According to general scientific consensus, human life (in the sense at what point does an individual physical being comes into existence) starts at conception. This consensus is, for example, reflected in the Encyclopedia Britannica article:
“”Although organisms are often thought of only as adults, and reproduction is considered to be the formation of a new adult resembling the adult of the previous generation, a living organism, in reality, is an organism for its entire life cycle, from fertilized egg to adult, not for just one short part of that cycle.
—Encyclopedia Britannica 1998, v 26, p 611
Why is it this way? Concerning when life begins, a particular aggregate of hereditary tendencies (genes and chromosomes) is first assembled at the moment of fertilization when an ovum (egg) is invaded by a sperm cell. This restores the normal number of required chromosomes, 46, for survival, growth, and reproduction of a new composite individual. And we say that it is a human being, because...
This being is alive. The fertilized egg has the characteristics of life. He can reproduce his own cells and develop them into a specific pattern of maturity and function.
This being is human. He is a functioning individual organisms that genetically belong to the species