Washington, Oct 19 : Scientists now have physical proof of how species evolve and the fittest survive, after a 21-year study in which they documented the evolution of single-celled E. coli bacteria over 40,000 generations.

Richard Lenski, Hannah professor of microbial ecology at Michigan State University (MSU), said: "It's extra nice now to be able to show precisely how selection has changed the genomes of these bacteria, step by step over tens of thousands of generations."

Lenski's team periodically froze bacteria for later study, and technology has since developed to allow complete genetic sequencing. By the 20,000-generation midpoint, researchers discovered 45 mutations among surviving cells in the bottled bacteria.

Those mutations, according to Darwin's theory, should have conferred some advantage, and that's exactly what the researchers found.

The results "beautifully emphasise the succession of mutational events that allowed these organisms to climb toward higher and higher efficiency in their environment", noted Dominique Schneider, molecular geneticist at the Universite Joseph Fourier in Grenoble, France.

Lenski's long-running experiment itself is uniquely suited to answer some critical questions -- such as whether rates of change in a bacteria's genome move in tandem with its fitness to survive.

A mutation involved in DNA metabolism arose around generation 26,000, causing the mutation rate everywhere else in the genome to increase dramatically.

The number of mutations jumped to 653 by generation 40,000, but researchers surmise that most of the late-evolving mutations were not helpful to the bacteria, said an MSU release.

Gene mutations involved in human DNA replication are involved in some cancers. Many of the patterns observed in the experiment also occur in certain microbial infections, "and cancer progression is a fundamentally similar evolutionary process", observed collaborator Jeffrey Barrick.

"So what we learn here can help us better understand the course of these diseases."

The paper involved collaboration with scientists from South Korea as well as France and MSU.

The findings were published in Nature. (IANS)