Early stone tools represent one of the most important technological milestones in human evolution. The production and use of sharp stone tools significantly widened the ecological niche of our ancestors, allowing them to exploit new food resources. However, despite their importance, it is still unclear how these early stone technologies emerged and which behaviors served as stepping-stones for the development of systematic stone tool production in our lineage. One approach to answer this question is to collect comparative data on the stone tool making and using abilities of our closest living relatives, the great apes, to reconstruct the potential stone-related behaviors of early hominins. To this end, a team of researchers from the University of Tübingen, the Max Planck Institute for Evolutionary Anthropology, the University of Barcelona and the University of Oslo tested both the individual and the social learning abilities of orangutans (Pongo pygmaeus) to make and use stone tools.
“Given their resilience to destructive taphonomic processes, the most abundant hominin artifacts in the archaeological record are stone tools,” said lead author Dr. Alba Motes-Rodrigo from the Department of Early Prehistory and Quaternary Ecology at the University of Tübingen and her colleagues.
“Early stone tools typical of the Early Stone Age record include intentionally modified stones with sharp-edges (e.g. flakes and cores) and unmodified stone tools such as hammerstones and anvils dating as early as 3.3 million years ago.”
“The production and use of these and later stone tools is often highlighted as a milestone in human evolution: stone tools widened the early hominin ecological niche by enabling, for example, butchering, meat processing, bone marrow extraction and plant tissue modification.”
“As a result, the production and use of stone tools elicited over time major changes in hominin dentition, hand morphology and brain size.”
“However, despite the clear ecological importance of stone tools in human evolution, it remains debated how the skills associated with their production and use emerged and how they were learned by naïve individuals.”
In their research, Dr. Motes-Rodrigo and co-authors used five untrained orangutans.
In the first experiment, they tested tool making and use in two captive male orangutans at Kristiansand Zoo in Norway.
Each orangutan was provided with a concrete hammer, a prepared stone core, and two baited puzzle boxes requiring them to cut through a rope or a silicon skin in order to access a food reward.
Both orangutans spontaneously hit the hammer against the walls and floor of their enclosure, but neither directed strikes towards the stone core.
In the second experiment, the orangutans were also given a human-made sharp flint flake, which one orangutan used to cut the silicon skin, solving the puzzle.
This is the first demonstration of cutting behavior in untrained, unenculturated orangutans.
To investigate whether apes could learn the remaining steps from observing others, the researchers demonstrated how to strike the core to create a flint flake to three female orangutans at Twycross Zoo in the United Kingdom.
After these demonstrations, one female went on to use the hammer to hit the core, directing the blows towards the edge as demonstrated.
This study is the first to report spontaneous stone tool use without close direction in orangutans that have not been enculturated by humans.
“Our observations suggest that two major prerequisites for the emergence of stone tool use — striking with stone hammers and recognizing sharp stones as cutting tools — may have existed in our last common ancestor with orangutans, 13 million years ago,” the authors said.
“Our study is the first to report that untrained orangutans can spontaneously use sharp stones as cutting tools.”
“We also found that they readily engage in lithic percussion and that this activity occasionally leads to the detachment of sharp stone pieces.”
The results appear in the journal PLoS ONE.
A. Motes-Rodrigo et al. 2022. Experimental investigation of orangutans’ lithic percussive and sharp stone tool behaviours. PLoS ONE 17 (2): e0263343; doi: 10.1371/journal.pone.0263343
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