Anthropologists have long suspected our ancestors were pushed into their role as masters of technology when the forests they lived in turned to savannah. This hard-to-prove hypothesis has gained support from a study of chimpanzees, showing our nearest living relatives respond to frequently changing environments by developing wider behavioral diversity.
Fifty-five years ago, the discovery chimpanzees made tools shocked the world. Today, we know that not only do they use a variety of tools but many are specific to certain populations. The same is true of behaviors – some chimpanzees escape the heat in caves or by bathing, while such activities are unknown to others. These are cultural traits, passed on from adult to child within certain groups but unknown in others.
An international team of researchers led by Dr Ammie Kalan of the Max Planck Institute for Evolutionary Anthropology tested the influence of the environment on chimpanzee behaviors. Inevitably, some behaviors only evolve under specific conditions – methods for extracting a food source will not be preserved long when living where that food is absent. The team wanted to know if all chimpanzee populations have a similar number of options in their toolkit or if certain environments inspire a larger array of distinctive behaviors. In particular, the team note in Nature Communications that this would test “the critical assumption that population diversification precedes genetic divergence” and the formation of new species.
Using a combination of their own observations and data from other researchers, Kalan and co-authors investigated whether 31 behaviors, such as nut-cracking and termite-fishing, are present or absent in 144 chimpanzee populations.
"Chimpanzees experiencing greater seasonality, living in savannah woodland habitats and located further away from historical Pleistocene forest refugia were more likely to have a larger set of behaviors present," Kalan said in a statement.
“Environmental variability also supports cultural diversification in chimpanzees," Kalan concludes. As much sense as this makes, "this is some of the first cross-population data within a single species to support this idea.”
Behavior doesn't fossilize, so we know almost nothing about our ancestors' culture before they started making stone tools. Much as modern chimps may differ from the hominins of 3-4 million years ago, they represent the best guides we have.
It seems likely then that when once-stable forests started to dry out and be replaced by grasslands, the Australophicenes living there had to either retreat further into the forest or develop the diversity of skills to survive in a changing environment. Those that stuck with the old ways are long extinct, while those that broadened their capacities evolved into us.
Co-author Dr Hjalmar Kühl thinks there may be more information to be wrung from the observations, including whether "there may be other demographic and social factors that have also played an important role in the process of behavioral diversification."