Out of Thin Air: Dinosaurs, Birds, And Earth's Ancient Atmosphere [Anglais] [Relié]

Mountain climbers struggling to breathe astride the 29,029 foot (8,848 meter) summit of Mt. Everest routinely see birds gracefully flying above them, engaging in nonchalant aerial acrobatics at altitudes where humans risk hypoxia (oxygen starvation) while standing still.

The avian respiratory system is at least 33% more efficient than any mammalian lung. Birds combine lungs with an extensive system of air sacs - permitting a unidirectional airflow of 'fresh' air with a higher oxygen content. Mammals are saddled with bidirectional lungs that mix 'fresh' and 'stale' (carbon dioxide-laden) air.

Since birds descended from dinosaurs - they are avian dinosaurs - what does this say about dinosaurian respiration, the world in which they evolved, and more specifically the atmospheric chemistry of the planet they came to dominate?

"Out of Thin Air: Dinosaurs, Birds, and Earth's Ancient Atmosphere" by Peter Ward hypothesizes that the history of atmospheric and oceanic oxygen levels throughout geologic time has profoundly impacted the nature of animal life on Earth - everything from morphology (body plans) and physiology to evolutionary history and diversity - was contingent on oxygen levels which have varied radically over time.

Ward, a paleontology professor at the University of Washington, and a NASA staff astrobiologist, is an expert in paleo-atmospheric chemistry and supports his claims with ample and compelling evidence.

Earth's atmosphere presently consists of 78% nitrogen and 21% oxygen, the final 1% composed of various gases; carbon dioxide being the most notable and problematic. 4.54 billion years ago Earth's atmosphere was a hothouse dominated by carbon dioxide. Oxygen was so scarce that Iron could not rust. Photosynthetic cyanobacteria introduced oxygen into Earth's atmosphere - precipitating an oxygen crisis - the first known mass extinction.

Since the advent of photosynthesis atmospheric oxygen levels hare varied considerably. Only 5 million years ago (MYA) oxygen levels hit 28%. The early Cambrian (544 MYA) averaged 13% and levels peaked during the Carboniferous - Permian transition (299 MYA) at 35%. By the Permian - Triassic boundary (251 MYA) oxygen levels plummeted to less than 12%.

Mass extinctions periodically reshape life on Earth. The best known, the Cretaceous - Tertiary (K-T) boundary, ended the reign of the non-avian dinosaurs approximately 65 MYA when an asteroid roughly 10 kilometers wide gouged the Chicxulub crater near the Yucatan Peninsula, setting the stage for mammals, including Homo sapiens, to become the dominant terrestrial vertebrates.

Another extinction event, the Permian - Triassic (P-Tr), some 251 MYA, is informally known as 'the Great Dying.' Up to 96 percent of all marine species and 70 percent of terrestrial species were erased as global ecosystems crumbled. Life itself nearly died as a greenhouse gas spike caused temperatures to soar 10 - 30 degrees Celsius (18 - 54 degrees Fahrenheit), and oxygen levels plummeted when the oceans became the anoxic (without-oxygen) abode of methanogenic and sulfate-reducing microorganisms - amplifying global warming (methane is 10 times more efficient than carbon dioxide at trapping heat in the atmosphere) and poisoning plant and animal life with deadly hydrogen sulfide (rotten egg gas). The sky literally turned a sickly shade of green, a topic ably covered in Ward's superb Under a Green Sky: Global Warming, the Mass Extinctions of the Past, and What They Can Tell Us About Our Future.

Since dinosaurs evolved in the early Triassic - a period of suffocatingly low (to mammals) oxygen levels - any evolutionary innovations that enhanced respiratory efficiency would provide a compelling advantage. Ward contends that dinosaurs eclipsed the dominant Therapsida (mammal-like reptiles) and early mammals by evolving the unidirectional airflow lung and air sack respiratory system utilized by their avian descendents. In the Cynognathus vs. Eoraptor world of the early Triassic the race was to the swift and battle to the strong - our ancestors lost. Therapsids went extinct, early mammals retreated to niches where their respiratory and metabolic systems could cope with Triassic atmospheric conditions, and the reign of the dinosaurs began.

