Chimpanzees’ stone tool choices may mirror ancient human ancestors’ techniques

An international team of paleobiologists, anthropologists and behavioral scientists has found that the process used by modern chimps to select tools for cracking nuts may be similar to how ancient human ancestors chose their tools. In their paper published in the Journal of Human Evolution, the group describes the way chimpanzees choose rocks to use as tools for cracking nuts, and how they compared what they found with prior research regarding how ancient human ancestors chose tools for various purposes.

Previous research has shown that the oldest known use of stone tools is by Oldowan hominins, dating back to approximately 2.5 million years ago. They used rocks as choppers, scrapers and simple cutting implements. Study of the tools they left behind has shown that they were intentional when selecting rocks for such uses and that there may have been some learning of how to craft them passed down through generations.

In this new effort, the research team wondered if similar patterns of stone selection might be found in modern chimpanzees. To find out, they conducted a thorough study of the processes the chimps use when choosing rocks for cracking open nuts.

To crack open a nut, a chimpanzee chooses to use two tools: a hammer, and an anvil. The hammer is held in the hand and brought down heavily onto a nut sitting atop the anvil. Rocks used as a hammer are chosen for their heft and shape, while those used as an anvil are chosen for their ability to withstand blows from the hammer and to be less likely to allow a nut to careen off to the side rather than split when struck.

The study began with the researchers providing test chimps with premeasured rocks, noting how elastic they were, how hard, how much they rebounded during strikes, and their size, weight, and shape

In closely watching how chimps chose rocks as their tools and how well the tools they chose worked as intended, the researchers found that the chimps chose their rocks based on their mechanical properties, rather than how they looked. They chose harder stones for hammers, for example, and softer stones for anvils. They also noted that younger chimps would use the tools that had been used by the older chimps, a hint at the possibility of tool choice being learned and passed down.

The researchers contend that the results of their observations suggest that ancient hominins and modern chimpanzees share some similar techniques when choosing stones for use as tools.

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