Scientists have discovered the first evidence that the life span of an animal -- a type of roundworm that lives in the ground -- can be lengthened dramatically by altering the function of a normal gene.

The discovery, described in today's edition of the journal Science, may help geneticists in their search for a similar gene in other animals, including humans, the report's author said.

"Most optimistically, it could mean that identical genes could be found in humans," said Thomas E. Johnson, a geneticist at the University of Colorado. "If that's the case, we may be able to dramatically intervene in lengthening life span. But it's probably a far-fetched scenario. There are fundamental differences {between the species}."

Johnson's study looked at different strains of the nematode known as C. elegans, a type of roundworm. Strains with a mutated version of the gene, called age-1, had an average life span at least 63 percent longer than any other strain. Some worms with the mutant gene lived twice a normal life span.

Scientists have long known about mutations of genes that shortened life, such as the human gene that, when defective, causes the disorder known as phenylketonuria, or PKU. And other studies have found that strains of some animals, such as the fruit fly, lived longer than normal by carrying mutations of several different genes. But this is the first time that anyone has pinpointed one specific gene that shortens life when in its normal form, Johnson said.

The Science article describes two studies Johnson conducted in 1986. The first study involved 144 worms with the mutated gene and 383 with a normal gene. It found that the mutant worms lived an average of 37 days, while the others ranged from 18 to 23 days.

The second study, which gave even more dramatic results, involved 145 mutant worms and 752 with the normal gene. It found that the mutants had an average life span of 56 days, compared with 19 to 27 days for the others.

Johnson said he doesn't know the exact genetic function of age-1. But his study suggests the mutant gene may slow the rate at which mortality increases over time. Starting at age three days, the nonmutant worms' death rate doubled every 4 to 5 days, on average, while the mutant worms' death rate doubled every 8 to 14 days.

But Johnson also said he was extremely skeptical that similar life-extension could be achieved with larger species. Nematodes have relatively few types of cells and the mutant gene may not have any effect in many of the additional types of cells in more complex animals. Also, little is known about the worms' natural causes of death.

But he added that just knowing about such a gene might help human geneticists in their search for life-shortening genes.

"It may have absolutely no relevance to anything having to do with humans," he said. "On the other hand, most of what we have learned about basic genetics come from studies of lower organisms."

He said he waited a few years to publish his initial data because he was so surprised that one gene could have such an impact and wanted to do more studies to complete his findings.

"The data was sitting around because I was amazed that a single mutation could have such an effect," he said.