Microbial Products Color Ancient Art, Study Shows

Early hunter-gatherers harvested and heated microbially-produced iron oxides to make pigments for rock art, according to a new paper published by Senior Research Scientist Dave Emerson and colleagues in Scientific Reports.

"This research followed a path like a great detective story, in which we put together clues from the cave paintings and pigment minerals to uncover how the iron materials were processed thousands of years ago," Emerson said. "Cutting-edge research today is exploring the potential uses of microbially-produced iron oxides as nanomaterials, and our findings show they have actually been used in technology for thousands of years."

Emerson has spent years studying iron-oxidizing bacteria, which live in environments as extreme as the deep ocean and as common as roadside ditches. They gain energy by transferring electrons between these two elements, and capturing the tiny spark of energy that results. This process produces rust minerals as byproducts, which help form twisted stalks, ribbon-like structures that grow out of the microbes. Emerson has found these stalks in ancient fossils, ecosystems around the globe – and most recently, in a flake of ancient paint.

A team of scientists led by Brandi MacDonald at the University of Missouri used archaeological techniques to uncover how hunter-gatherers created ochre paint for rock art located at Babine Lake in British Columbia. Ochre, one of Earth’s oldest naturally occurring materials, was often used as a vivid red paint in ancient rock art known as pictographs around the world. Despite broad use throughout human history and a modern focus on interpreting the artistic symbolism of pictographs, little is known about the paint itself and how it was produced.

MacDonald used a scanning electron microscope to image the ochre used in the Babine Lake drawings. She noticed numerous microscopic tubular structures in the paint, and wondered if they might have been produced by bacteria. MacDonald sent images of the structures in question to Emerson, who immediately recognized them as iron-oxidizing bacteria. He sent back samples of freshly produced microbial iron oxides, and the similarity confirmed her initial suspicions.

"Ochre is one of the only types of material that people have continually used for over 200,000 years, if not longer," said MacDonald, who specializes in ancient pigments. "We have a deep history in the archeological record of humans selecting and engaging with this material, but few people study how it’s actually made."

The scientists used modern technology, including the ability to heat a single grain of ochre and watch the effects of temperature under an electron microscope. They determined that individuals at Babine Lake deliberately heated the bacteria to a temperature range of approximately 750°C to 850°C to initiate the color transformation.

This is the first study of the rock art at Babine Lake. It shows that individuals who prepared the ochre paints harvested iron-rich bacteria out of the lake — in the form of an orange-brown sediment.

"It’s common to think about the production of red paint as people collecting red rocks and crushing them up," MacDonald said. "Here, with the help of multiple scientific methods, we were able to reconstruct the approximate temperature at which the people at Babine Lake were deliberately heating this biogenic paint over open-hearth fires. So, this wasn’t a transformation done by chance with nature. Today, engineers are spending a lot of money trying to determine how to produce highly thermo-stable paints for ceramic manufacturing or aerospace engineering without much known success, yet we’ve found that hunter-gatherers had already discovered a successful way to do this long ago."

MacDonald and Emerson also collaborated with scientists and engineers at University of Arizona, University of Northern British Columbia and McMaster University for help with this technical reconstruction. The authors would like to acknowledge the permission and support of the descendant Lake Babine Nation, upon whose traditional territory the rock art resides.