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OVER the years, palaeontologist Peter Ward has become an able populariser of the Earth鈥檚 history and a big-picture thinker who pulls together diverse threads to weave intriguing theories about the history of life on Earth and the potential for life on other planets. Among his most provocative ideas is that simple life may be common in the universe, but complex organisms are extremely rare, a position well argued in an earlier book written with Donald Brownlee, Rare Earth: Why complex life is uncommon in the universe.

In Out of Thin Air, Ward offers another bold hypothesis: that variations in atmospheric oxygen levels drove animal evolution. His opening scene describes flocks of geese flying thousands of feet over Mount Everest. It鈥檚 a dramatic illustration of how birds have evolved lungs that extract oxygen from the air more efficiently than humans or other mammals. From there, Ward argues that the evolution of particularly efficient lungs gave early dinosaurs a leg up in the evolutionary race some 200 million years ago, when the air contained as little as 12 per cent oxygen. Birds evolved from dinosaurs and retain that advantage, he writes, but now that oxygen levels have reached 21 per cent, mammals can compete despite their less efficient lungs.

The logic linking animal evolution to oxygen levels is compelling. Animals need oxygen as much as they need food. Though we have no direct way to measure past oxygen levels, Yale University geologist Bob Berner has spent his career developing indirect methods of measurement and plotting variations in atmospheric oxygen over the past half-billion years. Comparing Berner鈥檚 data to the fossil record, Ward found that new species were most likely to emerge when oxygen levels dropped to their lowest values, forcing animals to evolve better ways to use oxygen in order to survive. Ward also correlates high-oxygen periods with the evolution of big animals, from giant dragonflies to the biggest dinosaurs.

鈥淣ew species emerged when oxygen levels were lowest鈥�

Ward鈥檚 best evidence comes from the dramatic rise of atmospheric oxygen levels to a peak of more than 30 per cent about 280 million years ago and the subsequent dip to a minimum of 12 per cent at the end of the Triassic period roughly 200 million years ago. In the middle came the biggest extinction of all time, an event Ward blames on massive eruptions of hydrogen sulphide from the oceans, which poisoned the air and left the ancestors of mammals gasping for breath. The Triassic period that followed, when oxygen levels were at a minimum, was a hotbed of evolution, bringing forth the two-legged theropod dinosaurs which later gave rise to birds, other types of dinosaurs, crocodiles, the flying pterosaurs and marine reptiles.

A key question that remains open is how important respiration was compared to other factors, and how it correlates with success today. Ward says nothing about how widely lung efficiency varies among birds. Do high-flying long-distance migrators make better use of oxygen than non-migratory birds? What about flightless birds? How do the lungs of bats compare with those of other mammals? Have modern oxygen levels reduced the importance of efficient respiration? I鈥檇 like to know what Ward thinks.

The book鈥檚 biggest weakness is a sloppiness that at times can be disconcerting. No book can cover all aspects of animal evolution, but it鈥檚 unfortunate Ward did not examine the enigmatic Ediacara fauna that lived from about 580 to 540 million years ago; their unique flat-bodied design might have illuminated the origins of respiration. The writing and editing feels rushed. A misquote has another writer saying that some dinosaurs 鈥渆xhibited no vertebrate body鈥�. And I chuckled to read that an asteroid impact produced 鈥渟hocked quarts鈥� rather than shocked quartz. It鈥檚 a good, thoughtful read, but it should have been better.