- Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere
- Joseph Henry Press, 282 pp.
The Role of Oxygen In Evolution
It is easy to think we know – that science knows – pretty much all there is to know about the past. Aside from the great mystery of the Big Bang, and how the initial spark of life started us off on the planet, it’s all detail work, right?
In an age when ad agencies regularly apply “revolutionary” to new car models and digital toys, it is wise for the rest of us to avoid the word, but Peter Ward’s Out of Thin Air comes as close to meriting the label as anything I’ve seen of late. Paleontology does involve a lot of detail work, from tiny picks and toothbrushes to radioactive dating; however, some details may not only inform but overturn and reinvent the much bigger picture.
Ward is a professor at the University of Washington in the fields of biology, earth and space studies, and astronomy, and therefore well suited to think about how small things like the skeletal structure of reptiles and the breathing mechanisms of tiny sea creatures may tell us about something as large as changes in the planet’s atmosphere.
If you saw “Winged Migration,” you may recall a brief sequence of birds tucking calmly into the snow for a nap in the high winds atop the Himalayas. What are they doing up there? How can they breathe?
Humans who struggle to the summit of Mt. Everest’s 29,029 feet have looked up and seen birds flying unconcernedly above them, doing just fine at an altitude where humans can barely get enough oxygen standing still and gasping. How do they do it?
In fact, bird lungs are at least 33 percent more efficient than any mammal lung. Why should this be? And more intriguingly, knowing they have descended from the dinosaurs, is it possible that birds’ ability to operate in (for us) mortally challenging levels of low oxygen might say something about their ancestors’ methods of respiration and how they evolved?
I’ll cut to the chase. Ward’s main thesis is:
The history of atmospheric (and hence oceanic) oxygen levels through time has been the most important factor in determining the nature of animal life on Earth – its morphology and basic body plans, physiology, evolutionary history, and diversity.
He contends that dinosaurs evolved and prospered – in fact, came to dominate the planet – during eras when oxygen levels fell to unprecedentedly low levels and extinguished thousands of species in other forms, including mammals. Contrary to our notion that mammals took over when a catastrophic event or mélange of events crippled the dinosaurs, those bygone giants may have begun their ascent to the top of the food chain through an earlier disaster that wiped out most mammals of the time.
Ward observes that we assume other ages and creatures were much like ours and us, but by now we should know better. After all, there are animals today who live without oxygen – at the bottom of the ocean and within the earth – and those pesky birds who flap over Chomolungma.
Today, Earth’s air consists of 78 percent nitrogen, 21 percent oxygen, and another 1 percent that’s a variety of gases from carbon dioxide to water vapor which can have huge effects despite their relative scarceness. This is not typical of Earth’s atmosphere through past eras, however.
Obviously, there was a time, early in the planet’s life, when there was little or no oxygen. The atmosphere was CO2-rich, hothouse conditions prevailed, and iron could oxidize only partially. Where did O2 come from? Probably via photosynthesis performed by single-celled bacteria.
But that doesn’t mean it’s been a steady march upward to our current 21 percent: 5 million years ago, oxygen levels were as high as 28 percent, and less than 100 million years ago, they were much lower than today. And things got even more bumpy before that: at the start of the Cambrian Explosion, 544 million years ago, oxygen levels were about 13 percent; by the Carboniferous/early Permian 300 million years ago, it topped out at 35 percent. By the early Triassic, however, they were down to 12 percent or lower!
The planet’s oceans have undergone periods of hypoxia (low oxygen) and even anoxia (no oxygen), and, Ward notes, small changes in the temperature or chemistry of the oceans can significantly alter the atmosphere.
A significant portion of Ward’s analysis parallels recent discoveries about prehistoric waves of mass extinctions. Most people know the dinosaurs were wiped out by something – a meteor strike or some other big climate change – and may possibly be aware of a previous general die-off: the Permian Extinction, 250 million years ago, nearly four times as old as the Cretaceous Extinction that was the dinosaurs’ swan song.
But there have been many more. Over the past 500 million years, life on Earth has been punctuated by at least 15 waves of mass deaths, during which huge numbers of plants and animals quickly went extinct. Five involved more than half of all fossil-producing species.
The Cretaceous Extinction, also known as the K/T extinction event, was clearly sparked by an asteroid strike, probably the one that left the Chicxulub Crater on the Yucatan Peninsula of Mexico. But extinctions from the K/T event were selective: though most dinosaurs and photosynthesizing organisms suffered heavy losses, ominivores, insectivores, and carrion-eaters didn’t do too badly.
