A New View of Evolution

How did life begin on the primordial earth? Scientists looking for clues within the living cell have been stymied by a catch-22. All living organisms are composed largely of proteins, which consist of strings of amino acids manufactured within cells in small granules called ribosomes. But ribosomes are themselves highly complex protein structures that obviously evolved long after protein first appeared. Then how, without the complex ribosome "factories," was primitive protein produced? Last week, in a report published in the journal Origin of Life, a team of molecular biologists suggested an answer. If the hypothesis is correct, says one of the researchers, it could alter Darwin's theory of natural selection and current concepts of genetic engineering.

The report's findings are highly technical and abstruse. But the names of the authors ensure that the paper will be taken seriously by scientists around the world. The team, which did its work at Cambridge, England, was led by Nobel Laureate Francis Crick, who with James Watson determined the structure of life's master molecule, DNA. His co-workers were Sydney Brenner, who discovered the "start" and "stop" signals in the genetic code; Aaron Klug, who first determined the crystalline structure of transfer RNA (tRNA), the molecule that brings amino acids to the ribosome for assembly into protein; and George Pieczenik, 32, a biochemist now at Rutgers University in New Brunswick, N.J.

The quartet based its work on the discovery by Pieczenik that there are certain constraints on the way that the nucleotides, or "letters," of the genetic message are arranged. These patterns could have arisen, they found, if primitive tRNA molecules each had five nucleotides interacting with the genetic message instead of the three that now do. With five nucleotides, tRNA molecules--each lugging along its distinctive amino acid--could link up firmly with a messenger RNA molecule (which brings the genetic instructions from the DNA molecule). The amino acids could thus be assembled into the appropriate protein without the aid of a ribosome. Contemporary tRNA molecules, unaided, cannot form a stable linkage with messenger RNA; a ribosome is needed to hold them in place until their amino acids are assembled. While the scientists admit that their concept is "fairly speculative," they note that it can be tested in the laboratory if researchers can construct some five-nucleotide tRNA molecules. If the theory is correct, these engineered molecules, placed in a test tube with amino acids and messenger RNA, should synthesize proteins.

Updating Darwin. Pieczenik believes there is further significance in the DNA patterns he discovered. In his view, the constraints suggest that a process of natural selection occurs at the molecular level long before organisms develop. If this is true, some additions will have to be made to the Darwinian theory that natural selection takes place only after the organism is formed and begins adapting to the world around it. That notion does not seem to bother Pieczenik. "What this means," he says, "is that the DNA sequences exist to protect themselves and their own information. It's not the organism that counts. The DNA sequences don't really care if they have to look like a lowly assistant professor or a giraffe." If that is indeed the case, he concludes, the DNA sequences will also resist outside attempts to recode them and thus probably thwart attainment of many of the long-sought goals of genetic engineering.

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