Using standard rates of mutation in other animals, they estimate that the common female ancestor of all bdelloid rotifers lived 50 million to 100 million years ago. That's a long time to go without sex, which biologists define not as copulation, but as DNA exchange between organisms. As early as 1887, August Weismann recognized the cost of sex, and hypothesized that sex must therefore confer important advantages on its practitioners. To this day, Weissman's argument for why sex is costly, and his hypothesis about what benefits sex confers, remain popular. Cabot professor of the natural sciences Matthew Meselson and David Mark Welch, Ph.D. '99, have found evidence that bdelloid rotifers, a class of multicelled animals, have survived for 50 million to 100 million years without sexual reproduction.
Biologists call it an evolutionary scandal.
A different type of hypothesis to account for the predominance of sex, one that Meselson's laboratory is currently exploring, holds that sex somehow purges the genome of deleterious mutations. But many biologists who have pondered the problem, says Meselson, would agree that we do not yet know why sex exists. Research New England Books Etc.