As the Sun grew brighter, the oxygen produced by living organisms destroyed these compounds, decreasing the greenhouse effect and thus compensating for the increased radiation from the Sun. In this model, the faint warmth of the early Sun was aided by a greenhouse effect produced by ammonia and methane in the atmosphere (both of these, like the more familiar carbon dioxide, CO2, are greenhouse gases). In 1978, the astrophysicist Michael Hart, then at Trinity University in Texas, published a computer model that described the history of Earth’s atmosphere. Others, however, adapted and were able to use the oxygen to drive the respiration cycle that keeps you and every other animal on the planet alive today. Presumably, many original inhabitants of the planet that could not tolerate oxygen then went extinct, drowned in their own waste products.
At first, this oxygen was removed by chemical reactions, such as the rusting of iron in surface rocks, but about 2.5 billion years ago, its abundance began to increase in what some scientists call the Great Oxidation Event.
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At that time, there was virtually no free oxygen in the atmosphere, but the bacteria were giving off oxygen as a waste product of photosynthesis (as plants still do today). To take just one example of atmospheric change, we know that 3.5 billion years ago Earth’s oceans were home to thriving colonies of cyanobacteria-much like what we call green pond scum.
Yet in spite of all of this, it appears that the oceans stayed just a few degrees above freezing throughout Earth’s history. As time went on and the Sun poured more energy onto Earth, the makeup of the planet’s atmosphere changed as well, influencing the temperature through the greenhouse effect. When the oceans first formed on Earth, about 4 billion years ago, the Sun was about 30 percent dimmer than it is now, so the planet had to retain a lot more of the incoming solar energy to keep its oceans from freezing. Look at it this way: Like all stars of its type, our Sun has grown gradually brighter over the 4.5 billion years since it formed. It’s not surprising, then, that when scientists began thinking about the fact that Earth’s oceans had to stay liquid for billions of years in order for life to survive-the planet’s temperature had to be not too hot and not too cold but just right-they christened it the first “Goldilocks planet.” We all remember the nursery story “Goldilocks and the Three Bears.” We delight in telling our children and grandchildren about how Papa Bear’s porridge was too hot, Mama Bear’s porridge was too cold, but Baby Bear’s porridge was juust right. Plus49/Construction Photography/Avalon/Getty Images Scientists seeking extraterrestrial life in the universe (above: a radio observatory in New Mexico) seek the answer to what is called the Fermi paradox: “Where is everybody?”
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