Embryology textbook on fertilization. Once zygote is formed it transforms into multicellular human being.
Keith L. Moore, T.V.N. Persaud, Mark G. Torchia, The Developing Human: Clinically Oriented Embryology, 11th ed. (Amsterdam: Elsevier, 2020), 11
Human development begins at fertilization when a sperm fuses with an oocyte to form a single cell, the zygote. This highly specialized, totipotent cell (capable of giving rise to any cell type) marks the beginning of each of us as a unique individual. The zygote, just visible to the unaided eye, contains chromosomes and genes that are derived from the mother and father. The zygote divides many times and becomes progressively transformed into a multicellular human begin through cell division, migration, growth, and differentiation.