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Of all the riddles that have stirred the human imagination, none has provoked more lyrical speculation, more religious awe, more contentious debate. No other moment in time, aside from the Big Bang that began the universe, could be more central to the understanding of nature than the instant that life began. "Scientific" theories on the subject are as old as civilization. The ancient Egyptians believed frogs and toads arose from silt deposited by the flooding Nile. The Greek philosopher Aristotle taught that insects and worms were born of dewdrops and slime, that mice were generated by dank soil and that eels and fish sprang forth from sand, mud and putrefying algae. In the 19th century, electricity, magnetism and radiation were believed to have the ability to quicken nonliving matter.
It took the conceptual might of Charles Darwin to imagine a biologically plausible scenario for life's emergence. In an oft quoted letter, written in 1871, Darwin suggested that life arose in a "warm little pond" where a rich brew of organic chemicals, over eons of time, might have given rise to the first simple organisms. For the next century, Darwin's agreeable hypothesis, expanded upon by other theorists, dominated thinking on the subject. Researchers decided that the "pond" was really the ocean and began trying to figure out where the building blocks of life could have come from.
In 1953 University of Chicago graduate student Stanley Miller provided the first widely accepted experimental evidence. In a glass jar he created a comic-strip version of primitive earth. Water for the ocean. Methane, ammonia and hydrogen for the atmosphere. Sparks for lightning and other forms of electrical discharge. One week later he found in his jar a sticky goop of organic chemicals, including large quantities of amino acids, Lego blocks for the proteins that make up cells. Case closed, or nearly so, many scientists believed.
Now this textbook picture of how life originated, so familiar to college students just a generation ago, is under serious attack. New insights into planetary formation have made it increasingly doubtful that clouds of methane and ammonia ever dominated the atmosphere of primitive earth. And although Miller's famous experiment produced the components of proteins, more and more researchers believe that a genetic master molecule -- probably RNA -- arose before proteins did.
Meanwhile, older and older fossils have all but proved that life did not evolve at the leisurely pace Darwin envisioned. Perhaps most intriguing of all, the discovery of organisms living in oceanic hot springs has provided a Stygian alternative to Darwin's peaceful picture. Life, says microbiologist Karl Stetter of the University of Regensburg in Germany, may not have formed in a nice, warm pond, but in "a hot pressure cooker."
If scientists have, by and large, tossed out the old ideas, they have not yet reached a consensus on the new. The current version of the story of life is a complex tale with many solid facts, many holes and no shortage of competing theories on how to fill in the missing pieces.
ONCE UPON A TIME