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Thursday, Dec. 26, 2002

NATURAL SELECTIONS

It came from the alphabet soup


"In the beginning God created the heavens and the Earth. And the Earth was without form and void. And darkness was on the face of the deep. And the Spirit of God moved on the face of the waters."

This is how the Bible describes the beginning of the world. For a long time that, and other creation myths, were the best explanations we had. For the subject of the beginning of life was the airy concern of philosophers and theologians. Scientists didn't have much to go on.

The earliest fossils on Earth, clumps of bacteria called stromatolites, are found in Western Australia, and are 3.5 billion years old. They might be simple compared to life now, but single-celled bacteria have a cell membrane, allowing them to regulate their internal environment and metabolize chemicals to gain energy.

Old as stromatolites are, though, the Earth is more ancient -- around 4.5 billion years old. Stromatolites were unlikely to have been this planet's first life form: Something able to do all that they could is unlikely just to have popped into existence. So what was?

Don't expect a quick answer. The origin of life is one of the three great mysteries remaining in biology (the other two are determining why organisms have sex, and building a theory of consciousness). But scientists have found ways to tackle the problem of the origin of life experimentally, and last week a paper in Nature authored by two Californian scientists showed a possible solution.

All known life forms (apart from a few viruses) use DNA to make RNA, which in turn makes proteins and enzymes that carry out operations directed by DNA. DNA stores information using strings of four nucleotides, identified as "A," "G," "C" and "T." The puzzle of this system, from an origin-of-life point of view, is its chicken-and-egg nature: Which came first, the enzymes that carry out the instructions, or the instructions themselves? The current favored theory is that neither came first.

RNA is the single-stranded version of the more famous DNA, whose double helix structure was elucidated by James Watson and Francis Crick in 1953. RNA has been considered a likely "origin of life" candidate. This is because it can store genetic information on its single strand (using the same building blocks that DNA uses) and, crucially, it can self-catalyze the reading of this information. RNA can act like an enzyme as well as carry the instructions to make the enzyme.

"It's pretty clear that there was a time when life was based on RNA," said Gerald Joyce, of the department of molecular biology at The Scripps Research Institute, San Diego, and one of the authors of the Nature paper. "Not just because it's feasible that RNA can be a gene and an enzyme and can evolve, but because we really think it happened."

However, RNA is probably not the very first molecule of life, because one of its four bases -- "C" -- is chemically unstable. It easily degrades into another base, "U" ("U" is the RNA equivalent of DNA's "T"). Moreover, it's thought that there was not enough "C" on early Earth for a four-base genetic system to have been feasible.

Francis Crick saw this problem nearly 40 years ago and suggested life might have started with two bases instead of four. Now Joyce, and colleague John Reader, have demonstrated that a two-base RNA system is chemically workable.

Joyce and Reader created an enzyme based on a binary genetic code: one containing only two different nucleotides, not four as in DNA and contemporary RNA. Their binary enzyme, using the nucleotides "A" and "U," is able to string together pieces of RNA that are composed of the same two nucleotide symbols. In the test tube, the binary string works as an enzyme by folding into a three-dimensional structure and using a portion of the string as a template. On the template it ligates subunits together, copying the template. Darwinian evolution by natural selection occurs in the test tube, because strings that are better at replicating themselves start to outnumber the others.

The rate at which the binary string catalyzes the formation of linkages between two RNA molecules is slow compared to "real," modern-day enzymes, but Reader and Joyce show that it is 36,000 times faster than the uncatalyzed reaction. It is easy to understand how a molecule with this catalytic ability arising in the soup of the early Earth would rapidly copy itself and spread.

The binary string has the simplest possible composition for an information-containing molecule. Could it be the same as the molecule that started life?

Reader and Joyce don't go that far. They emphasize that their study does not prove life started this way. It does, however, demonstrate that it is possible to have a genetic system of molecules capable of undergoing Darwinian evolution with only two distinct subunits.

And it allows us to visualize the spirit of life moving across the primeval waters: RNA molecules, quietly replicating.

Rowan Hooper welcomes comments at rowhoop@gol.com


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