Evolution News - Evolution of Skin, Fur and Feather Colours

Summary of Current Understanding of the Location of Melanosomes in Different Body Tissues and Organs and Putative Functions in Major Vertebrate Classes.

Functions of Melanin in Ancient Vertebrates

Fossil pigments shed new light on vertebrate evolution, UCC study finds | News and Views | University College Cork

A team of palaeontologists from University College Cork (Coláiste na hOllscoile Corcaigh), Ireland have uncovered how melanin, the pigment that now gives mammals and birds the colours in their skin, fur and feathers, once had a different function in our evolutionary ancestors, but, because of one of those effects of the utilitarian 'design' process of evolution by natural selection, it was also toxic. Unless, as a creationist who believes in intelligent [sic] design, you believe toxicity was a problem deliberately created so it could be solved with a clunky work-around later.

Of course, the work debunks intelligent [sic] design creationism simply by revealing the facts, especially its latest ploy - evolution = devolution!

Melanin initially evolved as part of a mineral storage system which also played a role in immunity, and was stored in several internal organs in the bodies of early amphibians and reptiles. But it has a tendency to produce free radicles and is thus mildly toxic. The benefits of a functional, albeit primitive, immune systems outweighed the damage done, but as later animals evolved improved immune systems - given added impetus by the toxic melanin, the dynamic shifted so that the net result of melanin in the internal organs was detrimental, producing selection pressure to remove it to a place of safety - namely, outside the body in the outer layer of the skin, in hair and feathers of the evolving mammals and birds.

This in turn created selection pressure favouring colouration for camouflage, display, etc, as well as protection from harmful solar radiation, so we now have a rich diversity in hair and feather patterns in the mammals and birds.

The UCC news release explains:

This new twist in the story of animal evolution is based on cutting-edge analyses of melanin granules – melanosomes – in many different fossil and modern vertebrates, including fish, amphibians, reptiles, birds and mammals. Melanin and melanosomes have traditionally been linked to outermost body tissues such as skin, hair and feathers, with important roles in UV protection and stiffening of tissues. Analyses of where different animals store melanin in the body, however, show that different vertebrate groups concentrate melanin in different organs, revealing shifts in how animals have used melanin over the last 400 million years.

The study, published in the journal Trends in Ecology and Evolution, was led by UCC palaeontologists Prof. Maria McNamara, Dr Chris Rogers, Dr Valentina Rossi and PhD student Tiffany Slater, with an international team of evolutionary biologists from Switzerland.

“Most studies of fossil melanin have focussed on melanin in fossil feathers and skin, and what colours ancient animals had,” said study leader Prof. McNamara.

“By comparing melanin in different animals – how much melanin they have, where in the body it occurs, what melanin type and composition is present – and by studying fossils, we discovered new evidence for changes in the functions of melanin through deep time.”

The research shows that amphibians and reptiles concentrate melanin in internal organs, where it supports the immune system and stores metals. In birds and mammals, however, almost all melanin occurs in hair and feathers. This difference has an unexpected source – evolution of the immune system and of warm-blooded lifestyles.

“There are pros and cons to having melanin in the body,” said team member Dr Rossi. “Melanin is hugely beneficial, but it also generates free radicals, which are harmful. This creates a major problem for animals.”

During the evolution of hair and feathers, mammals and birds evolved more sophisticated immune systems than in amphibians and reptiles. This meant that large amounts of melanin were no longer necessary in internal organs. Melanin storage then shifted to hair and feathers, which are dead tissues, thereby removing harmful metals and free radicals from living body parts.

“Melanin is a two-sided coin,” said Prof. McNamara. “It’s useful, but toxic. Birds and mammals basically came up with an ingenious solution during the early Triassic – pump melanin into new, outer, dead skin tissues that were evolving at the time. This set the scene for the evolution of the incredible diversity of plumage and fur patterning which we see today.”

The study also shows that key genes can be mapped onto colour patterns in fossils, tracking the genetic evolution of melanin through time, and that animals preferentially use less toxic forms of melanin. “There’s still a lot about melanin genetics and physiology that we don’t understand,” said Dr Ducrest of Lausanne University. What’s clear, however, is that the fossil record is a valuable source of information that we can use going forwards.”

The team's findings were published open access a few days ago in Trends in Ecology & Evolution:

Melanin-Based Phenotypes Reported in Fossil Vertebrates and Possible Genomic Events Underlying the Functional Evolution of Melanin in Vertebrates.

Evolving Role of Melanin in Vertebrate Evolution

Highlights

In extant vertebrates melanin fulfils diverse roles including visual communication, photoprotection, antioxidation, and mechanical strengthening of tissues, but the evolution of these functions is debated.

