The search for life-extending interventions has been often perceived as a purely academic pursuit, or as an unorthodox medical enterprise, with little or no practical outcome. Yet, in fact, these studies, explicitly aiming to prolong human life, often constituted a formidable, though hardly ever acknowledged, motivation for biomedical research and discovery. At least several modern biomedical fields have originated directly from rejuvenation and life extension research: (1) Hormone replacement therapy was born in Charles-Edouard Brown-Séquard's rejuvenation experiments with animal gland extracts (1889). (2) Probiotic diets originated in Elie Metchnikoff's conception of radically prolonged "orthobiosis" (c. 1900). (3) The development of clinical endocrinology owed much to Eugen Steinach's "endocrine rejuvenation" operations (c. 1910s-1920s). (4) Tissue transplantations in humans (allografts and xenografts) were first widely performed in Serge Voronoff's "rejuvenation by grafting" experiments (c. 1910s-1920s). (5) Tissue engineering was pioneered during Alexis Carrel's work on cell and tissue immortalization (c. 1900-1920). (6) Cell therapy (and particularly human embryonic cell therapy) was first widely conducted by Paul Niehans for the purposes of rejuvenation as early as the 1930s. Thus, the pursuit of life extension and rejuvenation has constituted an inseparable and crucial element in the history of biomedicine. Notably, the common principle of these studies was the proactive maintenance of stable, long-term homeostasis of the entire organism.