The onset of individual human life has fascinated thinkers of all cultures and epochs, and the history of their ideas may enlighten an unsettled debate. Aristotle attributed three different souls to the subsequent developmental stages. The last, the rational soul, was associated with the formed fetus, and entailed fetal movements. With some modifications, the concept of delayed ensoulment - at 30, 42, 60, or 90 days after conception - was adopted by several Christian Church Fathers and remained valid throughout the Middle Ages. The concept of immediate ensoulment at fertilization originated in the 15th century and became Catholic dogma in 1869. During the Enlightenment, philosophers began to replace the rational soul with the term personhood, basing the latter on self-consciousness. Biological reality suggests that personhood accrues slowly, not at a specific date during gestation. Requirements for personhood are present in the embryo, but not in the preembryo before implantation: anatomic substrate; no more totipotent cells; decreased rate of spontaneous loss. However, biological facts alone cannot determine the embryo's moral status. Societies must negotiate and decide the degree of protection of unborn humans. In the 21st century, fertilization, implantation, extrauterine viability and birth have become the most widely accepted landmarks of change in ontological status.