Stone artifacts painstakingly shaped into spheres were part of the daily lives of early humans for more than two million years. They have been unearthed by archaeologists in East Africa, humanity’s ancestral home, and they litter prehistoric sites across Eurasia from the Middle East to China and India. Yet experts have been puzzled by their function since the early days of research into our evolutionary history.

Now, an international team of archaeologists led by Tel Aviv University archaeologist researcher Ella Assaf, has produced evidence that these enigmatic artifacts were used for a very specific purpose: breaking the bones of large animals to extract the nutritious marrow inside.

The study, published last week in the journal PLOS ONE, highlights how an elegant technological solution that allowed hominins to increase their calorie intake endured for hundreds of thousands of years and continued to be used even as our ancestors developed new techniques and created more complex societies.

The researchers analyzed shaped stone balls, also called spheroids, found in Qesem Cave, a prehistoric site just east of the modern city of Tel Aviv that was inhabited from 400,000 to 200,000 years ago. The discovery of around 30 of these artifacts in this particular cave was a puzzle wrapped in an enigma for archaeologists. Not only did the function of the spheres remain obscure, but their presence there was considered anachronistic, because these artifacts are usually found at much older sites.

Mastery of fire

Qesem Cave was uncovered during road works in the year 2000. Since then, excavations led by Tel Aviv University archaeologists Avi Gopher and Ran Barkai has uncovered a treasure trove of hundreds of thousands of flint tools and animal bones as well as 13 hominin teeth, belonging to the as-yet-unidentified group that lived at the site.

Whoever they were, these distant ancestors of ours were relatively ahead of their time in much of the behavior they displayed, experts say. The people of Qesem Cave (whose modern name somewhat appropriately means “magic” in Hebrew) were among the first hominins to master controlled fire to cook meat, and they learned how to preserve food for a rainy day.

Open gallery view An archaeologist uses a reproduction of a shaped ball to crack open an animal bone Credit: Ella Assaf

The locals were also capable of producing sophisticated stone tools and pass on their knowledge to the next generation by schooling children in the art of flint knapping.

Initially, archaeologists were a bit surprised by the presence at Qesem of stone balls, which are generally associated with an earlier chapter in our evolution, explains Assaf.

These spherical objects first appear in Africa at sites that are nearly 2.6 million years old, often in association with Homo erectus. They were found, among others, at excavations in Tanzania’s Olduvai Gorge by Mary Leakey, the renowned British archaeologist.

In a 1971 book, Leakey suggested these artifacts may have been used as primitive bolas to hunt animals, while other researchers have speculated they may have served as projectiles, hammer stones or grinding tools.

In the Middle East, the spheres appear at sites dated to between 1.4 million and 500,000 years ago. So by the time hominins first entered Qesem Cave, these artifacts had fallen out fashion by at least 100,000 years in this region.

Open gallery view A view of Qesem Cave, a prehistoric site near Tel Aviv Credit: Ariel David

It turns out that their presence was linked to another behavior that researchers have spotlighted at Qesem: recycling. The residents of the cave, as well as other prehistoric populations, were very dedicated to collecting, retouching and reusing old tools, possibly made by even earlier groups of hominins.

“In Qesem we see a regular pattern of collecting stuff from outside the cave and reusing it,” Assaf says. In other words, the stone balls were not made at Qesem: they were spotted at nearby, likely much older prehistoric sites – of which there are several known to archaeologists in the area – and brought back to the cave. We know this because the stone balls are made of dolomite or limestone of a kind not present in the immediate vicinity of the cave, Assaf says.

The artifacts also have a patina, a nacreous layer that forms on objects as a result of chemical reactions when they are exposed to the elements, that is different from that of other tools that were found in the cave. This means that the balls were exposed to a different environment for a very long time before being brought into Qesem.

Fragile balls

So why did these hominins visit ancient sites and carry back home stone balls that weighed up to a kilogram each? Were they perhaps attracted by the craftsmanship and the symmetric beauty of the spherical shapes?

While previous research by Assaf has suggested that the people of Qesem liked to collect shiny, colorful pebbles purely for their aesthetic value, this is not the case for the spheres, the researchers conclude.

In the PLOS ONE study, microscopic analysis of the organic residues and the signs of wear on the Qesem spheres was conducted by Isabella Caricola and Emanuela Cristiani, of Rome’s La Sapienza University.

Open gallery view Prehistoric shaped stone balls found at Qesem Cave, near Tel Aviv Credit: Pavel Shrago

The artifacts are not perfect spheres, and their creators intentionally maintained some rough ridges. It was around these wide angles that the signs of wear on the stones were concentrated, along with residues of fat, collagen and bone. This suggested that the stones were used to crack open large bones (such as from elephants) and extract the marrow, Assaf says.

To verify that hypothesis, Javier Baena of Madrid’s Unversidad Autonoma produced modern versions of the stone balls and Jordi Rosell, of the The Catalan Institute of Human Paleoecology and Social Evolution in Tarragona, Spain, tested them by breaking open modern animal bones. The residues and the signs of wear on the reproductions matched those on the original spheres, the team found.

The experiment also highlighted why the people of Qesem would recycle used spheres rather than make their own, Assaf says.

“Javier can knap with his eyes closed, but he still struggled. It’s very difficult to make such objects,” she says. “One little mistake and the sphere can break in half, or you can keep fixing the ridges and end up with a very tiny, useless ball,” the archaeologist says.

The ridges on the tool were all-important features because they made it more accurate, opening the bones with a clean break and without mashing the precious spongy tissue inside.

Open gallery view A broken half-sphere from a prehistoric shaped stone ball from Qesem Cave Credit: Sasha Flit

No need to reinvent the wheel

Assaf does not rule out that the symmetry of the spheres was considered aesthetically pleasing by those wielding them, or that collecting old tools may have also been a display of respect for the distant ancestors who created them. Most likely, the interest in these artifacts mixed form and function.

“The people of Qesem used advanced and innovative techniques, and had a very wide tool kit, but sometimes having knowledge and skill means picking up something old and reusing it because it’s still useful,” the archaeologist adds. “Being smart also means acknowledging that those who came before you were smart too: you don’t have to reinvent the wheel every time.”

Being composed mainly of fat, bone marrow was an important source of calories for prehistoric populations, and could easily be preserved inside the bone for times when food was scarce, which, as previous research has shown, the Qesem people probably knew how to do.

“The spheroids phenomenon is a big puzzle that we don’t understand, and there has been very little research on their function,” says Ofer Marder, an archaeologist from Ben-Gurion University in Be’er Sheva and an expert on prehistoric tools. “Combining the analysis of residue and wear with experimental archaeology is a breakthrough in determining the connection between these tools and their use.”

We also cannot rule out that the spheroids may have been used for other purposes as well, such as processing plant material, notes Marder, who did not take part in the study.

“Most prehistoric tools were more like a Swiss Army knife and didn’t have a single function,” he says. “More research is needed in order to understand if the tool had other, possibly more complex, functions and whether the conclusions of this study can be applied to spheroids found elsewhere.”

The evidence that the stone spheres functioned as marrow-extractors is, strictly speaking, only applicable to their use in Qesem, Assaf agrees. In other words, there is no direct proof that their original makers intended them for the same purpose, or that the other spheres found across Africa and Eurasia were used thusly.

“We cannot be sure yet, but my assumption is that this was always their main function, because they are so efficient when you use them for this specific purpose,” Assaf says, adding that she is already planning to test that hypothesis on other stone balls from different sites.