The discovery that melanosomes in fossil and modern vertebrates are associated with tissue-specific suites of trace metals supports hypotheses that melanin has ancient functions in metal homeostasis and antioxidant regulation.

Shifts in melanosome biology across the dinosaur–bird transition reveal intimate links between adaptive and pleiotropic processes relating to the evolution of endothermy, metal homeostasis, photoprotection and the lymphatic system.

Key genes can be mapped onto color pattern phenotypes in fossil vertebrates.

Melanin-based coloration in vertebrates is dominated by melanin forms associated with low cytotoxicity, possibly reflecting selective adaptation against forms linked with greater oxidative stress or co-option of melanin forms with specific metal binding behavior for coloration.

Melanins are widespread pigments in vertebrates, with important roles in visual signaling, UV protection, and homeostasis. Fossil evidence of melanin and melanin-bearing organelles – melanosomes – in ancient vertebrates may illuminate the evolution of melanin and its functions, but macroevolutionary trends are poorly resolved. Here, we integrate fossil data with current understanding of melanin function, biochemistry, and genetics. Mapping key genes onto phenotypic attributes of fossil vertebrates identifies potential genomic controls on melanin evolution. Taxonomic trends in the anatomical location, geometry, and chemistry of vertebrate melanosomes are linked to the evolution of endothermy. These shifts in melanin biology suggest fundamental links between melanization and vertebrate ecology. Tissue-specific and taxonomic trends in melanin chemistry support evidence for evolutionary tradeoffs between function and cytotoxicity.

McNamara, M.E.; Rossi, V.; Slater, T.S.; Rogers, C.S.; Ducrest, A.-L.; Dubey, S.; Roulin, A.
Decoding the Evolution of Melanin in Vertebrates
Trends in Ecology & Evolution doi: 10.1016/j.tree.2020.12.012

Copyright: © 2021 The Authors. Published by Elsevier Ltd.
Open access
Reprinted under a Creative Commons Attribution 4.0 – NonCommercial – NoDerivs License (CC BY-NC-ND 4.0)

And, like all good science which recognises that research findings such as this raise more question to be answeres because science is an advancing and widening front of knowledge, the team list several outstanding questions:

• How have the functions of melanin evolved in vertebrates through time? Was melanin co-opted for integumentary coloration only after other functions were already established?
• Given the cytotoxicity of pheomelanin, why did it evolve? Were early functions in cysteine metabolism expanded to include new functions in communication, and when?
• What is the significance of changes in the location, morphology, and chemistry of internal melanosomes during vertebrate evolution?
• Why are melanosomes in different tissues associated with different metals?
• Why do mammals and birds have less internal melanin than amphibians and reptiles? Is this a crown group feature or apparent in early representatives of both groups?
• What is the genetic basis for the evolution of specific melanosome geometries associated with iridescent and noniridescent structural colors in the feathers of birds and nonavian dinosaurs?
• Are shifts in the geometry, chemistry and spatial distribution of melanosomes across the evolutionary transition from reptiles to birds mirrored, or controlled by, associated transformations in melanogenic genetic regulatory networks?
• What is the anatomical distribution, morphology and chemistry of melanosomes in basal vertebrates, and what are the implications of these for the functional evolution of melanin, vertebrate origins and the transition to life on land?
• Do melanosomes in the skin of amphibians show selection for extreme morphologies as in many extant bird feathers? Conversely, do melanosomes in bird feathers show less derived morphologies when not under selection for coloration; for example, where the melanosomes serve as a backing pigment for feathers colored by carotenoids or other pigments?
• How representative are data from individual fossils? Can we gauge this using fossilization experiments and comprehensive testing of statistically robust datasets on extant vertebrates?

There we have a perfectly rational explanation for the appearance of melanin in the internal organs of ancient fossils which fits well with the idea of a utilitarian process that can come up with sub-optimal solutions that, when they are replaced by better ones, create redundancy that then becomes a problem and an opportunity to utilise the redundant structures and processes for a novel function.

This, of course, is the basic mechanism by which creationism's beloved 'irreducible complexity' arose and why their latest attempt to explain away evolution as a degenerative process where each change is 'devolutionary' (© 2019 Michael J Behe) because it moves away from some assumed initial perfection of special magic creation, is such biological and factual nonsense. Nowhere in the process described above can any of the changes be described as 'devolutionary' or degenerative, unless you regard a toxic, primitive immune system and a lack of skin, fur and feather colours and patters as 'perfection'.

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