Anatomy and Physiology: The Root of Multicellular Organism Diversity

Multicellular life begins when a single sperm (male) out of hundreds of sperms penetrates the membrane of an egg (female) and fertilizes it.  The sperm and egg fuse together to form a zygote.  The zygote divides into two cells, the two cells divide into four cells, the four cells divide into eight cells. . .  At this point, the cells are still unspecialized cells.  However, as the cell division, the cells become specialized when they eventually divide into different types of cells which perform different functions.  When the same type of cells group together to perform the same function, they form a tissue.  In different multicellular organisms, the tissues perform different functions, and the diversity of multicellular organisms result from all these different specialized cells/tissues, even though they all started out as a single unspecialized cell.