Oxygen: The Molecule that Made the World by Nick Lane

Natural History Magazine Review, July/August 2003

Professor Lawrence A Marschall, M.D., Ph.D., WK. T Sahm professor of physics at Gettysburg College in Pennsylvania

Our Earth is an odd place. No other planet in the solar system has so much oxygen in its atmosphere. Although oxygen is a relatively common element in the universe, its atoms are so reactive that they never float around by themselves. Pump an atmosphere full of monatomic oxygen, chemists will tell you, and every unattached oxygen atom will immediately rush off to find a mate, combining with iron to form rust, with carbon to form carbon dioxide, with hydrogen to form water. Even the diatomic molecule ([O.sub.2]) that occurs in the air we breathe is reactive enough to form ferric and carbonate compounds. Those compounds should be abundant on a planet, but pure oxygen, whether monatomic or molecular, should be vanishingly scarce--as it is, in fact, on Mars and Venus.

When Earth's atmosphere formed 4 billion years ago, pure oxygen was probably rare here, too. Hyperactive volcanoes, more common during Earth's early years than they are now, cloaked the primordial planet with nitrogen, sulfur dioxide, carbon dioxide, and water vapor. But sometime around 2 billion years ago the oxygen presence began to increase, soon reaching levels of between 5 and 18 percent of what we have today. The evolution of life, of course, was what made that possible. Photosynthetic organisms, using chlorophyll and other pigments to capture sunlight, were turning carbon dioxide and water into molecular oxygen. As those new forms of life flourished, oxygen began to be pumped ever faster into the atmosphere. Earth blossomed with life, the composition of its atmosphere closely coupled to the evolutionary process that was taking place in its seas and on its continents. With every breath we take, we benefit from that momentous chain of events.

Nick Lane, an English biochemist, has written a meticulously detailed history of oxygen on our planet, organized around two major themes. The first is the tricky problem of how the presence of atmospheric oxygen has risen to its present level over time. Alternating layers of red- and black-banded ironstone, as well as coal beds, provide some benchmarks, because the rusting of iron and the fossilization of coal depend on how much oxygen is present in the air. Pockets of "ancient atmosphere," trapped in 100-million-year-old amber, may add direct evidence: in the mid-1980s geochemists using a quadrupole mass spectrometer measured the oxygen in amber samples from various geologic periods and reported that atmospheric [O.sub.2] levels had been higher than 30 percent during the Cretaceous Period, nearly twice what they are today. (According to Lane, however, the controversy is still raging within the scientific community about whether the air inside amber has been hermetically sealed since the time it solidified from drops of tree sap.) And the size of fossilized insects provides tantalizing, albeit indirect, clues that the atmospheric oxygen level may have been much higher during the age of the dinosaurs.

Only with such high levels of oxygen, Lane argues, could a Carboniferous dragonfly called Meganeura, which had a wingspan of almost a yard, have been able to fly. (The largest modern dragonflies, by comparison, are no more than four inches across.) In the end, such high oxygen levels may have been lowered by a worldwide firestorm that ended the age of the dinosaurs 65 million years ago. According to this scenario, the highly oxygenated atmosphere was sparked by an incoming asteroid, and the forests lit up as if they were tissue paper.

Lane then jumps to a second major theme: the role of oxygen in health and longevity. Fragments of molecules containing oxygen, called free radicals, form during the natural process of respiration. Free radicals can chemically alter the normal functioning of cells. According to a growing body of evidence, the accumulated damage plays an important role in aging. Oxygen, in effect, is a highly reactive fuel; the cell, in relying on oxygen to promote life, courts death and, sooner or later, succumbs. Lane's book ranges widely over a host of topics, from the usefulness of antioxidants such as vitamin C in curing colds, to the potential for prolonging human life with enzymes that repair damaged DNA. And it turns out that the jump from the geologic theme in the first part of the book to the medical theme in the second is not as great as it seems. A unifying thread of Lane's narrative, fascinating in its irony, binds it all together: oxygen, essential element of life, is also an agent of death.

Laurence A. Marschall, author of The Supernova Story, is the WK. T Sahm professor of physics at Gettysburg College in Pennsylvania, and director of Project CLEA, which produces widely used simulation software for education in astronomy.