A glue formula used by people in South Africa 70,000 years ago required more intelligence than archaeologists normally attribute to Stone Age men. It was made by mixing red ochre with the gum of acacia trees. It turns out that the red ochre serves more than a decorative purpose, as researchers found out when they made some of the glue themselves.

"We discovered that when we used ochre, the glue is much more robust, and the stone tool doesn't come off the shaft," said study team member Lyn Wadley of the University of the Witwatersrand in Johannesburg, South Africa.

But making the glue wasn't easy for the ancient Africans.

It was mentally taxing work that would have required humans to account for differences in the chemistry of gum harvested from different trees and in the iron content of ochre from different sites.

"They couldn't possibly have known about chemical pH or iron content … but they knew that certain combinations of things worked very well," Wadley said.

Link