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How Do We Know When A Hunk Of Rock Is Actually A Stone Tool?

The first time archaeologist John Shea looked at what might be the oldest stone tools ever found, he almost blew them off. “Are you kidding me?” he remembers asking Sonia Harmand, his colleague at Stony Brook University who found the tools in 2011 along the shores of Kenya’s Lake Turkana, at a site now known as Lomekwi 3. Harmand’s analysis suggested that the tools were 3.3 million years old — 700,000 years older than the previously known “oldest” tools. And they were huge. The mean weight of the pointy flakes — the cutting tools that are “flaked” off larger rock cores — was 2 pounds. In comparison, the next-largest group of ancient hominin tool flakes have a mean weight of 0.06 pounds. It seemed like a lot for the hands of a primate that was probably half the size of the average modern human. Maybe, this time, a rock was just a rock. But then Shea looked at the Lomekwi tools more closely, and he saw multiple fractures, all running in the same direction — a telltale sign that the flakes weren’t just the lucky product of one rock bumping into another as it tumbled off a cliff or rolled through a stream. Something had pounded one rock against another in the same place over and over and over … until a sharp piece broke off.

The archaeologists who study lithic technologies — “stone tools” to us lay primates — are used to making these kinds of determinations. For generations they’ve been sorting out what is a tool, what isn’t, and what those tools mean, and they’ve been doing it largely by sight, by context of the objects and the site where they were found, and by experience. They learn the signs. They use dating techniques and look for evidence of how tools were being used. They test hypotheses by making their own stone tools and using them to see, for instance, how an ax might wear down after it’s hacked through a few deer femurs. “But here’s the problem,” Shea said. “Archaeologists don’t have measurements to answer these questions. The decision rests on visual assessment. I’ve made stone tools for 45 years, started as an 11-year-old Boy Scout. So is this an artifact or not? Truthfully, I have no idea. I can’t tell you based on measurements, and my opinion shouldn’t count for science.”

Archaeological analysis always involves some amount of informed interpretation, and not all archaeologists are this skeptical of the traditional methodology. Harmand, for one, thinks that visual assessment is crucial. But only during the past decade or so has there been any real alternative. Called geometric morphometrics, it’s a suite of digital tools that can turn one of Lomekwi’s 4-inch-wide flakes of volcanic basalt into a 2-D or 3-D map of its curves, cracks, shape and size. These maps could be put into databases that could, eventually, allow scientists to compare the minute variations among tools from different periods and different locations. This technology is still in the early stages of adoption. Many tools have been scanned; far more have not. It’s primarily used to give archaeologists around the world access to artifacts they would otherwise have to travel long distances to see. People such as Shea hope that it will, one day, allow archaeologists to more definitively answer questions that today are addressed using circumstantial evidence and logical conjecture.

The central issue is not necessarily whether scientists can tell a tool from a naturally broken rock. The more complex question is how to tell the difference between tools intentionally created (probably by humans or our ancient ancestors) and those that another primate made by accident. The difference matters. Once upon a time, we considered tool use the factor that separated animals and humans. As scientists discovered tool use among more and more animals — including chimpanzees and octopuses — the goal post shifted. Today, as far as anyone yet knows, humans and our ancestors are the only animals that create sharp-edged tools specifically for cutting things. Humans have taught captive bonobos to make stone cutting tools,1 but we have no evidence of any living primates intentionally doing this in the wild.

But the line is awfully fuzzy. Especially when you’re looking at really ancient tools such as those at Lomekwi. Somebody somewhere had to invent stone tool making. And “probably the first tools ever made by a hominin were made by accident,” Harmand said. Imagine your less-upright ancestor going to town on a nut with a rock, trying to get at the good stuff inside. “Of course, you will sometimes hit the stone with your other stone and then detach something,” she said. And although hominins seem to be the only primate to think, “Hey, I’ll take that bit of stone and go butcher me a hog,” there’s evidence that other primates are still accidentally making “tools” — and that those tools are very difficult to distinguish from the ones archaeologists attribute to our predecessors.

