Like many people, chimpanzees have an odd affinity with crystals.

Researchers have shown that chimpanzees regularly pick crystals over regular pebbles, carefully examining both when they are within easy reach. This attraction is akin to the behavior of our predecessors who gathered similar stones long before they were understood to be useful.

A 14-inch clear crystal and a regular rock of comparable size were placed on identical pedestals close to Madrid. Professor Juan Manuel García-Ruiz of the Donostia International Physics Center (DIPC) saw how the chimps immediately abandoned the rock and concentrated on the crystal.

Many people lifted the crystal, turned it in their hands, and carried it into their sleeping quarters long after the initial inspection. It is unclear exactly which aspects of the crystal attracted the chimpanzees' attention because they appeared to be drawn to some aspects of it.

Hours of meticulous examination

The researchers observed extended periods of looking and grip shifts, and close handling was just as important as selection. Light can penetrate through a crystal held at eye level, making it simpler to compare sharp faces and interior lines.

According to García-Ruiz, "the chimpanzees started studying the transparency of the crystals with extreme curiosity, holding them up to eye level and looking through them." The crystal became more than a toy after hours of silent examination, and it established a pattern that warranted further investigation.

Chimpanzees sort crystals fast.

Subsequently, circular pebbles that appeared dull and unremarkable were mixed with tiny crystals that had entered the yard. Even when multiple crystal patterns presented simultaneously, chimpanzees were able to extract crystal fragments from a pile of twenty in a matter of seconds.

Sandy, a chimpanzee, transported crystals and pebbles in her mouth to a wooden platform where she divided them. According to García-Ruiz, "she separated the three crystal types from all the pebbles, which themselves differed in transparency, symmetry, and luster."

Such rapid sorting revealed that the chimp was following a rule rather than making a haphazard grab for bright things.

Connections to human conduct

As their decisions were monitored over time, the DIPC team discovered that transparency and form directed the chimps' attention in both trials. Clean edges and reflections are produced when light is allowed to flow through a crystal; these cues promote careful examination and hand rotation.

In woodlands and savannas, where the majority of natural objects curve or branch, flat faces and straight lines also stand out. The chimpanzees' preference, when understood as a reaction to surfaces and transparency, is similar to earlier human behavior without implying the same motivations.

Why crystals are unique

A crystal can develop with repeated faces, which gives it a neat geometry that other rocks don't exhibit. This shape, which has multiple flat sides and sharp edges that naturally form without human molding, is referred to by scientists as polyhedral.

Crisp angles were strange in early human settings since trees, clouds, and animal bodies provided curves and branches. Crystals may have influenced how minds learned to classify and compare forms if they focused attention toward straight patterns.

Crystals and early humans

Crystals were nevertheless carried home by humans long before jewelry, and they were stored in shelters with tools, ash, and bones. Across sites spanning 780,000 years, archaeologists identify those collectors as hominins, our close human cousins in the ape family.

A 2021 study detailed crystals found 105,000 years ago at a rockshelter in the Kalahari region of South Africa. The crystals appear to be selected objects rather than cave detritus because the closest known source was located far from the rockshelter.

The endeavor was not motivated by practical work because there were no drilling holes or scrape marks to explain why people saved crystals.

Chimpanzees that are impacted by people

Unlike wild apes, chimpanzees raised by humans can be enculturated—that is, shaped by daily interactions with people and objects. Even after the novelty wore off, nine adults in two groups continued to carry the crystal and chew lesser stones.

According to García-Ruiz, "some people may find the transparency of crystals fascinating, while others are interested in their smell and whether they're edible." Because the same crystal cues may compete with food, competitors, and predators in natural life, wild tests will be important.

Crystals and wild chimpanzees

Instead of considering groups as a single entity, future work at DIPC can monitor which individuals guard, share, or abandon crystals. Testing gorillas and bonobos may reveal whether crystal curiosity is unique to chimpanzee living or is shared by all big apes.

It would also be possible to distinguish between a deeper inclination toward straight edges and acquired curiosity by comparing wild and sanctuary groups. Crystals appear to be objects that draw attention even when they are doing nothing, as shown in chimpanzees and ancient sites.

Improved data from wild primates may reveal whether that pull is a human-trained habit or a reflection of common impulses.