more claim to crystals than others, although future studies should take chimp personalities into account, the team said. “There are Don Quixotes and Sanchos: idealists and pragmatists. Some may find the transparency of crystals fascinating, while others are interested in their smell and whether they’re edible,” García-Ruiz pointed out. The chimps tested here also are used to contact with humans and familiar with objects not found in the natural world. Therefore, the same experiments should be carried out with less enculturated species, ideally wild apes.

The combined observations from the experiments identified both transparency and shape as alluring properties. It might have been the same qualities attracting early humans to these rocks. The clouds, trees, mountains, animals, and rivers of the natural world surrounding our ancestors were defined by curvature and ramification, so few items had straight lines and flat surfaces. Crystals are the only natural polyhedral, meaning the only natural solids with many flat surfaces. When early humans tried to make sense of their environment, their cognitive processes might have been drawn to patterns that were unlike what they knew.

“Our work helps explain our fascination with crystals and contributes to the understanding of the evolutionary roots of aesthetics and worldview,” concluded García-Ruiz. “We now know that we’ve had crystals in our minds for at least six million years.”