The real cause of genetic instability, which is the hall-mark of most cancers, is poorly understood. Specific gene mutations and acquired aneuploidy have been implicated as the root causes of genetic instability. Here we propose and cite evidence for the hypothesis that genetic instability of cancer cells is caused by telomere dynamics, erosion and/or amplification of the TTAGGG repeat sequences present at chromosomal termini. Since telomeres determine the domain of individual chromosomes within a nucleus and protect them from internal and external challenges, their erosion will destabilize the cell karyotype. Our hypothesis predicts that telomere dynamics provides the single unifying mechanism playing a major role in speciation, aging and cancer development. It was found that metastatic cancers of different histologic phenotypes, as well as mammalian taxa with active speciation and larger numbers of species exhibit amplification of their telomeric DNA as compared to non-metastatic counterpart cancers and taxa with only a limited number of species. The dynamic nature of this DNA can be found not only in the cancer cells but also in the peripheral lymphocytes of cancer patients. Human syndromes such as Down, Turner, Bloom, Werner, Fanconi, ataxia and many others, show aneuploidy and also are prone to develop various malignancies and premature aging. We have found that <normal cells> of all these syndromes have a reduced amount of telomeric DNA associated with specific mitotic catastrophes as compared to cells of age- and sex-matched normal individuals. From these and additional data generated by our group concerning speciation, aging and cancer karyotypes, we conclude that aneuploidy, which is responsible for birth defects, cancer initiation and is a major player in natural speciation, is a consequence of telomere dynamics. Because telomere reduction is linked to the aging process, which is a risk factor for cancer development in the human population, our hypothesis offers a unifying mechanism for the initiation of both hematologic and solid cancers, as well as for the origin of new species.