Out of Thin Air

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Out of Thin Air by Peter D. Ward

If you love “Gosh-Wow!” science – science that expands the mind, presents the big picture, stuns the imagination – then this is one of the “Gosh-Wowiest!” of “Gosh-Wow!” science.

Author Peter D. Ward starts with an unusual and striking premise that leaves the reader walking on existential egg-shells. Ward posits that the big, big driver of evolution has been the periods when the oxygen content of the Earth's atmosphere plunged to levels of extreme oxygen deprivation, such that a creature during some of the Earth's geological epochs living at sea level would experience the same oxygen deprivation that today someone climbing to the top of Mr. Everest would experience. Ward offers this example:

“At the end of the Permian just living at sea level would have been equivalent today to breathing at 15,000 feet, a height greater than that found atop Mount Rainer in Washington State. Thus even low altitudes during the Permian would have exacerbated this, so that even a modest set of hills would have isolated all but the most low-oxygen-tolerant animals. The result would be a world composed of numerous endemic centers hugging the sea-level coastlines. The high plateaus of many continents may have been without animal life save for the most altitude tolerant. This goes against expectation based on continental position.”
And:

“The drop in oxygen did more than make mountain ranges barriers to migration. It made most areas higher than 3,000 feet uninhabitable during the late Permian-Triassic time interval.”

Stop there for a moment....basically, a low coastal mountain range would have been an impassible barrier to life in the past. What kind of alien planet are we talking about?

Ward explains that scientific modeling of atmospheric oxygen content provides circumstantial evidence of oxygen levels. The reason for the changes in atmosphere levels has to do with the rate of burial of organic carbon and sulfur bearing compounds. Sometimes carbon is buried by plate tectonics, or in the case of the Carboniferous, by the massive evolution of vegetation and the lag in the evolution of bacteria able to consume the complex chemicals in the “recently” evolved trees. The result was the massive lock-up of carbon in buried forests that we now exploit as coal. The result of that, plus the massive production of oxygen by plankton and vegetation, raised oxygen levels to an unseemly 30% which was responsible for the development of huge insects and massive forest fires. Ward describes the period as follows:

“The first noticeable characteristic is the color of the sky. It is a polluted yellow-brown, irrespective of weather; only in high winds does the air clear and then soon it muddies again. This is due to smoke from giant fires perpetually raging and new ones set alight with each lightning strike hitting the extensive forests of the temperate and tropical regions. But there is more than soot in the air; there is fine wind-blown dust as well, accumulating as loess, or glacial front deposits. The air is cold over much of the planet, for this is the time of one of the most extensive glaciations in Earth's history, with ice caps at each pole and continental glaciers reaching icy fingers across the land as they snake downward out of the mountains onto the plains and river valleys. Where there are forests we find unending vistas of conifer-like trees, for the gymnosperms have by now evolved and have swept away many of the earlier dominant plants. No longer do tall but shallow-rooted primitive trees like Lepidodendron dot the landscape. It is much more familiar, or familiar at least to those of us who now live in the high northern or southern latitudes where pines and fir trees dominate the forests.”

The result of this was mayflies with 19 inch wingspans and spiders with 18 inch legs.

Alternatively, there have been times when volcanoes and traps have polluted the atmosphere with carbon dioxide and other noxious gases, and oxygen levels have lowered to catastrophic levels. Ward's thesis is that the times when oxygen levels have been reduced to extremely low levels are the periods marked by the “Great Extinctions.” Further, during these periods, evolution favors those species that obtain better results in respiration. Sometimes these changes involve changes in the apparatus of respiration, but other times, the advantage is given by morphological changes. Thus, for example, lizards have a poor morphology for breathing since their side to side motion means that they cannot run and breathe at the same time. This is not much of a problem when oxygen levels are high, but when they drop the organism that can make an improvement on respiration can hunt, evade and reproduce better than the competition.

One morphological change that worked was the development of an upright stance on the part of the early dinosaurs. Being upright, their breathing was not affected by the lizards side-to-side motion and they were able to run and breath, which was a major advantage when oxygen levels increased and those animals that were already pre-adapted to better respiration were able to exploit their new morphologies for other advantages.

One of Ward's thesis is that the primary reason for the change in body plans is respiration, with all other considerations coming in as a lucky second. Thus, crabs evolved during another downturn in oxygen. The primary advantage of the crab morphology isn't the obvious advantage of armor and an unexposed thorax, but the fact that it creates a better plan for sucking more water over the gills in order to extract more oxygen out of more water.

Ward also provides an idiosyncratic theory on the “warm blooded dinosaur” debate. Ward does not think that dinosaurs were necessarily warm-blooded. They evolved in a unique period of low oxygen and high temperature, such that being warm blooded all the time would not have been an evolutionary advantage. Ward writes:

“And the conditions of the Triassic may have been such that, thanks to highly elevated carbon dioxide levels, greenhouse heating may have kept the temperatures virtually equal day and night—and hot to boot. The Triassic climate was one suited for reptiles—hot. That heat would actually have been a problem for very large endotherms. Large dinosaurs (greater than a ton, such as most sauropods) would have overheated in even moderate temperatures, and the Triassic environment was anything but moderate. So here it is suggested that cold-blooded dinosaurs would have been a condition actually more favorable than warm-bloodedness for dinosaurs, mainly because of the large differ ence in oxygen needed while at rest. In the Triassic and into the Jurassic, ectothermy would not have relegated dinosaurs to a sluggish life style. With the absence of nasal turbinals characteristic of modern-day endotherms, the case for ectothermy is stronger than that for endothermy.”

And:

“What are we left with? A kind of animal unknown on Earth today. An ectotherm with phenomenally rapid growth rates and a lung system that, while inferior to the best of modern-day birds, was more efficient at extracting the thin oxygen available than those of other denizens of the day. Superiority of the dinosaurs in the latest Triassic and then into Jurassic through the Cretaceous was made possible by being better than everyone else. A return to those times might be surprising indeed, with animals showing behavior that is not mammalian or avian, not a sluggish existence but something in between. Perhaps the earliest dinosaurs were something like lions, sleeping 20 hours a day to conserve energy because of the low oxygen, but when hunting doing so actively, more actively than any of their competitors, which would have included the nondinosaur archosaurs, the cynodonts, and the first true mammals. All they needed to be was better than the rest. Clearly they were.”

This is fascinating, engaging book. The writing is journeyman. I found the first part somewhat lackluster, but one Ward got into the evolutionary perspective it was absolutely fascinating. For me the changing oxygen levels seems somewhat speculative, and I wondered how much crackpottery there is in Ward's thesis, but Ward hammers away with example after example, such that, by the end, granting him his inconstant atmosphere, I'm willing to keep an open-mind. I wonder whether 30 years from now, Ward's view of Earth's atmosphere is going to be as commonplace as the idea that the continents slide around like pool balls. My biggest take-away was the inconstant nature of the Earth's atmosphere, and the possibility that the Earth's future might, like it's past, be oxygen-starved.