Probing ancient pheomelanin

Living organisms create spatial patterns of trace elements in their bodies. Clever means of detecting those spatial patterns are arriving. These have given new avenues into the biology of extinct organisms, by pulling information from exceptional fossils.

A new paper in Nature Communications by Phillip Manning and colleagues describes a new way to look at the distribution of the red pigment, pheomelanin, in the preserved fossil soft tissues of ancient organisms: “Pheomelanin pigment remnants mapped in fossils of an extinct mammal”.

They have applied XRF (X-ray fluorescence) and XAS (X-ray absorption spectroscopy) at a very tiny scale by using a synchotron. This provides a way of building a microscopic matrix of trace element distributions across a fossil.

Similar approaches have been looking at eumelanin distributions for some time, exploiting the fact that the process for generating eumelanin relies upon a copper-containing enzyme. Recently, it has become possible to quantify pheomelanin versus eumelanin by focusing on zinc. The results described in this paper focus on a particular fossil mammal, a Pliocene mouse. Those are interesting as a proof of concept but I am more fascinated by their description of the chemistry involved:

Trace metals are key components of melanin and play important roles in melanogenesis. Melanins are complex molecules formed from the aromatic amino acid tyrosine via the action of the Cu-containing enzyme tyrosinase. Because Cu is the metal cofactor in the enzymatic process forming eumelanin, elevated concentrations of organically bound Cu can typically be correlated with eumelanin-rich tissue. After Cu, Zn is the second most abundant metal in mammal melanosomes. Both metals may be complexed within the interior ring structure of eumelanin, attached to the diol functional group of dihydroxyindoles, or attached to terminal carboxylate groups, but in all cases, the Cu and Zn within eumelanin are strictly light-element coordinated (O/N): in eumelanin, there are no sulfur groups to which trace metals can bind. Importantly for this study, pheomelanin synthesis additionally requires the sulfur containing amino acid cysteine as a substrate. Sulfur in pheomelanin is contained within benzothiazole (or benzothiazine) units that are accessible for metal attachment. While Cu is strongly associated with eumelanin, Zn correlates with pheomelanin pigment. Previously, work on fossil integument concluded that organosulfur–Zn complexes may be the residue of pheomelanin; however, detailed coordination chemistry for Zn in extant pheomelanin to use in comparison with the fossils was unavailable. Subsequently, Edwards et al. applied detailed XRF and XAS to extant pheomelanin-rich feathers. The results showed that these feathers possessed a distinct chemical signature for Zn and S, with a significant portion of the Zn inventory bonded to S, almost certainly through the S contained within the pheomelanin molecule. This conclusion was based on the fact that there was a strong, spatially resolvable correlation between Zn and pheomelanin-associated sulfur groups.

Biochemistry is complicated, and bodies do other things with heavy metals besides make melanin. Besides, fossils get trace elements in them from post-depositional processes, not only from the intrinsic biochemistry of the living organism. So it is extremely necessary to validate these kinds of approaches using large samples of tissues from extant organisms as well as various controls from non-fossil bearing sedimentary situations.

That kind of work has so far been pretty light. For understandable reasons, research teams are working to validate their approaches upon exceptional fossils that preserve details of soft tissue anatomy, and preferably tissue microanatomy.

The point of using exceptional fossils in such analyses is that the chemical patterns can be compared to anatomical patterns. When they correspond to each other, that gives some confidence that the chemistry is seeing something real.

My hope would be that once the techniques are validated more widely, they may be able to bring some information out of the much larger samples of less exceptional fossils.