A new study suggests that the fundamental abilities underlying human language and technological culture may have evolved before humans and apes diverged millions of years ago. The findings will be published 5th December 2024 in the journal PeerJ.
Many human behaviours are more complex than those of other animals, involving the production of elaborate sequences (such as spoken language, or tool manufacturing). These sequences include the ability to organise behaviours by hierarchical chunks, and to understand relationships between distantly separated elements.
For example, even relatively simple human behaviours like making a cup of tea or coffee require carrying out a series of individual actions in the right order (e.g. boiling the kettle before pouring the water out). We break such tasks down into solvable chunks (e.g. boil the kettle, get the milk and teabag, etc), composed of individual actions (e.g. ‘grasp’, ‘pull’, ‘twist’, ‘pour’). Importantly, we can separate related actions by other chunks of behaviour (e.g. you might have to stop and clean up some spilt milk before you continue). It was unknown whether the ability to flexibly organize behaviours in this way is unique to humans, or also present in other primates.
In this new study, the researchers investigated the actions of wild chimpanzees — our closest relatives — whilst using tools, and whether these appeared to be organised into sequences with similar properties (rather than a series of simple, reflex-like responses). The research was led by the University of Oxford with an international collaboration across the UK, US, Germany, Switzerland, and Japan.
The study used data from a decades-long database of video footage depicting wild chimpanzees in the Bossou forest, Guinea, where chimps were recorded cracking hard-shelled nuts using a hammer and anvil stones. This is one of the most complex documented naturally-occurring tool use behaviours of any animal in the wild. The researchers recorded the sequences of actions chimps performed (e.g. grasp nut, pass through hands, place on anvil, etc.) — totalling around 8,260 actions for over 300 nuts.
Using state-of-the-art statistical models, they found that relationships emerged between chimpanzees’ sequential actions which matched those found in human behaviours. Half of adult chimpanzees appeared to associate actions that were much further along the sequence than expected if actions were simply being linked together one-by-one. This provides further evidence that chimpanzees plan action sequences, and then adjust their performance on the fly.
Understanding how these relationships emerge during action organization will be the next key goal of this research, but these could involve behaviours such as chimpanzees pausing sequences to readjust tools before continuing, or bringing several nuts over to stone tools that are then cracked in one long sequence. This would be further evidence of human-like technical flexibility.
Additionally, the results suggest that the majority of chimpanzees organise actions similarly to humans, through the production of repeatable ‘chunks’. However, this result did not hold for every chimpanzee, and this variation between individuals may suggest that these strategies for organising behaviours may not be universal in the way they are for humans.
Lead researcher Dr Elliot Howard-Spink (formerly Department of Biology, University of Oxford, now Max Planck Institute of Animal Behavior) said: “The ability to flexibly organise individual actions into tool use sequences has likely been key to humans’ global success. Our results suggest that the fundamental aspects of human sequential behaviours may have evolved prior to the last common ancestor of humans and chimpanzees, and then may have been further elaborated on during subsequent hominin evolution.”
Co-senior researcher Professor Thibaud Gruber (University of Geneva) said: “There has been a renewed interest in the co-evolution of language and stone tool use in human evolution, and our study contributes to this debate. While the connection between our results and early hominin stone tool use can be made more readily, how this connects with the evolution of other complex behaviours, like language, remains an exciting avenue of future research.”
Co-senior researcher Professor Dora Biro (University of Rochester) said: “There is increasing recognition that preserving cultural behaviours in wild animals — such as stone-tool use in West-African chimpanzees — should be incorporated into conservation efforts. Wild chimpanzees and their cultures are critically endangered, yet our work highlights how much we can yet learn from our closest relative about our own evolutionary history.”
As many great apes perform dextrous and technical foraging behaviours, it is likely that the capacity for these complex sequences is shared across ape species. More research is needed to validate this theory, and is a key goal for the team moving forward.
The researchers also plan to investigate how actions are grouped into higher-order chunks by chimpanzees during tool-use. This research will aim to clarify the rules that chimpanzees follow when generating their tool use behaviours. They will also investigate how these structures emerge during development and are shaped across adult lives.
