Sophisticated remote-sensing technology aids scientists in their quest for life on far-off planets. Telescopes measure how these planets reflect both the visible light and heat of their stars, which can reveal the chemical makeup of the planets' atmospheres. Some scientists believe that this information can confirm whether the planets have telltale "fingerprints," or specific blends of chemicals, that could potentially support life—at least life as we know it to exist on Earth.

But these scientists don't just consider the atmospheric makeup that supports the types of life that exist on Earth today. Along with the life that has inhabited it, Earth's atmosphere has also evolved. Scientists think that it was completely oxygen-free for the first 2 billion years or so. In fact, the gases that made up our initial atmosphere, including the carbon dioxide and ammonia compounds that came from ancient volcanic eruptions, would be toxic to most of today's life forms. It wasn't until about 2.5 billion years ago that primitive blue-green bacteria began producing oxygen through the process of photosynthesis. Once the oxygen began to accumulate in the atmosphere, an ozone layer formed. Concentrated ozone shields Earth from the Sun's harmful ultraviolet rays, allowing life to survive on land. Once terrestrial plants started to flourish, Earth's atmosphere began to change to the oxygen-rich blend that exists today.

That Earth's atmosphere has supported different kinds of life as it has evolved demonstrates that the presence of atmospheric oxygen isn't a prerequisite for a planet to support life. In fact, scientists generally point to carbon as being more fundamental to life. Carbon is the molecular basis of amino acids—life's building blocks. Among life's other essentials is the presence of liquid water, or some other fluid medium in which chemical reactions can occur. Within our solar system, Earth is the only planet whose atmosphere regulates temperatures such that liquid water can exist on its surface.

Earth's oldest discovered cellular fossils were single-celled bacteria—just about the simplest of all life forms, but ones that would later diversify and evolve into multicellular organisms. Knowledge of the wide range of conditions in which microbes flourish on Earth also guides scientists as they consider where life might exist elsewhere within the solar system. For example, so-called "extremophiles" are organisms that inhabit environments with conditions at temperatures, pressures, and pH more extreme than those typically found on Earth's surface. These environments include caves containing highly acidic hydrogen sulfide in the air, and deep-sea vents, where the water reaches temperatures of 375°C (707°F), and is rich in dissolved minerals, including iron.

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