This 1.4 million-year-old hand axe was chipped off a hippo femur

Hand axes are fairly common finds at sites dating between 2 million and 1 million years old. These sturdy tools have two sides (also called faces) and a sharp edge at one end. But hand axes are usually made of stone, so archaeologists working at the Konso Formation in southern Ethiopia were surprised to find a hand axe worked from a large chunk of bone buried in a 1.4 million-year-old layer of sediment. When Tohoku University archaeologist Katsuhiro Sano and his colleagues compared the bone to a collection of bone samples from large mammals, they found that their ancient hand axe had once been part of a hippopotamus femur (thigh bone).

From hippopotamus to hand axe

The Konso find is only the second bone hand axe archaeologists have ever found, and one of just a handful of bone tools from sites older than 1 million years. Based on fossils found at Konso, the hominin who flaked off a chunk of hippo femur and worked it into a nice, sharp hand axe was probably a Homo erectus. Members of the species walked upright and were built a lot like modern humans, and they eventually spread from Africa, across Europe and Asia, and all the way to modern Indonesia.

At least one member of this species left behind a 13cm-long hand axe that is, according to Sano and his colleagues, an excellent piece of craftsmanship. The toolmaker apparently flaked a large, flattish piece of bone off the side of a hippo femur; you can still see the outer surface of the bone on one side of the hand axe. That fits the standard Acheulean approach to making hand axes and other tools; the first step is to make a large “blank” in the right general shape, then gradually flake off smaller pieces to shape the finished product.

It’s a relatively advanced technique to begin with, compared to some earlier stone toolmaking styles, because it requires planning and also really good control over what breaks (and how it happens) when you hit one piece of stone with another, so that you knock off a flake in the size and shape you want. That kind of control is even harder to accomplish with bone than with stone, and it’s also harder to find bone big enough to make the right size blanks. It’s not for nothing that the only other Acheulean bone hand axe ever found—a 1.3 million to 1.6 million-year-old tool from Olduvai Gorge in Tanzania—was shaped from a piece of elephant bone.

No bones about it—early hominins were pretty sharp

“Finely-shaped bone tools like bone hand axes are extremely rare,” wrote Sano and his colleagues. But the toolmakers at Konso knew what they were about.

The hominin who made the hippo bone hand axe chipped the large bone “blank” into the right general shape, then chipped a series of tiny flakes off one end to make a sharp edge. By alternating those flakes between one face and the other, the toolmaker made a remarkably straight 5cm-long cutting edge at the working end of the hand axe.

“This bone handaxe shows that at Konso, […] H. erectus individuals were sufficiently skilled to make and use a durable cutting edge,” wrote Sano and his colleagues. That lines up pretty well with other evidence that H. habilis and H. erectus understood material properties like sharpness and durability well enough to choose the right material for the right job. Our early hominin cousins were already very resourceful, very competent, and very intelligent.

In fact, that earlier study about material properties raises the question of why this particular hominin chose to make a hand axe out of hippo bone, of all things. There would have been plenty of stone available in the area. Actually, Sano and his colleagues suggest that the abundance of workable stone around Konso may have helped Acheulean toolmakers there hone their craft, since they had plenty of material to work with.

Maybe they decided to take advantage of a hippo femur that just happened to be available at the site, or maybe they faced a temporary shortage of stone; volcanic activity in east Africa sometimes changed hominins’ access to stone deposits for centuries at a time. And 1.4 million years ago, the area around Konso would have been a patchwork of wetlands, woods, and grasslands surrounding a large lake, so hippos could have been available to brave-enough hunters.

Or perhaps the ancient toolmaker decided bone was actually the best material for the task at hand.

A handy gadget

The hand axe offers some clues about what that task may have been. Its 5cm working edge bears some signs of wear from use. The edge is rounded in places near the tip, and under the microscope, Sano and his colleagues noticed patterns of striations and polished patches that look similar to the patterns seen on stone tools used for butchering animals—work that involves a lot of cutting and sawing motions. Since that’s what most archaeologists believe Acheulean hand axes were made for, the explanation makes sense.

Interestingly, one side of the edge seems a bit more worn than the other. That may suggest something about how the tool was used or about the dominant hand of the user.

Because bone tools are so rare in the archaeological record from the period just before 1 million years ago, it’s hard to say exactly how significant this find is or what it tells us. But it’s a relic of hominin technology from a very interesting time: an early phase of the Acheulean style, when toolmaking techniques were rapidly being refined.

“At Konso, this is a time period when significant technological developments in lithic technology were occurring,” wrote the authors. It would have been an interesting time to be a tech reviewer (but also very frustrating, since you couldn’t write anything down).

PNAS, 2020 DOI: 10.1073/pnas.2006370117  (About DOIs).