Were Stone Age humans the first geologists?

Humans living in what is now South Africa 70,000 years ago possessed a surprisingly sophisticated understanding of the various kinds of rock that made up their world, indicates a new study published in the Proceedings of the National Academy of Sciences of the United States of America.

Not only did hunter-gatherers of the time know how to make finely crafted stone tools, but they understood exactly which rocks would yield the best combinations of ease of shaping and wear resistance for the task at hand, said Patrick Schmidt, an archaeologist at the University of Tübingen in Germany and study coauthor.

“If you want to make something out of something, you choose the right material for it,” Schmidt said. “Those processes were already in place in the Middle Stone Age in South Africa.”

Schmidt and his colleagues looked at various physical properties to quantitatively compare different types of rocks used by early humans.

The approach may open new avenues of inquiry for scientists hoping to assess the capabilities of our Stone Age ancestors, said Alex Mackay, an archaeologist at the University of Wollongong in Australia who wasn’t affiliated with the research. “You can unpack a whole lot more complexity and decision making,” he said. “Even from something as simple as rock choice you’re seeing dimensions in human thought that we just weren’t getting at before.”

Stone Age Geology

You may look at a rock and see a hunk of stone. Ancient people looked at a rock and saw a world of possibility. Rocks could be knapped, or shaped, into knife blades, spear points, ax heads, and more, allowing hunter-gatherers to take on new prey and use animal remains for clothing and other things.

Rocks give archaeologists one of our only glimpses into culture in the Stone Age, a time when “modern humans start becoming really interesting,” Mackay said.

One long-standing question among archaeologists is what drove people to use different kinds of rocks for their tools. Were they simply using whatever stones were at hand? Or were they making more specific choices about what to use?

To begin to answer these unknowns, Schmidt and his colleagues created a mathematical formula to describe rock properties, including strength, fracture toughness, and fracture variability, among different kinds of rocks to show how much force is needed to flake them.

They applied this flaking force calculation to four different kinds of rock found at the Diepkloof site in South Africa, which was inhabited by Stone Age humans for tens of thousands of years beginning about 130,000 years ago. Some samples were collected from the site itself, whereas others were taken from locations tens of kilometers away. The researchers compared their data on rock properties to existing stone tools from Diepkloof to learn more about how ancient humans selected rocks for toolmaking.

Inhabitants of the shelter fashioned various kinds of rocks, including quartz, quartzite, hornfels, and silcrete. After testing samples of each of these rocks, clear differences emerged, Schmidt said. Some rocks took more than 3 times as much force to fracture than others.

“So there are really big differences. Some of those rocks, you have to hit them really hard to make a flake. Some of those rocks, you just have to tap them lightly,” he said.

Fracture strength wasn’t the only factor important to ancient humans. How predictably a rock flakes and how strong the resulting tool would be also mattered. Looking at their data, Schmidt said they saw distinct patterns emerge at different points in time.

Members of the Still Bay technocomplex, who were making stone tools at the site around 71,000 years ago, seemed to prefer quartzite for their tools. The metamorphic rock is relatively hard and fractures predictably, meaning it’s possible to make finely crafted, durable tools from it. Still Bay points tend to be long, double-edged points likely used as spear tips and sometimes for cutting. Crucially, Schmidt said, the quartzite used to make these tips fractures easily in small volumes—like when you are knapping the edge of a flake to make a tip—but is very strong when force is applied lengthwise to a larger volume, such as when you stab something with a spear.

Later humans inhabiting the site around 65,000 years ago preferred silcrete. The rock fractures more easily, which Schmidt said makes sense given the types of tools those people were making. Members of the Howiesons Poort technocomplex made many small blades for arrowheads and spearpoints that may have been meant to break off inside prey, making them easier to hunt down.

“So you don’t care about the resistance to fracture, because the thing is supposed to break off anyway,” he said. “What you rather want is something that is really making it easy for you to make those segments.”

Showing that changes in the kinds of tools line up with the qualities of the rocks they were made from adds quantitative heft to hypotheses that humans were making nuanced decisions involving stone properties more than 60,000 years ago, Schmidt argued.

“They were testing materials and trying to understand the trade-offs that come from different materials,” he said.

Refining Calculations

It may be too soon to make those kinds of statements about early human tool use, however, said Marina de Araújo Igreja, an archaeologist who specializes in use-wear analysis of ancient stone tools who was not involved in the study.

She said the researchers’ analysis adds promising data, but knapping the rocks they sampled and testing them as tools would help to paint a fuller picture of how ancient humans used these rocks. Studies that re-create stone tools and use them for activities like cutting can help reveal how particular stones performed. That kind of practical testing is an important component of understanding ancient human toolmakers, de Araújo Igreja argued.

“It’s only when you hold a stone tool that you understand the dynamic of the use,” she said.

In addition, there can be a broad variation in rock properties even within rock types such as quartzite and silcrete, she said. Mackay agreed, saying future studies should take into account differences among rocks of the same category taken from different places. He added that Schmidt and colleagues’ formula may help with this. Archaeologists have often assumed that rocks of one type are all the same. With a new way to quantify the properties of stone tools, they can now begin to look more critically within those categories to tease out new insights.

“I see this as a really important step forward…in the capabilities it gives us to access human decision making in the past,” Mackay said.

This article was originally published in Eos. Read the original below.

Nathaniel Scharping (@nathanielscharp), Science Writer

Citation: Scharping, N. (2024), Stone Age humans chose their rocks with care, Eos, 105, https://doi.org/10.1029/2024EO240124. Published on 19 March cc 2024.