Scientists have found that today’s chimpanzees select stones to use as tools in a similar way to human ancestors who lived about 2.5 million years ago.

The research, published in the Journal of Human Evolution, compares modern chimpanzee tools to those of the Oldowan hominins.

Oldowan is the name given to the type of ancient technology used by various human species 2.9–1.7 million years ago across Africa and Eurasia. It is believed to have emerged with the species Australopithecus garhi and thrives with Homo habilis and Homo ergaster.

Oldowan hominins used rocks as choppers, scrapers and simple cutting tools. Archaeological studies of Oldowan tools shows that the ancient humans were selecting and crafting rocks to produce the tools. They probably used knowledge passed down over generations.

Homo erectus inherited Oldowan technology, then refined it into the Acheulean tools about 1.7 million years ago.

Chimpanzees are our closest living relatives. Other studies have compared what we know of the modern great apes’ behaviour to make inferences about our ancestors.

The new research asked the question: do modern chimpanzees select rocks in a similar way?

A team of palaeobiologists, anthropologists and behavioural scientists analysed the way chimps crack open nuts.

To get to the edible nut inside the shell, chimps combine two tools: a hammer and an anvil. The hammer, held in the hand, is used to smash a nut sitting on top of the anvil.

Hammers must be the right size and shape to fit in the hand, as well as being heavy enough to hit the nut with force. The anvil has to be able to withstand the force of the hammer while also having a shape which means the nut won’t roll off once struck.

The researchers tested how the chimps chose their rocks by giving them stones with known characteristics. The rocks were of different size, weight, shape, hardness and elasticity.

They found that the chimps arrived at their tools of choice based on their mechanical properties rather than how they looked.

Harder stones were preferred for hammers, and softer ones for anvils.

The researchers also found that younger chimps used the tools that had previously been used by older individuals. This suggests that tool choice is learned and passed down through generations.

“These patterns of stone selection parallel what is documented for Oldowan hominins,” the authors write. “The processes identified in this experiment provide insights into the discrete nature of hominin rock selection patterns in Plio-Pleistocene stone artifact production.”