The beginning of human personhood is the moment when a human is first recognized as a person. There are differences of opinion as to the precise time when human personhood begins and the nature of that status. The issue arises in a number of fields including science, religion, philosophy, and law, and is most acute in debates relating to abortion, stem cell research, reproductive rights, and fetal rights.
Traditionally, the concept of personhood has entailed the concept of soul, a metaphysical concept referring to a non-corporeal or extra-corporeal dimension of human being. In modernity, the concepts of subjectivity and intersubjectivity, personhood, mind, and self have come to encompass a number of aspects of human being previously considered to be characteristics of the soul. With regard to the beginning of human personhood, one historical question has been when the soul enters the body. In modern terms, the question could be put instead at what point the developing individual develops personhood or selfhood.
Related issues attached to the question of the beginning of human personhood include both the legal status, bodily integrity, and subjectivity of mothers, as well as the philosophical concept of "natality", i.e. "the distinctively human capacity to initiate a new beginning", which a new human life embodies.
Biological markers
Fertilization
Fertilization is the fusing of the gametes, that is a sperm cell and an ovum (egg cell), to form a zygote. At this point, the zygote is genetically distinct from either of its parents.
Fertilization was not understood in ancient times. Hippocrates believed that the embryo was the product of male semen and a female factor. But Aristotle held that only male semen gave rise to an embryo, while the female only provided a place for the embryo to develop, (a concept he acquired from the preformationist Pythagoras). In 1651 William Harvey refuted Aristotle's idea that menstrual blood could be involved in the formation of a fetus, asserting that eggs from the female were somehow caused to become a fetus as a result of sexual intercourse. Sperm cells were discovered in 1677 by Antonie van Leeuwenhoek, who believed that Aristotle had been proven correct. Some observers believed they could see an entirely pre-formed little human body in the head of a sperm. The human ova was first observed in 1827 by Karl Ernst von Baer. Only in 1876 did Oscar Hertwig prove that fertilization is due to fusion of an egg and sperm cell.
Fertilization is a process lasting around 24 hours, after which more than a third of zygotes are lost in the first few days. Many fail to implant into the uterine wall, sloughing off with the endometrial lining during menstruation. Of those which implant, around 50% are miscarried, usually without the woman knowing anything has happened. A zygote that is not lost will sometimes split into two zygotes. Separately, two individually fertilized zygotes will sometimes fuse into one zygote.
A single-cell embryonic human zygote can be described as a "whole living human being" in contrast to sperm or egg cells, which are also human life but are only parts of other human beings and are not genetically unique. Even then, the twinning process occurring after fertilization "weakens the possibility of seeing individuality as something irreversibly resolved" after fertilization.
Some members of the medical community accept fertilization as the point at which life begins. Dr. Bradley M. Patten from the University of Michigan wrote in Human Embryology that the union of the sperm and the ovum "initiates the life of a new individual" beginning "a new individual life history." In the standard college text book Psychology and Life, Dr. Floyd L. Ruch wrote "At the time of conception, two living germ cells—the sperm from the father and the egg, or ovum, from the mother—unite to produce a new individual." Dr. Herbert Ratner wrote that "It is now of unquestionable certainty that a human being comes into existence precisely at the moment when the sperm combines with the egg." This certain knowledge, Ratner says, comes from the study of genetics. At fertilization, all of the genetic characteristics, such as the color of the eyes, "are laid down determinatively." James C. G. Conniff noted the prevalence of the above views in a study published by The New York Times Magazine in which he wrote, "At that moment conception takes place and, scientists generally agree, a new life begins—silent, secret, unknown."