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submarine hot springs situated some distance from the deep oceanic ridges. Scientists thought that four billion years ago life could have emerged there from a mass of bubbles, each bubble containing hot mineral-laden solutions.
    Around the turn of the twenty-first century, the research vessel Atlantis and its human-occupied submersible Alvin found this exact type of geyser about nine miles (fifteen kilometers) from the Mid-Atlantic Ridge. Dubbed the Lost City, these vents stood like ornate structures up to two hundred feet (sixty meters) in height, poking up into the vast darkness. At this depth hydrogen could more freely bind to carbon dioxide to form organic molecules. First life was not a single cell but a rocky labyrinth of mineral cells that produced complex molecules, including the formation of proteins and eventually DNA molecules, generated by the energy of the warm vent fluids.
    As we came to the end of our bird walk at Cary, Schlesinger said that this made sense. He had one caveat: he favored a more neutralsolution for first life. “Life can tolerate a wide range of pH, but really acid conditions [low pH] are likely to oxidize organic materials and really alkaline conditions break down cell membranes,” he explained.
OXYGEN MAKES IT HAPPEN
    Most scientists agree that, for the first few billion years, life was largely microbial. Yet these little critters were responsible for most of the genetic heavy lifting. Though we marvel at the size and anatomical complexity of large animals, these features were made possible by cell biology and genetics that were developed in single-cell creatures in much earlier times. According to Harvard’s Andy Knoll, when complex life first evolved, it had the majority of its DNA already worked out.
    For life to really get going, to produce the complex forms of more evolved beings, it had to have oxygen. Two and a half billion years ago, “life” was still in bacterial forms. It had its genetic architecture, but it survived in oxygen-free environments, so it stayed small. But then some of the oxygen-free bacteria evolved into cyanobacteria or blue-green algae, the stuff you sometimes see on polluted waters, commonly referred to as “pond scum.”
    These guys promoted photosynthesis, a different type of metabolism from what their archaic brethren employed. Photosynthesis used sunshine, water, and carbon dioxide to produce carbohydrates and, finally, oxygen. At last, the giraffes and basketball players of the world had a chance at survival!
    Oxygen was the critical element in the burst of evolution that occurred during the Cambrian explosion about 570 to 530 million years ago, when most of the major animal groups suddenly appear in the fossil record. At the time the air was murky, since there wasn’t enough oxygen to scrub the atmosphere of haze and dust. Without enough oxygen, there was no ozone, either, so the searing intensity of ultraviolet light from the sun could fall without obstruction. Ultravioletlight breaks up water (H 2 O), and since hydrogen (H) is so light, it can slip into space, and there goes your ocean. Without oxygen holding on to hydrogen, the world today might look a lot like Mars: a dry, dusty, pockmarked planet with no seas, lakes, rivers, or streams and no visible sign of life.
    Oxygen gradually accumulated on earth from the photosynthesis of plants. Once oxygen reached critical mass, changes were sudden. If you look at the paleontological record in the soil, there is evidence of oxygen-free microbes in one layer, followed closely by oxygen-dependent microbes in another layer. This introduction of oxygen, though a boon to most life, spelled destruction for a good deal of earth’s early ancestors who excelled without it.
    Oxygen made the planet livable. Once established, oxygen patrolled the atmosphere capturing all the hydrogen atoms trying to get away and turned them back into water and rain. Now an ozone shield could form, dampening the intensity of