The answer is a little more complicated than you may think. It may have a lot to do with rocks, phosphorous and ancient algae, according to a new study.
For the first two billion years of Earth's history or so, the sky was probably orange. We're not sure whether that's really true -- no one's been able to hop in a time machine and go back and check -- but based on what we know about the chemistry of that time period, there's a good chance the atmosphere's primary component was methane (CH4), which would've cast a strange pall over our young planet.
These days, the atmosphere is mostly nitrogen and oxygen. Sunlight is made up of all the colors of the rainbow (as well as many wavelengths we can't see); as it jostles through air molecules, blue light is most efficiently reflected, so our eyes end up experiencing a beautiful azure shade.
How did it change from orange to blue? About 2.5 billion years ago, the newest fad in organisms was photosynthesis -- the ability to to turn sunlight, carbon dioxide (CO2) and water into sugar. Armed with the latest evolutionary accoutrement, ancient algae had it made -- an everlasting food source and all the world's oceans to expand into.
Only one problem. Algae need more than sugar for a balanced diet; they need nutrients like phosphorous, too. Dominic Papineau of the Carnegie Institution for Science thinks they got it in a burst of erosion from 2.5 to 2 billion years ago, a period of time when Earth's atmosphere got its first big injection of oxygen.
The way Papineau sees it, the "Great Oxidation Event" lines up nicely with a rise in continental rifting and widespread glacial deposits. So it's possible that enhanced tectonic activity and a change in climate eroded large amounts of phosphorous-rich rocks, which washed into the ocean over a period of several hundred million years.
With plenty of phosphorous to munch, algae were off to the races, churning out oxygen that flooded the atmosphere, Papineau reasons in this press release. It's not unlike humans' prodigious use of fertilizers today, which can cause large algal blooms in rivers, lakes and even the Gulf of Mexico:
"Today, this is happening very fast and is caused by us," he says, "and the glut of organic matter actually consumes oxygen. But during the Proterozoic this occurred over timescales of hundreds of millions of years and progressively led to an oxygenated atmosphere."
The first episode only got us about 10 percent of the way toward present-day oxygen levels, though. It wasn't until about a billion years ago that the atmosphere got another hit of O2, bringing us to the air we breathe today. This period, from 1 billion to 540 million years ago, is known as the "Cambrian explosion" after the riot of diverse life found in the fossil record.
In some ways, it's one of the most important moments in the history of life on Earth. Organisms went on a rampage of evolutionary innovation, giving rise to complex life forms the likes of which the planet had never seen before, and Papineau thinks phosphorous was behind it:
"This increased oxygen no doubt had major consequences for the evolution of complex life. It can be expected that modern changes will also strongly perturb evolution," (Papineau) adds. "However, new lineages of complex life-forms take millions to tens of millions of years to adapt. In the meantime, we may be facing significant extinctions from the quick changes we are causing."
by "environment clean generations"
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