In a 20-year experiment, researchers repeatedly cloned mice from a single original donor, producing more than 1,200 animals across 58 generations. The mice appeared healthy and lived normal lifespans. But with each round of cloning, mutations quietly accumulated in their DNA, eventually reducing success rates and bringing the process to a halt. By the final generations, cloning succeeded less than 1 percent of the time. The findings, published in Nature, show that while cloning can sustain individuals, it may not sustain a species.

The results point to a fundamental limit. “Mammals rely on sexual rather than asexual reproduction to eliminate genetic anomalies caused by clonal reproduction,” the authors stated in the study.