Monkey tool design changed over millennia

The satellites humans fling into space and the massive accelerators that smash together subatomic particles are a far cry from the stone tools our ancestors were making three million years ago.

But it turns out we’re not the only ones to change our technologies over time. Stone tools used by capuchin monkeys have changed at least twice over the past 3000 years, according to a study in the journal Nature Ecology & Evolution.

The finding is based on a capuchin monkey site in the World Heritage listed Serra da Capivara National Park in northeast Brazil. 

Today, bearded capuchin monkeys (Sapajus libidinosus) at the site mostly use stone tools to pound open cashew nuts. Elsewhere in the park, monkeys wield stone tools to smash apart seeds and fruit, dig for roots and spiders, and bang together as a threat to others.

Several modern-day primates are known to use tools, but few archaeological sites exist showing non-human tool use in prehistory. The oldest example, from chimpanzees in Côte d’Ivoire, dates to 4000 years ago.

The Brazilian site is unique because not all artefacts at the site are the same. The types and sizes of rocks used changed over time.

During excavations, more than 1500 stones were dug out of the earth. Of these, 122 had impact marks, crushed surfaces, stuck-on residue or other signs they had been used as tools. 

Radiocarbon dating of charcoal excavated alongside the stones indicate that the earliest period of occupation occurred between 2400 and 3000 years ago – roughly 450 capuchin generations away. 

Hammer-stones from this time were low-weight rocks, likely used to process foods smaller and less resistant than cashew nuts.

By around 600 years ago, anvils had been added to the capuchin toolkit; by 250 years ago, low-weight rocks had been replaced by sturdier ones, probably for opening harder foods.

A final change, over the last 100 years or so, saw tools revert to a smaller size, similar to the cashew-pounding tools used by modern-day capuchins.

“This discovery presents the first example of long-term tool use variation outside of the human lineage,” write Tiago Falótico from the University of São Paulo in Brazil, Tomos Proffitt from University College London in the UK, and colleagues.

The exact reasons behind the technological change for the monkeys is currently unknown, according to the authors.

The region is home to numerous capuchin groups that are able to learn stone tool use from each other. The changes in the archaeological record could mean that different groups of the species – with different favourite foods – occupied the site at different times.

Alternatively, a single group could have occupied the site more or less continuously. Changing tools might reflect a change in the availability of different foods. The abundant cashew trees of today, for instance, might not always have been so common.

Similar explanations – of differing cultural traditions or raw materials – have been used to explain tool changes in the early human record.