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Some stone tools attributed to prehistoric humans may in fact have been made by horses, according to researchers in Spain. They aren鈥檛 claiming that horses make tools deliberately, but as an accidental by-product of trimming their hooves on rocks. The discovery means that archaeologists will have to be more careful about declaring objects to be ancient human-created artefacts.

Stone tools are common in the archaeological record from about 2.6 million years ago onwards, usually consisting of small heaps of sharp-edged flakes and the cores they were chipped off. Until recently, archaeologists thought they could reliably distinguish human-made flakes and cores from naturally broken stones by tell-tale fracture patterns. But now they aren鈥檛 so sure.

In 2016, researchers at the University of Oxford discovered that . The monkeys aren鈥檛 deliberately making tools, but are thought to break rocks open to obtain a nutrient, possibly silicon, that they lick off the freshly exposed surface. The resulting accumulation of discarded stone flakes and source rocks can look exactly like a human tool factory.

Now the 鈥渦nintentional toolmaker鈥� club has been joined by horses and their relatives, collectively called equids. According to 聽and his colleagues聽at the University of Alcal谩鈥檚 Institute of Evolution in Africa (IDEA) in Madrid, equid hooves grow rapidly and need to be trimmed, which the animals do by kicking and stamping on rocks. This can produce fractured pieces that also look exactly like toolmaking debris.

Equid rock-smashing was already known from observations of semi-feral ponies. The latest evidence comes from an experiment on a farm near Cuenca involving three donkeys, a horse and 14 lumps of flint and quartzite, which Dom铆nguez-Solera聽put into their enclosure. The farmer reported seeing the animals kicking and stamping on the rocks and after 52 days Dom铆nguez-Solera collected the debris. Much of it strongly resembled flakes and cores created by human flint knapping, he says. 鈥淔or us it was a surprise that donkeys can make flakes like human ones,鈥� says Dom铆nguez-Solera.

Capuchin monkeys only live in the Americas, so present little obstacle to assigning flakes to early humans. But equids lived alongside our ancestors in Africa and Eurasia for millions of years, so their 鈥渢ools鈥� throw a much bigger spanner into the works. 鈥淲e say 鈥榖e careful鈥� 鈥� this flake could be made by an [equid],鈥� says Dom铆nguez-Solera.

According to , an independent archaeologist based in Berlin, Germany, the study is interesting but doesn鈥檛 suggest that all tool-like objects were actually made by horses. 鈥淚 wouldn鈥檛 go so far,鈥� he says. 鈥淚t is a fascinating study, but I wouldn鈥檛 change textbooks yet.鈥�

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