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Issue 293: Clonal expansion of blood stem cells (clonal hematopoiesis or CH) is a hallmark of aging. Blood cancers can develop from such clonal expansions. However, CH without any clinical implications is observed in a large fraction of the aging population. Why is CH pervasive in healthy individuals? What are the origins of CH? These questions are of importance not only to understand the blood system and associated disease risks, but also to shed light on the evolution of somatic variants throughout a human life. In our preprint, we provide an answer to the origins of clonal hematopoiesis. We show that a model comprised of stem cell clones arising during development and evolving throughout life under weak selection is fully consistent with the data, and gives rise to CH as we grow old.