"Almost all higher animals start their lives from a single cell, the fertilized ovum (zygote). ... The time of fertilization represents the starting point in the life history, or ontogeny, of the individual." (Carlson, Bruce M., Patten's Foundations of Embryology, 6th edition. New York: McGraw-Hill, 1996, p.3.)
It's just too bad that according to numerous pro-choicers "no one knows when life begins."