"Out of Thin Air": A breath of fresh air in scientific writing

"Out of Thin Air: Dinosaurs, Birds, and Earth's Ancient Atmosphere"
by Peter D. Ward
Joseph Henry, 282 pp., $27.95

Through several previous books, including "Life As We Do Not Know It" and "Rare Earth" (the latter in collaboration with Donald Brownlee), University of Washington paleontologist Peter D. Ward has developed a loyal readership. His audience has come to expect books built around original questions that lead to new insights about the origin and evolution of life — and about the worlds on which such life can develop.

Rarely will scientists propose new hypotheses in popular books, but Ward does so comfortably, as if trying them out among friends before publishing them in peer-reviewed journals. His readers always relish his vivid descriptions and insights into cutting-edge science.

His latest effort, "Out of Thin Air," will reward them with more of the same. The central question of this book is this: How have changing levels of oxygen in Earth's atmosphere influenced the path of evolution?

His conclusion is that respiratory demands have played a dominant role in driving the evolution of novel body structures. Earth's primordial atmosphere had virtually no oxygen. Animal life developed only after photosynthetic bacteria and plants began producing oxygen faster than geologic processes could remove it.

Other than life itself, the dominant processes influencing Earth's atmospheric oxygen concentration are the carbon and sulfur cycles. Trying to deduce the oxygen level at a given epoch in the planet's history is difficult, relying on limited geological information and extensive computer modeling. The models rely on what scientists have been able to deduce about changing continental arrangements, the variability of global climate, periods of increased volcanism and variations in solar light and heat due to changes in Earth's orbit and in the sun itself.

The models indicate that during the roughly half-billion years of animal life on Earth, atmospheric oxygen content has ranged between 12 and 30 percent. At the lower limit, animal life at sea level had to adapt to respiratory conditions similar to those tolerated by today's high-altitude species, such as the many species of birds that can fly over the highest mountain ranges.

The book traces Earth's changing ecology, both on land and in the water, through periods of mass extinction and explosive speciation, examining the potential evolutionary effects of changing oxygen levels. When oxygen was low, many species went extinct and diversity decreased, while evolutionary innovation enabled other species to survive. As oxygen content increased, improvement of existing body structures, rather than innovation of new ones, drove evolution. Life became more diverse as new species radiated to fill ecological niches that were vacated in the preceding extinction.

Scientists agree that changing oxygen level was a factor in the history of Earth's evolving life, but many will dispute Ward's argument for its primacy. Still, even his scientific critics will appreciate his question and the productive research directions it opens up.

Fred Bortz (www.fredbortz.com), the author of 15 children's science books, frequently visits schools to talk about life on a changing planet.