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Warning: All rights reserved. This article appeared in the issue of the National Finch and Softbill Society Bulletin. Volume 21. No. 2. Mar/April. 2004. (p. 14-15). Anyone wishing to reproduce this article for another bulletin, newsletter, article, journal, CD, or any other public forum needs express written consent of the NFSS and of the author michael@exoticfinches.com

I must admit that the thought of feather evolution hardly comes to mind when I am chasing feathers with my vacuum cleaner the day I clean the bird room; and when I rest my head on my goose down pillow at night, I may be thinking of which birds to pair up this season - but never do I think about how feathers came about. For decades scientists, while knowing the benefits of feathers on birds, have wondered where they fit in the scheme of evolution. Recent discoveries have shed important light on this interesting question. As is true with most scientific inquiries, they are surrounded by controversy. Review-ing all theories is well beyond the NFSS Journal, so I present here what is considered one of the more believable scientific opinions.

It has been with discoveries in the last ten years of fossilized primitive feathers on non-avian theropod dinosaurs in China that the dinosaur ancestor of birds has gained one more line of support. These dinosaurs pre-date flight and birds, making scientists ponder about the reason these structures evolved. Arguments against the belief that birds evolved from theropod dinosaurs are becoming less valid and it is with the assumption that birds evolved from dinosaurs that feather study is making progress. A current topic that remains unresolved is whether structures found in other dinosaur classes are feather progenitors and if this is so, did the “feather” pop up at different times in evolution in unrelated dinosaur lines. After reading several papers, my inclination is to believe a single evolutionary “feathered” line descended from one theropod group. Future studies will resolve this issue.

To understand more about feather evolution, the chemical and developmental structure of feathers is studied. Feathers are branched filamentous keratin (a type of protein) structures which form a cylindrical invagination around the emerging “feather bud”. So, feathers begin as a tube of protein. It is believed that the recently discovered fossils show enough homology to modern feathers to be considered primitive feathers. The “early feathers” are found along the back ridge, face, and belly of these dinosaurs. Interestingly, there are mutants in current day poultry that display primitive feather morphology which may hearken back to the day of primitive dinosaur “feathers”. Such developmental mutants will be critical to study feather evolution because they may represent genetic and developmental steps that predate the modern complex feather.

Much of the conflict among scientists discussing feather origin and function is that some scientists did not think to study feather development by breaking it into stages that could be accommodated by a sequence of evolutionary events. The “sexier” line of thought was to think of flight as being the sole reason that feathers evolved. This narrow line of thought hinders evolutionary study. According to Prum and Bush (2002), “Concluding that feathers evolved for flight is like maintaining that digits evolved for playing the piano.” Instead, most serious evolutionary biologists are looking at a series of plausible functions like thermal insulation, thermal shielding, water repellency, tactile organs, or defense. These would not require that flight ever evolved but would still explain early function that predates flight. With only fossil records available it is unlikely that the early purpose or purposes for feather evolution will be resolved. Prum and Bush (2002) believe that if feather evolution had been under strong selection for a single function, the developmental plasticity we see today would not have evolved and the function of flight assistance, which was dependent on continuous structural evolution, might not have occurred. They believe that by studying the evolution of feathers from the simple to the complex, one can demonstrate an increasingly hierarchical complexity in form and function. They conclude that feathers likely originated by natural selection for a protein outgrowth from the skin that did not become part of the dermis itself, resulting in a tubular follicle that could continue to grow outward. Subsequent evolution led to novel shapes, forms, and function. Had flight not evolved, there is a good chance that feathers would still have evolved for other functions. That flight evolved is one of the most remarkable evolutionary events. Not only does it assist birds in invading new habitats and escaping predators, but it humbles man to see this effortless form of travel only accomplished by humans with the less elegant airplane. Flight, however, came at a cost for birds, since they have lost digits on two limbs. They have adjusted remarkably to this and live happy lives. But what happens in death? The question comedian Steven Wright poses remains unanswered – “Do bird angels have four wings?”

References

Brush, A.H. (2001). The beginnings of feathers. pp. 171-179 in New Perspectives on the Origin and Early Evolution of Birds, J Gauthier and L.F. Gall (eds.), Peabody Museum Special Publications, New Haven, CT.

Prum, R.O. and A.H. Brush. (2002). The evolutionary origin and diversification of feathers. Quart. Rev. Biol. 77: 261-295.

At the time of press a reprint of the Prum and Brush 2002 paper could be downloaded at:

http://www.mcorriss.com/Prum_&_Brush_2002.pdf

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