By contrast, at least 90 percent of all species were wiped out in the Permian Extinction, at a more gradual speed than the K/T, and the cause is still unknown. Ward has identified one big player, and perhaps even the prime culprit, however: the extinction followed one of the greatest drops in O2 and jumps in CO2 in Earth’s history. In fact, four of the five biggest mass extinctions in Earth’s known history correlate to periods of low O2, or at least a 10 percent drop in atmospheric oxygen. This usually correlated with high carbon dioxide and hot temperatures, which heighten the stress that low oxygen puts on aerobic organisms.
The discussion can get technical at times. Ward delves into the implications of trilobite gills, sulfur and carbon cycles, the evolution of endothermy (warm-blooded metabolism), segmented body designs as a reflection of respiratory needs, reproductive strategies (eggs or live birth?), effects of tectonic shifts on oxygen levels, alveolar versus septate lung design, development of turbinals (small, blade-like bones in the nasal passages that serve as potential evidence for endothermy), the significance of four-chambered hearts over three chambers, the effects of O2 levels on species migration and gene flow, upright posture in dinosaurs (Ward suggests the need to breathe while walking may have driven this innovation), and the possibility that structural evolution in chambered nautiluses, clams, and crabs were primarily for respiratory efficiency rather than locomotion or physical protection.
Fortunately, talk of such biological and geological minutiae, from the general reader’s perspective, never lasts long. The narrative periodically surfaces to take a snapshot of the planet’s conditions at a given period, and then it seems as if one has stepped into a science fiction novel.
Just before the Cambrian Explosion, land consisted of not much more than bare rock with little vegetation: no soil, no trees, no plants, no flowers, no bugs, no amphibians, no fish. The sun and moon were dimmer but the moon was also much closer, so there were huge tides. Air pressure was similar, but oxygen levels were low. The only life to speak of were bits of moss and red and brown algae on the rocks, although the oceans had photosynthesizing bacteria, sponges, jellyfish, anemone-like creatures, and worm-like forms on the bottom.
By the Carboniferous, the sky turned a dirty yellow-brown due to smoke from giant fires caused by lightning strikes on the forests. The continents had all collided into one super-continent. Dragonflies had 20-inch wings, mayflies were 19 inches across, spiders walked on 18-inch legs, scorpions were three feet long, and amphibians reached ten feet in length – all due to high oxygen levels.
At the end of the Permian, the world turned hot and desert-like. There was no ice at the poles, and great fields of dunes on land. Harsh, hot winds blew across them and there was little plant life. The largest animals were no bigger than a lamb; they were inactive and stayed close to sea level, and very few species lived in the ocean as well. Even insects were rare. The air reeked of rotten eggs.
Following the most devastating of all the mass extinctions, the Triassic Explosion saw the greatest evolutionary experimentation ever seen on Earth, resulting in the greatest variety of animal forms and species. Early on, carnivorous cynodonts resembled mammals in appearance and behavior, except that they moved very little and tired easily – in fact, most land creatures spent much of their time nearly motionless, yet panting – because of the oxygen-bleak atmosphere. The first dinosaurs appeared, and somehow proliferated quickly in number and exploded in physical size.
All of this constructs a fairly coherent narrative from a variety of disparate facts and theories we may have picked up about the ancient past in school and in the news – floating continents, Pangea, asteroid strikes, mass extinctions, and warm-blooded versus cold-blooded dinosaurs.
Ward even treats us, if that is the right verb, to a forecast of the future, given his theories of atmospheric and species development and their relationship to geologic change. In the next 250 million years, the northern land masses will move out of the high latitudes and Antarctica will shift from the south to converge in another supercontinent. As part of that change, the Mediterranean will disappear and a mountain range will spring up to reach from what we know as Europe to the Persian Gulf. Australia will close with Papua New Guinea and Indonesia. Baja will shift north along the Pacific Coast of the U.S. Mountain building will continue in the Appalachians and along the east coast of South America.
Sea levels will rise. Antarctic and Greenland ice will melt, and ocean levels will climb about 300 feet, but Antarctica and Australia will remain separated from the supercontinent by an inland sea.
Given what Ward has shown us of past changes, however, when the supercontinent begins to break up, oxygen levels will most likely plummet, and another Permian-style mass extinction could result. (You might want to sell your beachfront property at Malibu or Nice and relocate to high ground in Nunavut.)
Out of Thin Air may strike one initially as a catchy, trendy title. It is in fact scientifically precise. The book is not easy sledding, but it is fascinating and marvelously enlightening about massive changes in terrestrial geography, atmosphere, flora, and fauna throughout time, and inarguably offers “a new insight about the levels of that most necessary and most poisonous of gases, giver and taker of life, oxygen.”