Along the way Ward lucidly engages a wide array of topics to make his case. The impact of continental drift (plate tectonics) and geochemistry (sulfur and carbon cycles) on oxygen levels are explored. Segmented body plans as a respiratory strategy, gills (trilobite, cephalopod, and decapod), and lungs of every variety (from alveolar to septate) are contrasted. The advent of endothermy (warm-blooded metabolism), evidence for same (turbinal bones in mammal-like reptiles and early mammals), and associated reproductive strategies (eggs vs. live birth) also illuminate Ward's insights. Circulatory advances (four-chamber hearts), even the upright posture of dinosaurs (Ward suggests the need to breathe while walking drove this innovation) are deftly dropped into a compelling evidentiary mosaic.

"Out of Thin Air" is more than a trendy title - the science shows how the dinosaurs literally emerged as a result of 'thin' air due to near-hypoxic atmospheric oxygen levels prevailing throughout the P-Tr transition. Dinosaur enthusiasts will be enthralled and mystery lovers will applaud Ward's 'science as the ultimate sleuth' approach to deciphering the history of life on Earth. Ward's Gorgon: The Monsters That Ruled the Planet Before Dinosaurs and How They Died in the Greatest Catastrophe in Earth's History makes an excellent companion volume.

Ward's book is really quite interesting to explain the "logic" of life's development on the earth, starting with the first animals (540 MA). Everything is linked to a timeline showing the rise and fall of oxygen levels over the geologic eras. It would be fatal to the book's premise, I believe, if subsequent research drastically revises this timeline.

As for criticism of Ward's writing style, there were creative forays in his writing that I most enjoyed. On a number of occasions, he takes us on an imaginary trip to visit Earth at a particular era. We are in some sort of conveyance that is boat, submarine, and plane. Like a tour guide, he explains what we are seeing -- bare rocks covered with moss and lichens, the faint haze of hydrogen sulphide in the air, the first primitive pre-phyla of the Burgess shale slowly moving across the sea bottom.

There is some repetition -- this can be criticized, but can also be helpful if one does not whiz through the book rapidly, but goes back every few days for another bite. This is not a thriller, but a rather challenging book of lay science. It is filled with mouthfilling Latinate words. A little extra help by way of some selective repetion is not that objectionable, I think.

One aspect of the book that is radically new is the analysis of the physiology of various prehistoric families of creatures. Their livers, their lungs, their feathers, their bone structure. Only in fairly recent times has this sort of discussion even been possible, and the field is sort of a "terra incognita."

Because the book covers new ground, it will remain to be seen how will the findings hold up in decades to come.

I found it intelligent, lively, and filled with new assertions and new insights. I do NOT agree with one reviewer that the book is too expensive. I got my copy from Amazon for a considerable discount from the nominal price.

Buy it if you think you will enjoy it.

Did periods of low oxygen in Earth's ocean and atmosphere - `thin air' - drive the evolution of animals? Ward meticulously correlates oxygen levels with virtually all animal species' evolutions on land and sea from the Cambrian thru-out the following half-billion years. Altho the timings of the oxygen/carbon-dioxide levels versus ancient animals' ages are both still somewhat speculative, Ward's theory seems to be the most plausible explanation I've read so far.

What caught my attention and attracted me to this book was the realization that birds migrate over the Himalayas (the book's dust-jacket and chapter headings picture Eurasian cranes in flight) while the fittest of our species struggle in the thin air to reach those heights. What enables birds to do that? Ward traces birds' respiratory system's origin to the pre-avian dinosaurs and says that at sea level birds' is a third more efficient than mammals' and at a mile high theirs is two times more efficient. However I was disappointed that he doesn't explain why birds' dinosaur ancestors survived the K-T extinction 65mya which killed off all the other dinosaurs, or how they evolved into today's birds. His focus is more on us mammals.

Some reviewers grumble that Ward's prose is flawed which impeded their reading. Granted it's a little rough but the fact that he's breaking new ground and promptly delivering the results to us, should earn him some latitude. The scope and novelty of his research is impressive, let's not quibble about its form. Perhaps his fault is that he rushed to publish his `first draft' rather than take the time to polish it, but I'm glad he did altho as I said, I think he wraps it up too hastily. (His "Under a Green Sky" was published just 5 months later - I'll tackle it next.)