Alan Brush enjoys a compelling narrative on the discoveries that have illuminated the complexities and evolution of plumage.
Feathers: The Evolution of a Natural Miracle
- Thor Hanson
This book, about the natural history of feathers, begins with Archaeopteryx. This late-Jurassic (about 150-million-year-old) fossil, something between a reptile and bird, confounded and delighted evolutionists Charles Darwin and Thomas Huxley and palaeontologist Richard Owen. A small beast with reptilian teeth, long tail and skeletal features of both groups, it had clearly identifiable feathers with modern shape and structure. Archaeopteryx feathers were identical to those that today fascinate laymen, ornithologists, fashionistas and casual collectors.
Thor Hanson's storytelling is enhanced by his infectious excitement. In Feathers, he interviews the leading proponents on all sides of the controversies that surround the origin and evolution of feathers and the birds that produce them.
Hanson explains the physics of how feather structures interact with light to produce amazing iridescent colours. He catalogues how different feathers with the same hollow, branching structure provide insulation (down), protection (contour), aerodynamic surfaces (wing and tail) and sensory input (filoplumes). Plumage helps with species identification, dictates behaviour and provides the spectacular decorations that birders enjoy. Hanson also traces the long arguments between advocates of the 'ground-up' and 'tree-down' theories of how the first birds took to the air, and the alternative 'wing-assisted incline running' hypothesis.
Thanks to feathers' myriad qualities, people have used them as quill pens to sign significant historical papers, as badges and advertisements, on clothing and in fishing flies and as stuffing for garments, mattresses, quilts and cushions. Hanson's style makes the concepts of chemical morphogenesis, coherent scattering and reaction-diffusion waves accessible. His narrative is accompanied by a small number of diagrams and images and an appendix illustrating feather types. That it is not a picture book is an accomplishment when writing about colourful plumage and exotic behaviours.
In the 1970s, debate on the evolutionary origin of birds was reinvigorated by the US palaeontologist John Ostrom, who posited that birds, as vertebrates with feathers, were related to theropod dinosaurs. Ostrom's claim was based on fossil evidence and supplemented by others' work on metabolism and behaviour. But for decades, the controversial argument that birds and dinosaurs were related lacked a key element: an evolutionary history of feathers.
At the time, the dogma was that all birds — and only birds — have feathers. This changed in the 1990s, with the discovery of 'feathered dinosaurs' from the Yixian Formation in Liaoning Province in China. The rich fossil findings stimulated a re-evaluation of the evolutionary history of both feathers and the animals that bear them. Phylogenetic analysis confirmed that theropods and birds are sister groups, and the feather structures on the Yixian fossils provided direct evidence for the evolution of feathers. These findings complemented other data from ontogeny, molecular biology and morphology. Finally, a clear picture of the evolution of feathers has emerged.
Hanson's tale is comprehensive, accurate, timely and engaging. One thing missing is the story of the technical breakthroughs that led to the understanding of feather structure (keratin) and genomics. The fact that feathers are insoluble is partly because of their structure — they are made from highly organized filaments — and partly because of their amino-acid composition (they contain many stable intra- and intermolecular disulphide bonds).
In the late 1960s, a group in the protein-chemistry division of the Commonwealth Scientific and Industrial Research Organisation in Australia isolated and identified the soluble monomer of feather keratin, and revealed the characteristics of the gene family involved. Ornithologists quickly became interested. This accomplishment provided ways to test directly the 'feathers arose from scales' hypothesis and to map molecular evolution more widely onto lineages derived from other features. Comparative work on the proteins of the other epidermal structures, such as claws, scales and beaks, soon followed.
Feathers is a compelling introduction to one of nature's wonders.