by Leslie E. Orgel
Growing evidence supports the idea that the emergence of catalytic RNA was a crucial early step. How that RNA came into being remains unknown.
LESLIE E. ORGEL is senior fellow and research professor at the Salk Institute for Biological Studies in San Diego, which he joined in 1965. He obtained his Ph.D. in chemistry from the University of Oxford in 1951 and subsequently became a reader in chemistry at the University of Cambridge. While at Cambridge, he contributed to the development of ligand- field theory. The National Aeronautics and Space Administration supports his extensive research on chemistry that may be relevant to the origin of life. Orgel is a fellow of the Royal Society and a member of the National Academy of Sciences.
When the earth formed some 4.6 billion years ago, it was a lifeless, inhospitable place. A billion years later it was teeming with organisms resembling blue-green algae. How did they get there? How, in short, did life begin? This long-standing question continues to generate fascinating conjectures and ingenious experiments, many of which center on the possibility that the advent of self-replicating RNA was a critical milestone on the road to life.
Before the mid-17th century, most people believed that God had created humankind and other higher organisms and that insects, frogs and other small creatures could arise spontaneously in mud or decaying matter. For the next two centuries, those ideas were subjected to increasingly severe criticism, and in the mid-19th century two important scientific advances set the stage for modern discussions of the origin of life.
In one advance Louis Pasteur discredited the concept of spontaneous generation. He offered proof that even bacteria and other microorganisms arise from parents resembling themselves. He thereby highlighted an intriguing question: How did the first generation of each species come into existence?
The second advance, the theory of natural selection, suggested an answer. According to this proposal, set forth by Charles Darwin and Alfred Russel Wallace, some of the differences between individuals in a population are heritable. When the environment changes, individuals bearing traits that provide the best adaptation to the new environment meet with the greatest reproductive success. Consequently, the next generation contains an increased percentage of well-adapted individuals displaying the helpful characteristics. In other words, environmental pressures select adaptive traits for perpetuation.
Repeated generation after generation, natural selection could thus lead to the evolution of complex organisms from simple ones. The theory therefore implied that all current life-forms could have evolved from a single, simple progenitor – an organism now referred to as life’s last common ancestor. (This life-form is said to be “last” not “first” because it is the nearest shared ancestor of all contemporary organisms; more distant ancestors must have appeared earlier.)
Darwin, bending somewhat to the religious biases of his time, posited in the final paragraph of The Origin of Species that “the Creator” originally breathed life “into a few forms or into one.” Then evolution rook over: “From so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved.” In private correspondence, however, he suggested life could have arisen through chemistry, “in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc. present.” For much of the 20th century, origin-of-life research has aimed to flesh out Darwin’s private hypothesis – to elucidate how, without supernatural intervention, spontaneous interaction of the relatively simple molecules dissolved in the lakes or oceans of the prebiotic world could have yielded life’s last common ancestor.
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