Biologists have been startled in recent months to discover that the genetic code -- the language by which genes govern the processes of life -- is not the same for all species.

Since the 1960s, when the genetic code was cracked, biologists have been deeply impressed by evidence that every form of life they tested used the same code. Human genes, they found, speak precisely the same language as do bacterial genes that arose perhaps a billion years ago, although, of course, human cells write many additional sentences.

Because biologists knew other codes were theoretically possible, they saw this apparent "unity of life" as powerful confirmation that all living things evolved from a single ancestor. Had life started several times, several genetic codes might be in use today.

Now, however, researchers in the United States and France have found there is, after all, a second genetic code.

It is only slightly different, and it affects so few species that its discovery is likely to have no practical consequence. But the discovery does pose a difficult challenge to evolution theorists.

The similarity of the two codes strongly indicates that the second evolved from the first. There is no hint of a second origin of life. The challenge, then, is to explain how even a slight variation in the code could arise without garbling the message of all the genes.

Every aspect of an organism, from the structure of its various proteins to the nature and arrangement of organs, is dictated by genes. Change the code and every gene is threatened.

The variant code is used by two widely different groups of organisms. One is a bacterium called Mycoplasma. The other includes three members of a group of one-celled protozoans fringed with hairlike cilia, including Paramecium, a common pond-water species studied in many schools. The alteration in the genetic code involves the "punctuation marks" that the otherwise universal code uses to indicate the end of a genetic sentence -- the "stop" code, as geneticists call it by analogy with old-style telegrams.

All other species have three different "words" that mean "stop," but the variant code has changed the meaning of two of these to something different. Instead of stopping the reading of the genetic sentence, they act as words within the sentence, specifying certain building blocks to be used in the construction of the protein molecule for which the gene codes.

The presumed universality of the original genetic code was what led biotechnologists to put human genes in bacterial cells. The bacteria species they used have no trouble reading the code.

Now it appears it was simply luck that allowed such recombinant DNA experiments to succeed. Had the researchers chosen the bacterial species with the variant code, the bacterium would have read the "stop" code as meaning something else and produced useless products, if anything.

"When we first saw this, we thought we had made a mistake. It just couldn't be," said John R. Preer Jr. of Indiana University, leader of one of the American groups. "We kept doing the work over and over again."

A French team, working independently at that country's National Center for Scientific Research, had come up with the same puzzling results. Preliminary word of the findings filtered across the Atlantic and the teams met last year.

"We gave each other confidence that we were right and that we were on to something very surprising," Preer said. The two groups reported their findings in Nature, the British science journal.

The groups concluded that the variant code is real and that it somehow evolved from the original code shared by other species. The same variant must have arisen twice -- once among bacteria, which are thought to resemble the earliest forms of life, and then again among protozoans, which are more complex organisms.

"It's easy to imagine that different codes could work," Preer said. "But it's hard to imagine how one code could evolve into another without screwing up everything in the cell."

The genetic code is written in the structure of the DNA molecule, which is essentially a long chain of smaller molecules, called bases, each of which is a letter of the genetic alphabet. It is a short alphabet, containing only three letters, or bases. All genetic words consist of three letters; geneticists call them triplet codons. Thus, there are 64 possible genetic words.

Alhough its vocabulary is modest in size, the genetic language can specify the structure of tens of thousands of different protein molecules -- each with a special function in the cell's life -- because its sentences may be thousands of words long.

Each sentence, carrying the code for one kind of protein molecule, is a gene. Since a length of DNA typically contains many genes linked end to end, the code must have some way of indicating where each gene ends. This is the stop code, a three-letter word at the end of a gene that tells the code-reading machinery to stop.

The code-reading machinery is needed to carry out the gene's instructions because in all organisms more complex than bacteria, the cell's library of genes stays locked inside the cell's nucleus.

The gene's message can be implemented only after it is transcribed, base for base, into a complementary long-chain molecule called messenger RNA. This RNA is analogous to a rubbing from an engraved stone. It travels out of the nucleus to the cell's protein-making apparatus, which reads the code, gathers amino acids (the building blocks of proteins), and chains them together in a sequence corresponding to the sequence of words.

Each word specifies one of about 20 amino acids. Each kind of protein is simply a chain of amino acids that, once assembled, folds up into a shape dictated by the alignment of chemical bonds between the amino acids. The protein's final shape determines its role in the body.

Although mutations -- the gene-altering phenomena that allow evolution -- may affect any codon, scrambling the gene into nonsense, all evidence suggests they happen one at a time at random points in a cell's DNA, which typically codes for thousands of proteins.

To change the code and leave a viable organism, as must have happened at least twice, biologists say it would seem necessary to have many identical mutations occurring simultaneously.

To put it another way, changing the meaning of a word in only one sentence in a book would make little sense if the same word is used in every other sentence.