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NO HELPING HANDS: The spontaneous appearance of an RNA-like molecule without the assistance of a chemist faces immense implausibilities. "Metabolism first" theories suggest how simpler molecules could develop into living systems without needing that kind of replicator molecule.
© MEDIOIMAGES/CORBIS

Extraordinary discoveries inspire extraordinary claims. Thus James Watson reported that, immediately after they had uncovered the structure of DNA, Francis Crick "winged into the Eagle (pub) to tell everyone within hearing that we had discovered the secret of life." Their structure--an elegant double helix--almost merited such enthusiasm. Its proportions permitted information storage in a language in which four chemicals, called bases, played the same role as twenty six letters do in the English language.

Further, the information was stored in two long chains, each of which specified the contents of its partner. This arrangement suggested a mechanism for reproduction, that was subsequently illustrated in many biochemistry texts, as well as on a tie that my wife bought for me at a crafts fair: The two strands of the DNA double helix parted company. As they did so, new DNA building blocks, called nucleotides, lined up along the separated strands and linked up. Two double helices now existed in place of one, each a replica of the original.

The Watson-Crick structure triggered an avalanche of discoveries about the way in which living cells function today. These insights also stimulated speculations about life's origins. Nobel Laureate H. J. Muller wrote that the gene material was "living material, the present-day representative of the first life," which Carl Sagan visualized as "a primitive free-living naked gene situated in a dilute solution of organic matter." In this context, "organic" specifies material containing bound carbon atoms. Organic chemistry, a subject sometimes feared by pre-medical students, is the chemistry of carbon compounds, both those present in life and those playing no part in life. Many different definitions of life have been proposed. Muller's remark would be in accord with what has been called the NASA definition of life: Life is a self-sustained chemical system capable of undergoing Darwinian evolution.

Richard Dawkins elaborated on this image of the earliest living entity in his book The Selfish Gene: "At some point a particularly remarkable molecule was formed by accident. We will call it the Replicator. It may not have been the biggest or the most complex molecule around, but it had the extraordinary property of being able to create copies of itself." When Dawkins wrote these words 30 years ago, DNA was the most likely candidate for this role. As we shall see, several other replicators have now been suggested.

When RNA Ruled the World

Unfortunately, complications soon set in. DNA replication cannot proceed without the assistance of a number of proteins--members of a family of large molecules that are chemically very different from DNA. Proteins, like DNA, are constructed by linking subunits, amino acids in this case, together to form a long chain. Cells employ twenty of these building blocks in the proteins that they make, affording a variety of products capable of performing many different tasks--proteins are the handymen of the living cell. Their most famous subclass, the enzymes, act as expeditors, speeding up chemical processes that would otherwise take place too slowly to be of use to life.

The above account brings to mind the old riddle: Which came first, the chicken or the egg? DNA holds the recipe for protein construction. Yet that information cannot be retrieved or copied without the assistance of proteins. Which large molecule, then, appeared first in getting life started--proteins (the chicken) or DNA (the egg)?

A possible solution appeared when attention shifted to a new champion--RNA. This versatile class of molecule is, like DNA, assembled of nucleotide building blocks, but plays many roles in our cells. Certain RNAs ferry information from DNA to structures (which themselves are largely built of other kinds of RNA) that construct proteins. In carrying out its various duties, RNA can take on the form of a double helix that resembles DNA, or of a folded single strand, much like a protein. In 2006 the Nobel prizes in both chemistry and medicine were awarded for discoveries concerning the role of RNA in editing and censoring DNA instructions. Warren E. Leary could write in the New York Times that RNA "is swiftly emerging from the shadows of its better-known cousin DNA."