A team of archaeologists recently applied high-tech engineering tests to stone tools, and the results suggest that even very early members of our genus, like Homo habilis, knew how to select rocks with the right combination of sharpness and durability for the work at hand.

Species on the hominin family tree have made and used stone tools for about 2.6 million years that we know of; you could call it a family tradition. At Olduvai Gorge in northern Tanzania, sediment layers dating back to about 1.8 million years ago contain simple stone tools—the handiwork of a small hominin called Homo habilis. That species was an early member of our genus who walked upright and had a mixture of human and ape-like features. Starting around 1.2 million years ago, a later hominin species called Homo erectus made more complex stone tools, like hand-axes.

Think about a stone flake from the oldest layers at Olduvai. That simple tool exists because 1.8 million years ago, a Homo habilis picked out a rock, worked the stone into the right shape, and then used it to do something. Archaeologists can learn a lot about what ancient hominins knew and how they lived by studying the wear and knapping marks on such tools. But the rock itself has a story to tell. Why did a hominin 1.5 million years ago pick this kind of rock, and why this particular chunk of it?

Archaeologists working decades ago at Olduvai noticed that those now-extinct hominins had preferred particular kinds of stone for particular types of tools. For example, quartzite was a favorite for making the small, sharp-edged cutting tools called flakes, while basalt and other lavas seem to turn up more often in the form of large cutting tools like hand-axes. To figure out why, University of Kent anthropologist Alastair Key and his colleagues put Pleistocene-style stone tools through a battery of tests usually used in modern engineering research.

We have always been engineers

Key and his colleagues wanted to know whether H. habilis and H. erectus knew how to choose the most practical materials for specific tools or jobs. To find out, the archaeologists compared the sharpness and durability of Olduvai basalt, chert, and quartzite: three of the most common materials for stone tools in and around the gorge. And that required letting a robot play with some replica stone tools.

An engineering testing machine lowered each sharp stone flake onto a 2mm-wide section of PVC pipe and recorded how much force had to apply to cut through the pipe, and how much the pipe’s surface gave way before splitting. To test durability, Key and his colleagues put the machine to work cutting through oak branches with the replica flakes, then measured how well each blade held its edge between uses.

Quartzite turned out to be the sharpest stone on the list, with chert coming in a close second. Basalt’s sharpness wasn’t terribly impressive; if you want to cut something with a basalt flake, it’s going to take about twice as much energy as using a chert or quartzite flake. So it makes sense that hominins at Olduvai Gorge would pick quartzite or chert for small flakes—quickly made cutting tools that would have been tossed aside after use (they're the plastic sporks of the Pleistocene).

A brand-new basalt biface is a bit duller than a biface made of chert or quartzite, but the basalt tool will hold its edge much better over time. And it seems that H. habilis tool-makers knew that 1.8 million years ago, because dull but sturdy basalt was a more common choice for heavier work or for larger cutting tools, which would be used over and over. Key and his colleagues suggest that’s not a coincidence.

“Although Pleistocene individuals may not have been aware of doing so, a series of mechanical principles routinely applied during the design of modern metal cutting tools were being exploited,” wrote Key and his colleagues. Of course, H. habilis didn’t carefully measure how much force it took to cut a limb off a freshly killed antelope and then draft plans for a more efficient blade. But it seems that they noticed which materials made sharper blades and which ones offered a tradeoff between sharpness and durability, and then applied that knowledge to making tools.

That's no small cognitive feat, especially for such an early member of our family tree. You'd probably recognize H. habilis as a relative, but you'd be unlikely to mistake one for a modern human. They stood about a meter tall, with a mostly ape-like facial structure and long arms still adapted for a life partially spent in the trees. But it turns out they were pretty sharp.

Royal Society Interface, 2020 DOI: 10.1098/rsif.2019.0377 (About DOIs).