When ancient humans first invented stone tools, they may have been trying to emulate naturally formed sharp stones, meaning they would not have needed a huge leap of inspiration.
âWhat our hypothesis does is, it really turns the origin of technology entirely on its head,â says at Kent State University in Ohio. Instead of imagining a sharp tool and then figuring out how to make it, early hominins may have used for millennia before anyone tried crafting them. âRather than hominins creating the knife and then looking for something to cut, we propose that they were already exploiting carcasses,â he says.
One of the defining features of hominins is their ability to both make and use stone tools, which are useful for butchering animals and opening hard fruits. Creating a stone tool requires hitting two rocks together in precise ways, knocking flakes off one of them in order to shape it into a cutting edge.
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This is called âknappingâ, and hominins have been doing it for at least 2.6 million years. There are even older stone tools from Lomekwi in Kenya, , but these were made using a simpler method: bashing a single stone on the ground.
âItâs been traditionally thought that the very first stone flakes were produced intentionally or by accident, and then early hominins started to look for things to cut with these new sharp implements,â says Eren. He says this story doesnât make sense. âFor a creature to start to use an item, or to invent an item, there has to be a selective pressure first.â
Eren and his colleagues argue that hominins found naturally sharp stones, which they used as cutting tools. By doing so, they developed a habit of cutting and began seeking out such stones.
âMother Nature is producing knives all over the place,â says Eren. He calls these raw blades ânaturalithsâ.
The team has compiled multiple examples of naturaliths. Eren has studied stones from Antarctica, which resemble hominin tools but must have been made by natural processes, since no hominin ever lived in Antarctica. Experiments have also shown that tool-like artefacts can be produced when large animals like and horses trample on stones. Monkeys sometimes accidentally knock flakes off stones. There are also processes that donât involve living animals, such as waves crashing on rocky shores, frost fracture and glaciers grinding over bedrock.
If naturaliths were available in homininsâ habitat, says Eren, it would be easy to start using them. âAll they need to do is pick them up.â
For Eren, the appeal of this hypothesis is that it doesnât require a âeureka momentâ of inspiration. âIt shortens the cognitive distance between every step in the origin of technology,â he says. He calls it âthe most parsimonious proposalâ for how hominins invented stone cutting tools.
However, Eren emphasises that this hypothesis is a suggestion, not a fact. He calls it âa big swingâ. While the team has listed examples of naturaliths and processes that could make them, many other tests are needed. Eren wants to look for naturaliths from times and places where hominins lived, and find evidence that they were used as cutting tools. That could include distinctive wear patterns or traces of plant and animal material on the sharp edges. It might also be possible to find naturaliths that hominins have transported over long distances.
Such evidence might not be forthcoming. âSimply because the hypothesis is parsimonious doesnât mean itâs correct,â says Eren.
âI think itâs a really intriguing proposal,â says at the Smithsonian Institution in Washington DC. âMaybe the invention of stone tools wasnât this major cognitive leap,â she says, but instead a ânatural extension of what hominins were already doingâ. She also says that the biggest challenge will be testing the idea, in particular figuring out whether an apparent tool was made by a hominin, an animal or a non-biological process.
Identifying the sources of naturaliths is useful, says at the University of TĂźbingen in Germany. However, the researchersâ âspecific story just doesnât check outâ.
Tennie says the proposal relies on an assumption: that inventing stone tools from scratch is too cognitively difficult for hominins to have pulled off. âThatâs not true,â he says. Instead, several lines of evidence suggest that hominins were smart and creative enough to come up with knapping unaided.
For instance, in a 1994 study Gregory Westergaard and Stephen Suomi reported that and modified the stones by striking them against hard surfaces. If capuchins could make and use stone tools, presumably hominins could too. In a series of studies published in 2022, Tennieâs team showed that  without training; that , a prerequisite for knapping; and that without help.
âThese beings had available certain strategies, certain intelligences,â says Tennie. He thinks hominins didnât need a helping hand from naturaliths.
Eren thinks they did. He argues that those experiments may not tell us much about early hominins because several of them used captive primates, and modern humans are not the same as hominins that lived millions of years ago. âEven if hominins were clever enough to spontaneously invent stone tools at will, that does not negate our hypothesis that Mother Nature helped them along,â he says.
Archaeometry
Embark on a captivating journey through time as you explore key Neanderthal and Upper Palaeolithic sites of southern France, from Bordeaux to Montpellier, with New ĐÓ°ÉÔ´´âs Kate Douglas. 
Neanderthals, ancient humans and cave art: France
