Twenty-five years ago, Francis Crick, who codiscovcred the structure of DNA, published a provocative book titled Life Itself" Its Origin and Nature. Crick speculated that early in Earth's history a civilization from a distant planet had sent a spaceship to Earth bearing the seeds of life. Whether or not Crick was serious about his proposal, it dramatized the difficulties then plaguing the theory that life originated from chemical reactions on Earth. Crick noted two major questions for the theory. The first one--seemingly unanswerable at the time--was how genetic polymers such as RNA came to direct protein synthesis, a process fundamental to life. After all, in contemporary life-forms, RNA translates genetic information encoded by DNA into instructions for making proteins.

The second question was, What was the composition of Earth's early atmosphere? Many planetary scientists at the time viewed Earth's earliest atmosphere as rich in carbon dioxide. More important, they were also skeptical about a key assumption made by many chemists who were investigating life's origin--namely that Earth's early atmosphere was highly "reducing," or rich in methane, ammonia, and possibly even free hydrogen. In a widely publicized experiment done in 1953, the chemists Stanley L. Miller of the University of California, San Diego, and Harold C. Urey had demonstrated that in such an atmosphere, organic, or carbon-based, compounds could readily form and accumulate in a "prebiotic soup" But if a highly reducing atmosphere was destined for the scientific dustbin, so was the origin-of-life scenario to which it gave rise.

In Crick's mind, the most inventive way to solve both problems was to assume that life had not evolved on Earth, but had come here from some other location--a view that still begs the question of how life evolved elsewhere.

Crick was neither the first nor the last to try to explain life's origin with creative speculation. Given so many difficult and unanswered questions about life's earthly origin, one can easily understand why so many investigators become frustrated and give in to speculative fantasies. But even the most sober attempts to reconstruct how life evolved on Earth is a scientific exercise fraught with guesswork. The evidence required to understand our planet's prebiotic environment, and the events that led to the first living systems, is scant and hard to decipher. Few geological traces of Earth's conditions at the time of life's origin remain today. Nor is there any fossil record of the evolutionary processes preceding the first cells. Yet, despite such seemingly insurmountable obstacles, heated debates persist over how life emerged. The inventory of current views on life's origin reveals a broad assortment of opposing positions. They range from the suggestion that life originated on Mars and came to Earth aboard meteorites, to the idea that life emerged from "metabolic" molecular networks, fueled by hydrogen released during the formation of minerals in hot volcanic settings.

This flurry of popular ideas has often distracted attention from what is still the most scientifically plausible theory of life's origin, the "heterotrophic" theory. The theory holds that the first living entities evolved "abiotically"--or from systems of nonliving organic molecules present on the primitive Earth. (The term "heterotrophic" was originally coined to describe a kind of metabolism in which "nutrients" such as carbon and nitrogen must be obtained from nature as complex organic molecules such as amino acids, rather than from extremely simple compounds such as carbon dioxide.) According to the theory, organic molecules such as amino acids were chemically combined in a prebiotic soup and "cooked" by various sources of energy. True, some of the details of Miller and Urey's recipe for prebiotic soup presented difficulties, such as the ones Crick highlighted. But abandoning the premise of a prebiotic soup when new findings largely support its account of life's origin is to "throw the baby out with the bathwater."

One strong argument in favor of the heterotrophic theory is the surprising variety of biochemical constituents that emerge in laboratory simulations of Earth's prebiotic environment, and the remarkable similarity between them and the constituents of some carbon-rich meteorites. On September 28, 1969, for instance, a meteorite landed in Murchison, Australia, carrying nearly eighty kinds of amino acids. Among them were several amino acids that occur in proteins. Also embedded in the Murchison meteorite were purines, pyrimidines, carboxylic acids, and compounds derived from ribose and deoxyribose, the sugars present in RNA and DNA. (In fact, ribose is the "R" of RNA, deoxyribose the "D" of DNA.) Such relics of the early solar system provide insight into the kind of organic chemistry that took place some 4.6 billion years ago.