Aging is one of the most basic properties of living organisms. Abundant evidence supports the idea that cell senescence underlies organismal aging in higher mammals. Therefore, examining the molecular mechanisms that control cell and replicative senescence is of great interest for biology and medicine. Several discoveries strongly support telomere shortening as the main molecular mechanism that limits the growth of normal cells. Although cultures gradually approach their growth limit, appearance of individual senescent cells is sudden and stochastic. A theoretical model of abrupt telomere shortening has been proposed in order to explain this phenomenon, but until now there was no reliable experimental evidence supporting this idea. Here, we have employed novel methodology to provide evidence for the generation of extrachromosomal circular telomeric DNA as a result of abrupt telomere shortening in normal human fibroblasts. This mechanism ensures heterogeneity in growth potential among individual cells, which is crucial for gradual progression of the aging process.