Chimp love for crystals may explain shiny stones’ allure to early humans

By Stephen Beech

Chimps’ love for crystals could help explain our ancestors’ fascination with the shiny stones.

The forerunners of modern humans collected crystals for which they had no apparent use, say scientists.

Now they hope a new chimp study will finally unlock the roots of the infatuation.

Crystals have often been found at archaeological sites alongside early human remains.

Evidence shows hominins had been collecting the stones for up to 780,000 years.

But research shows our ancestors didn't use them as weapons, tools, or even jewellery.

Scientists in Spain investigated which characteristics of crystals may have made them so fascinating to our ancestors.

The research team 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.

Study lead author Professor Juan Manuel García-Ruiz, of the Donostia International Physics Center, San Sebastián, said: "We show that enculturated chimpanzees can distinguish crystals from other stones.

"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.”

He explained that modern humans diverged from chimps between six and seven million years ago, so we share substantial genetic and behavioural similarities.

To find out if fascination with crystals is one of them, the research team provided two groups of enculturated chimpanzees - Manuela, Guillermo, Yvan, Yaki, and Toti in Group One and Gombe, Lulú, Pascual, and Sandy in Group Two - with access to crystals.

In the first experiment, a large crystal – called "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 the 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, according to the findings published in the journal Frontiers in Psychology.

The researchers said 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 favoured snacks of bananas and yoghurt.

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 were still able to pick out crystal-type stones.

García-Ruiz said: “The chimpanzees began to study the crystals’ transparency with extreme curiosity, holding them up to eye level and looking through them."

The chimps repeatedly examined the crystals for hours, with Sandy carrying pebbles and crystals in her mouth to a wooden platform where she separated them.

García-Ruiz said: “She separated the three crystal types, which themselves differed in transparency, symmetry, and luster, from all the pebbles.

"This ability to recognise crystals despite their differences amazed us."

He said chimps don't usually use their mouths to carry objects, so it could mean they were hiding them - a behaviour consistent with treating the crystals as valuable.

García-Ruiz says the study didn't examine if some chimps were more interested or laid more claim to crystals than others, although future studies should take chimp personalities into account.

He 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.”

The researchers pointed out that the chimps tested in the study are used to contact with humans and familiar with objects not found in the natural world, so future 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 crystals, according to the research team.

García-Ruiz says 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.

He said crystals are the only natural solids with many flat surfaces, so when early humans tried to make sense of their environment they may have been drawn to patterns that were unlike what they knew.

García-Ruiz added: “Our work helps explain our fascination with crystals and contributes to the understanding of the evolutionary roots of aesthetics and worldview.

“We now know that we’ve had crystals in our minds for at least six million years.”