Thanks to advances in DNA technology, scientists can now reconstruct new copies of old viruses. Last year United States government scientists reconstructed the virus that caused the influenza epidemic of 1918. Now a team of French scientists has rebuilt a virus that infected our apelike ancestors several million years ago.

The scientists did not isolate a virus from a fossil. Instead, they examined vestiges of the virus that survive today within the human genome.

About 100,000 segments of human DNA are remarkably similar to retroviruses, a class of viruses that includes HIV. Retroviruses insert a copy of their genes into the genome of their host cell. Scientists estimate that 8 percent of the human genome is made up of this viral DNA, known as human endogenous retroviruses, or HERVs.

''Our genome is filled with retroviruses,'' said Dr. Thierry Heidmann, an expert on HERVs at the Gustave Roussy Institute in Villejuif, France. ''It's a hard idea to understand, but they are part of our genome.''

Many HERVs found in the human genome have counterparts in the genomes of other species. They infected our distant ancestors millions of years ago, and were passed down from generation to generation. They also produced new copies that could reinfect egg or sperm cells, adding more HERVs to the genome. Over time HERVs were crippled by mutations. Until now, scientists have never found a HERV that acts like a fully functional virus.

Dr. Heidmann reasoned that disabled HERVs must descend from working ancestors. He and his colleagues compared the DNA of HERVs from a family found only in humans, deducing the ancestor's genetic makeup from the differences. They built a corresponding piece of DNA and inserted it into human cells. Some of the cells produced new HERVs that could infect other cells.

The scientists named the reconstructed virus Phoenix. The journal Genome Research published the experiment on its Web site last week.

Dr. Heidmann plans to use Phoenix to study the role of HERVs in cancer.

''The procedure is ingenious,'' said Robert Belshaw, an expert on HERV evolution at Oxford University, who was not involved in the study. He speculates that some people still carry working versions of Phoenix. ''It's possible it's also still moving between individuals,'' he said.

Photo (Photo by Agence France-Presse)