Last month, Oxford archaeologist Tomos Proffitt published a paper that documents wild capuchin monkeys in Brazil cheerfully screwing with a whole scientific field by intentionally picking up rocks and banging them against other rocks over and over and over in the same place … until sharp pieces broke off.2 There’s no evidence that the monkeys were trying to make tools, and they generally ignored the flakes. Their behavior could be an aggressive display or possibly a means of getting at lichens or trace minerals in quartz dust. Either way, the accidental “tools” they produce from intentional rock-banging are virtually indistinguishable from what you might find in 3-million-year-old East African soil. “If you brought this to me [and said it came from] East Africa, I would have told you, ‘You’ve got a new Oldowan. Where’d you find it?’” Harmand said. “I’m convinced many of my colleagues would confirm.”

The context in which the tools or “tools” are found is really the only way to tell the difference. Where did they come from? How old are they? Were the sharp flakes being used for anything, like butchering, which often leaves cut marks on bones nearby?3

Geometric morphometrics could shed some light on this. For instance, what would Shea do about the capuchin monkey “tools” and their similarity to ancient hominin tools? “One of the claims is that these objects are indistinguishable from artifacts manufactured by humans. Well, let’s see. You could scan them, the ones we’ve watched be modified by capuchins. Then scan early human stone tools, and compare them. Do they differ in specific ways, and is that statistically significant?” he said. “We can argue about what it looks like, or we can measure it.”

Proffitt, the scientist who wrote the capuchin paper, is one of the archaeologists who have used geometric morphometrics for research. One paper he worked on investigated whether different methods of making stone tools might leave different signatures in the rock. The freehand technique is similar to what the capuchin monkeys do: hold a rock in each hand and mash them together. In what’s called bipolar knapping, on the other hand, you put your stone tool core on a stationary anvil rock on the ground and then hit the core with another rock. That may not seem like a lot of difference, but, theoretically, bipolar knapping gives you more control. “It speaks to intelligence and cognitive processes going on,” Proffitt said. “If you have only freehand knapping for a million years, and then you have bipolar knapping, you [now] have more varied behavior.”

The team digitized hundreds of flakes and examined their 2-D outlines — and found no statistically significant differences between the ones made with the freehand technique and those made with bipolar knapping. It’s possible that a 3-D model would show something more. But this paper highlights that you need more than just the tool — whether you’re eyeballing it or digitizing it — to know how it was made. That’s not a totally new conclusion, but it’s important, because it helps to clarify what morphometrics can and can’t do.

Some of the most detailed work in morphometrics has been with younger North American tools. Here, the questions are less about whether something is a tool — with 12,000-year-old artifacts, their human-madeness is obvious — but, rather, what differences in the styles of tools can tell us about the people who made them.

No two tools are identical, said Andy White, an archaeologist at the University of South Carolina. There wasn’t an Old Spear-Getting Factory churning out duplicate objects. But what we don’t know is how much of the difference between them can be accounted for by the vagaries of artisan work and how much represents real, cultural distinctions between groups of craftspeople. “You can take measurements in a computer that I can’t imagine how you’d ever take them with a pair of calipers. [You can] fit curves to things. Quantify things,” he said. “[I use] computer modeling and simulation to try and give myself some kind of credible avenue to interpretation.” White is compiling a database of projectile points, dating from 8,800 to 6,600 B.C., from across what is now the eastern United States.

Ultimately, though, Shea thinks geometric morphometrics hasn’t really had its first major test yet. It won’t really become widely used or relied upon for interpretation until it answers some kind of flashpoint, high-stakes question in archaeology, he said. And that could be Lomekwi. Some are still skeptical of whether those are actual, intentional tools. If there were another site from around the same time with tools that were either more clearly hominin-made or more clearly monkey accident, then digitization might be able to reveal, for certain, where on that spectrum Lomekwi falls. “They have this one site,” Shea said. “Somebody needs to find something almost, or slightly, older, and they need to go at each other.”


  1. Remember to thank the University of Haifa when the bonobo overlords destroy us all.

  2. If hominins were doing this, archaeologists would call it an example of the “passive hammer technique.”

  3. The Lomekwi site had bones. There were no cut marks on them. That fact contributes to major ambiguity for those tools.

Maggie Koerth was a senior reporter for FiveThirtyEight.