After millions of years of gloriously successful life on Earth, a dangerous new organism arose and spread rapidly across the planet. With unprecedented efficiency, this revolutionary life form pumped noxious fumes into the air, destroyed ecosystems and exterminated a substantial fraction of its fellow species. And the gasses it added to the atmosphere drastically altered global temperatures so that, between habitat destruction and climate change, the world was changed forever.
Mankind? No! These events happened 2 billion years ago. The delinquent organisms were cyanobacteria, the first photosynthetic life forms to give off pure oxygen gas.
Oxygen, a highly reactive chemical, was deadly to all extant organisms and also destroyed methane, a potent greenhouse gas that helped keep our planet warm under the fainter sun of the distant past. Cyanobacteria precipitated one of the biggest disasters ever to hit our biosphere, but this apocalypse was also a vital step in creating today's world: a world of large, energy-hungry organisms utterly reliant on oxygen, and a world where photosynthesis allows life to flourish in every nook and cranny on the face of the planet.
There may be surprising parallels between the, eventually positive, cyanobacteria impact 2 billion years ago and human impact today. Human beings too are a self-inflicted biosphere disaster in progress, but, in the extremely long-term, we could be just what the planet needs. The key to understanding this is climate change. By that, I don't mean the trifling meddling we're currently engaged in, disastrous though that may be for our grandchildren, but climate change on a much grander scale.
Across the billions of years of life's existence on Earth, our atmospheric composition has repeatedly changed whilst the ability of our planet to absorb heat has fluctuated as the amounts of sea, clouds, bare rock and verdant land have changed in response to geological and biological evolution. At the same time the aging sun has steadily grown brighter.
Despite these changes, which dwarf anything achieved by Homo sapiens, Earth has enjoyed 4 billion years of good weather because the fluctuations somehow cancelled each other out. I find that remarkable. Not once did Earth cool enough to freeze completely or become so warm that our seas utterly evaporated. Staying lukewarm and wet was not inevitable -- although it was essential for the continuation of life. Our nearest neighbors in space, Venus and Mars, show that global desiccation is possible since neither world has retained the liquid water of its youth; the waters of Venus have been lost into space whilst those of Mars are now permanently frozen beneath its surface.
The obvious explanation for Earth's good fortune is that there are stabilizing mechanisms, built-in processes that stop temperatures from getting too high or too low. But it's also true that planets wet enough to be suitable for life have inherently unstable climates. Water, when it freezes, is so white that cold, wet planets reflect a great deal of heat into space and become colder still. Water vapor, on the other hand, is a powerful greenhouse gas so that a bit of warming, and the resulting increased evaporation of seawater, is a self-reinforcing process.
This climate change amplification makes Earth's early habitability surprising. The young sun was only 70 percent as bright as it is now and our planet should have been deeply frozen. The mystery of how the young Earth nevertheless managed to be life-friendly was called the "faint young sun paradox" by Carl Sagan 40 years ago, but it has a straightforward resolution: either early Earth was relatively dark, and therefore absorbed a greater fraction of the sun's heat, or its atmosphere contained more greenhouse gasses.
As the sun aged and grew brighter, our world needed to become more reflective, or lose greenhouse gasses, to avoid overheating. How did it manage this trick? One reason is that those cyanobacteria, and the methane destruction they caused, appeared just in the nick of time. Other developments helped too. The steady growth of continents, as volcanic residues accumulated at Earth's surface, produced a steady increase in overall planetary reflectivity since land is brighter than sea. Increasing continental area also reduced carbon dioxide levels because, when dissolved in rainwater, the gas chemically reacts with rocks. The appearance of lichens 600 million years ago enhanced this process further as the roots of these early land-colonizers broke the rocks up. Then, 400 million years ago, land plants first appeared and these, too, cooled the planet since woody material was buried when the plants died and the carbon in that wood came from carbon dioxide in the air. We truly live on the best of all possible worlds.
The idea that this is nature finding its balance is beguiling and deeply ingrained in our culture -- despite being widely discredited by modern ecological research. In the context of long-term climate change, this idea finds expression as the Gaia hypothesis, the proposal that a complex biosphere naturally evolves in ways that maintain or enhance that stability. I disagree. Instead, I'd suggest that a complex biosphere is the consequence, rather than the cause, of environmental stability. The visible universe contains more planets than there are grains in a cubic mile of fine sand and so it is not surprising if, on a few worlds, fortuitous cancellations maintain life-friendly conditions. Earth is one of those rare, lucky planets.
Of course, this is a brutal and chilling vision of an uncaring universe. On most worlds where life appears it is wiped out, either by over-heating or over-cooling, long before anything as interesting as a multi-celled animal appears. Even worse, a few worlds may remain clement long enough for intelligent life to appear -- only to be wiped out by climate catastrophe before they have the technical ability to counter the threat. We, however, do have this ability and that changes everything.
Like cyanobacteria when they first appeared, we're a major hazard to other life forms, but we could eventually become an asset to Earth. Even if no earlier climate catastrophe threatens, our ever-warming sun will eventually render our planet uninhabitably warm and only we, or rather our distant descendants, can save it.
But that's for the unimaginably far-off future. In the meantime, we have much to learn before we become guardians rather than despoilers of Earth. We must take baby steps. What's a better way to start than habitat creation? Reversing, rather than halting, deforestation could be our first aim followed by constructing artificial reefs to repair some of the damage we've inflicted on the oceans. If our destiny is to safeguard life's future, it's time our apprenticeship began.