Colors of the Past: Pheomelanin's Timeless Hue in Evolutionary History
Dec. 2023
By Khadijah Dhoondia
Edited by Dannah Altiti
As rare as naturally red hair may seem to be, redheads have truly stood the test of time.
10 million years, to be exact.
This discovery was made by paleontologists at the University College Cork (UCC) who found the first molecular evidence of pheomelanin–a pigment that produces the ginger color–in fossils.
Background
To understand this discovery’s significance, it is important to understand the biochemical processes related to detecting fossil color. When prehistoric creatures undergo fossilization, the biomolecules that make up their unique features, such as their skin texture or color, are degraded into long, indistinguishable chains of carbon (Pinheiro et. al, 2019). However, one class of molecules that remain intact are melanins, which are key to revealing a prehistoric species color.
Figure 1: A comparison of the structural differences between eumelanin, eumelanosomes, pheomelanin, and pheomelanosomes (Earth Science Club of Northern Illinois, n.d.)
Melanin take two different forms: eumelanin and pheomelanin. Eumelanin is associated with black and brown hues, while pheomelanin is associated with pale yellow to rufous-brown tones. These melanins are produced and stored by organelles, specialized structures within cells, called melanosomes. Melanosomes are especially well preserved in fossils, and they are commonly found in keratinized soft tissues, such as feathers and hairs. In these soft tissues, phaeomelanosomes are more spherical (Fig. 1) while eumelanosomes have a more elongated shape (Fig. 1) (Slater et al, 2023). Due to the distinct differences in melanosome morphology, scientists have used this feature as a proxy for animal color.
A New Approach: Detecting Color in Fossils
Recent studies have questioned the accuracy of using melanosome morphology to determine fossil color. In fact, it was determined that fossil color “reconstructions based on morphological data alone are only 61.9% accurate” (Slater et al, 2023). Therefore, paleontologists are searching for a more accurate way to detect fossil color. One possibility is using chemical data to detect the presence of melanin monomers (small molecules that can bind to form more complex melanin structures). Specifically, scientists at UCC proposed the use of high-performance liquid chromatography (AHPO-HPLC), a type of chemical analysis that is successful in detecting melanin in biological samples. However, AHPO-HPLC has not been extensively tested on fossilized records; to confirm that AHPO-HPLC could also detect eumelanin and pheomelanin monomers in fossils, researchers conducted preliminary experiments on a more predictable model – artificially fossilized feathers.
The Discovery of Pheomelanin in Frogs
Researchers tested the effectiveness of AHPO-HPLC by conducting chemical analyses on black, rufous, and white fossilized bird feathers. The feathers were collected from currently living bird species (chickens and little egrets) and experimentally fossilized through exposure to intense heat. As UCC scientists had expected, the AHPO-HPLC analyses revealed that the black and rufous feathers had the greatest abundance of eumelanin and pheomelanin, respectively (Slater et al, 2023). Not only were the analyses consistent with the visually observed colors of the feathers, but the researchers also found additional markers to better distinguish between the feather colors (Fig. 2).
Figure 2. A schematic showing bird feather samples and their corresponding eumelanin and pheomelanin markers (Slater et al, 2023).
After demonstrating that AHPO-HPLC could be used to accurately assess the bird feathers, AHPO-HPLC was ready to be used on fossils. For their first sample, paleontologists used fossil frogs from the Miocene epoch. Upon conducting the analyses, the researchers made an exciting observation — pheomelanin was likely present in the frog's skin or tissues when it was alive.
Revolutionizing the Study of Color and Evolution
The discovery of pheomelanin in prehistoric creatures and the technique used to identify melanin monomers can revolutionize how paleontologists study the evolution of color. Through the preliminary experiments conducted on the feathers, researchers demonstrated that biochemical evidence of melanin remains even after species fossilization. Furthermore, the method of detecting color through chemical analyses proves to be more accurate than previous techniques that relied on the morphology of melanosomes, allowing researchers to refine their understanding of why certain colors have persisted through evolutionary processes. This is especially applicable to pheomelanin; evolutionary persistence of pheomelanin has eluded scientists because the pigment is actually highly toxic to animals when at high concentration.
Not only have innovative AHPO-HPLC analyses unveiled the enduring presence of pheomelanin through time, but this method of chemical analysis now enables paleontologists to answer the next pressing evolutionary question – “why ginger?”
References
Earth Science Club of Northern Illinois. (n.d.). Palaeocast Episode 52: Melanin. https://www.esconi.org/esconi_earth_science_club/2015/11/palaeocast-episode-55-pterosaurs.html
McGraw, K. J. (2003). Melanins, metals, and mate quality. Oikos, 402-406.
Pinheiro, F. L., Prado, G., Ito, S., Simon, J. D., Wakamatsu, K., Anelli, L. E., ... & Glass, K. (2019). Chemical characterization of pterosaur melanin challenges color inferences in extinct animals. Scientific reports, 9(1), 15947.
Slater, T. S., Ito, S., Wakamatsu, K., Zhang, F., Sjövall, P., Jarenmark, M., & McNamara, M. E. (2023). Taphonomic experiments reveal authentic molecular signals for fossil melanins and verify preservation of phaeomelanin in fossils. Nature Communications, 14(1), 5651.
