Modern chimpanzees select rock tools in similar ways to Oldowan hominins, early humans who used stone tools that date back to around 2.5 million years ago. According to a new study, these observations may provide new ways to understand the differences between technology use by nonhuman primates and ancient hominins, as the archaeological record extends further back in hominin evolution.

Human and primate tools

Studying human evolution is not easy. For a long time, the understanding of the emergence of our genus (Homo) was associated with the origins of tool use. However, the recent discovery of 3.4-million-year-old fossilized bones exhibiting butchery marks and 3.3-million-year-old Lomekwian tools suggest that tool usage predates any recognized fossil of the Homo genus by a long way.

Although there is debate among scientists over the origins of tools, it is clear they are significant for understanding our ancient ancestry and long dependence on technology. But this requires us to be able to distinguish between hominin and nonhuman primate tool use in the archaeological record. This may sound simple, but the field of primate archaeology has expanded our understanding of this material record to include dynamics around behavior and biology that complicate the process.

However, there are options. One promising way to get around this is to assess behaviors associated with the selection and transport of materials for tools. Chimpanzees, for instance, are known to incorporate size and weight of tools when they are selecting them. This is quite a sophisticated process. For chimpanzees who practice nut-cracking with tools, their selection relies on various factors that reflect a large set of conditional rules for deciding which rock is optimal right now and what might be useful in the future.

But then there is evidence that chimpanzees may learn to choose tools based on broader cultural and social factors, as sometimes the decision to use a specific type of tool – rock type, or a decision between wood or stone tools – is shared within a community, even when the choice is less efficient. Some research has even suggested that there may have been an element of information sharing between ancient hominin and primates in the distant past.

It seems selection and transportation of tools is a feature shared between hominins of the Pleistocene era (between 2.58 million years ago and around 11,700 years ago) and nonhuman primates.

Studies of the Oldowan stone tools – the oldest known stone tool industry – show that human ancestors selected stones for tools based on their specific properties, including their ability to fracture or their resistance to wear. At present, we do not know how our ancient ancestors identified these properties and therefore made their selections.

“The concurrence of tool use (especially percussive tool use) among primate and hominin lineages remains a topic that needs further investigation if we aim to explain the ubiquity of this behavior among nonhuman primate tool users,” David Royce Braun and colleagues explain in a new study.

Although chimpanzees choose stone types for nut-cracking for distinctly different reasons to hominins in the past, the degree of selectivity and the mechanisms used for identifying appropriate rocks could have valuable parallels.

Stone selection today

In their study, Braun and the team document the selectivity patterns of stone tools for nut-cracking among chimpanzees in Bossou, Guinea. They used controlled experiments to introduce various rock types that were unknown to the animals.

The team measured various properties of the stones – including their hardness, elasticity, and rebound hardness – before introducing them to the population. The aim was to explore the variability of the rock properties and see how they influenced the selection process. Analysis indicated that the rocks introduced for the experiment were typically harder and more durable than those commonly available in the Bossou forests.

“We measured rocks using three different standard mechanical tests. Although these measures were not all developed for stones, they have been adapted to allow for measurements of fine-grained rocks,” the team explain.

The researchers assessed the selection process of specific rock types based on a measure that included the total number of stones available for selection by an individual, and the number of stones available of the same raw material as being selected. This approach allowed for each selection to be ranked as high or low selectivity.

The details of individual animals were also included when they made selections, such as their age and sex, as well as contextual factors.

Rocks’ nut-cracking efficiency was also assessed by tallying the total number of strokes needed to crack a nut. This was calculated for each nut cracked and then average values were calculated for rock combinations – hammer and anvil – to create efficiency values for these combinations.

The results showed that chimpanzees selected rocks based on their mechanical properties, rather than their by their visible features. It seems chimpanzees selected harder stones for hammers and softer stones for anvils, suggesting an understanding of their specific properties for specific purposes.

“Selectivity of rock types suggests that chimpanzees assess the appropriate materials for functions by discriminating these ‘invisible’ properties. Adults identify mechanical properties through individual learning, and juveniles often reused the tools selected by adults,” the team explain.

Importantly, when compared to what we know about the Oldowan tools, it seems hominins and chimpanzees rely on similar factors when choosing rocks.

“Hominins may not have needed to directly understand the mechanical properties of stone to canalize the selection patterns on only a few specific rock types. Instead, as seen here in this experiment, a combination of individual learning with some social enhancement can lead to rapid enforcement of the most efficient tool use.”

This indicates that relatively simple social learning mechanisms may underpin the wider adoption of rock types that are best suited for specific tasks.

“Oldowan hominins may not have implemented the kind of high-fidelity social learning evident in modern humans to create the archaeological patterns of selection that are visible in earliest industries.”

“It is possible that Pleistocene hominins also reused toolsets on ancient landscapes. Accumulations of stones on ancient landscapes may have provided the necessary information about rock selection even in the absence of direct observation,” the team concludes.

The study is published in the Journal of Human Evolution.