The first single cell in the human body is the zygote. A zygote is formed when a sperm cell from a male fertilizes an egg cell from a female during sexual reproduction. This fertilized egg cell is the earliest stage of human development and is a single cell. It contains the complete set of genetic information (DNA) from both parents and has the potential to develop into a multicellular organism. As the zygote undergoes multiple rounds of cell division, it eventually gives rise to an embryo, which, in turn, develops into a fully formed human being through a complex process of cell differentiation and growth.