New research published in NeuroImage suggests that that selective pressures during the early Pleistocene resulted in an enhancement of working memory capacity in hominins. The study used imaging technology to observe the brain activity of modern humans as they learned to craft Stone Age tools.

“We know from fossil evidence that human brain size was increasing around 1.8 million years ago, particularly the prefrontal area of the cortex, which is largely responsible for complex cognitive behaviors, such as planning, working memory, personality, and decision-making,” said study author Shelby S. Jedele Putt, an assistant professor at Illinois State University and director of the ISU Biological Anthropology Lab (@ISUBioAnth).

“However, we are severely limited in what we can learn about the evolution of human cognition from fossil evidence alone. I began investigating modern humans’ functional brain activity while they recreate ancient stone tool types so that I could better understand the cognitive processes that might be involved or even necessary to make a simple Oldowan flake versus a more complex Acheulean handaxe tool.”

“By comparing the results of this study to what we already know about brain shape and size from the human fossil record, we can begin to make inferences about the cognitive abilities of earlier species of human, like Homo habilis and Homo erectus. More specifically, we can potentially see how particular brain areas and their functions changed over time in our human ancestors to accommodate increasingly complex social and technological skills, and investigate the factors that may have led to the evolution of modern intelligence,” Putt explained.

The researchers used functional Near-Infrared Spectroscopy (fNIRS) to measure the brain activity of 33 adults as they learned to make an Oldowan flake and Acheulean handaxe — two Stone Age tools of varying complexity.

Oldowan flakes first appeared about 2.6 million years ago and are little more than broken rocks with a jagged edge. Acheulean handaxes emerged as early as 1.8 million years ago and resemble a large arrowhead with symmetrical edges.

The participants learned how to manufacture the tools during seven 60-minute training sessions, in which they watched instructional videos and had time to practice constructing the items from rocks.

“Our study shows that as ancient stone technologies became more complex during the Early Stone Age, so did the cognitive processes that were necessary for their completion. We found that the left prefrontal cortex of the brain is activated to a much greater degree while learning to make an Acheulean handaxe when compared to learning to make simple Oldowan flake and core tools,” Putt told PsyPost.

“This most likely occurs because making a handaxe involves holding more items of information in working memory. Furthermore, those participants who were measurably better at the Acheulian toolmaking task than others showed even more activity in this region.”

“These results suggest that those ancient toolmakers with the largest working memory capacities were probably the most skilled and successful toolmakers, which in turn would have led to an increase in the amount and quality of food that they could provide for themselves and their offspring,” Putt said.

In previous research, Putt and her colleagues found that the co-ordination of visual attention and motor control networks were sufficient to remove simple flakes for Oldowan tools. The production of Acheulian tools required the integration of visual working memory, auditory and sensorimotor information, and complex action-planning.

But the studies — like all research — include some caveats.

“Of course we can never be certain that the cognitive operations of modern humans perfectly resemble those of early humans, even when they accurately replicate the same tools using the same order of steps. We can assume, however, that early humans possessed at least the minimum cognitive abilities needed by modern humans to complete the same tasks,” Putt said.

“Furthermore, while we show that working memory areas of the brain are relatively more activated during Acheulean toolmaking compared to Oldowan toolmaking, we will not know for sure that stone toolmaking involves working memory until a study is conducted which investigates the collocation of neural responses to both a working memory task and toolmaking task in the same participants.”

The study, “Prefrontal cortex activation supports the emergence of early stone age toolmaking skill“, was authored by Shelby S.J. Putt, Sobanawartiny Wijeakumar, and John P. Spencer.