Nature's Way

If living things existed on other earth-like worlds, what shape would they take? Some scientists have speculated that they would look like nothing on earth. Biochemist Joseph Kraut of the University of California at San Diego suggests a different view. He proposes that life evolving on planets with environments similar to the earth's would quite likely resemble familiar terrestrial forms.

Kraut says that his speculations are merely "vaporings," but he supports them with some striking laboratory evidence. For the past 15 years he has headed a scientific team looking into the three-dimensional structure of enzymes, the long-chained proteins that act as catalysts in all the chemical reactions necessary for life. The group's latest interest has been an enzyme called subtilisin, which is found in ordinary soil bacteria. As they investigated subtilisin's complex structure, the scientists realized that it had a curious similarity to another enzyme, chymotrypsin, common to all vertebrates, including man. While the overall molecular architecture of the two enzymes is quite different, they both have three identical groups of amino acids that form what Kraut calls their "business ends." It is at these spots that the chemicals involved in vital reactions are brought together.

Borrowing a phrase from classical biology, Kraut calls the discovery the first known instance of "convergent evolution" on the molecular scale. In other words, "nature has invented the same piece of molecular machinery to do a particular job in two separate and independent instances." Kraut speculates that this convergence in the evolution of the enzymes is more than a coincidence. The genetic code and the basic building blocks of life (amino acids and proteins) are already known to be universal, he says. Thus Kraut's discovery is further evidence of what may eventually be accepted as a scientific fact of life: given the same problem, nature will find the same solution.

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