Toxic pigment that causes red hair discovered in 10 million-year-old frog fossil

Two women pose with a frog.
Scientists Maria McNamara (left) and Tiffany Slater with the 10 million-year-old frog fossil. (Image credit: Pic Daragh Mc Sweeney/Provision)

Paleontologists have discovered the earliest molecular evidence of the toxic pigment that causes red hair in the fossil record — in 10 million-year-old frog fossils.

The ancient amphibians had preserved fragments of pheomelanin (also spelled phaeomelanin), a yellowish-red pigment that produces ginger-colored hair in animals, including humans, according to a study published Oct. 6 in the journal Nature Communications.

"It's the exact same pigment that causes red hair in us," lead study author Tiffany Slater, a postdoctoral researcher of paleobiology at University College Cork in Ireland, told Live Science. "But that doesn't mean that the frogs were necessarily ginger colored when they were alive."

Pelophylax pueyoi is an extinct species of large frog that lived in what is now Spain during the Early Miocene (23 to 5.3 million years ago). Their fossilized remains are part of a museum collection and were loaned to the researchers, according to a statement.

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To get a better understanding of how the pigments degrade during the fossilization process, the researchers examined liver tissues taken from the frog remains (the liver is known for containing high levels of pheomelanin) along with black, ginger and white bird feathers, which dovetailed with a previous study of theirs, according to the statement.

Their findings showed traces of pheomelanin contained within the liver tissues of the ancient frogs. 

"Fossils are invariably altered by the ravages of heat and pressure during burial, but that doesn't mean that we lose all original biomolecular information," study co-author Maria McNamara, a professor of paleobiology at University College Cork, said in the statement. "Our fossilization experiments were the key to understanding the chemistry of the fossils and prove that traces of biomolecules can survive being cooked during the fossilization process."

An illustration of pigments found in a fossilized frog.

A graphical abstract based on the new paper shows evidence of preservation of ginger pigments in a frog.  (Image credit: Science Graphic Design)

Slater said their experiments are "pushing the boundaries" of what they thought was possible regarding the information a fossil can contain. 

However, scientists are still learning how and why pheomelanin evolved in the first place, especially as it can be toxic to animals, Slater said. Pheomelanin is a type of melanin, a substance in the body that produces hair, eye and skin pigmentation. In humans, there are two types of melanin — eumelanin, which is responsible for dark colors, and pheomelanin, which produces blond and ginger hair colors and pale skin. While eumelanin helps protect from the sun's harmful UV rays, pheomelanin does not.

"It's toxic in the way that it interacts with sunlight, which can cause damage to certain cells," Slater said. 

The scientists hope that further study can lead to a better understanding of the evolution of different pigments within the fossil record, providing an insight into the colors of ancient animals.

"This is the first molecular record of pheomelanin in the fossil record, and we need to start looking for pigments in more and older fossils," Slater said. "Then we can start [to rebuild] what it says about evolution, and the big question of why ginger pigments evolve despite them being toxic to animals."

Jennifer Nalewicki
Live Science Staff Writer

Jennifer Nalewicki is a Salt Lake City-based journalist whose work has been featured in The New York Times, Smithsonian Magazine, Scientific American, Popular Mechanics and more. She covers several science topics from planet Earth to paleontology and archaeology to health and culture. Prior to freelancing, Jennifer held an Editor role at Time Inc. Jennifer has a bachelor's degree in Journalism from The University of Texas at Austin.