Homology is the existence of shared ancestry between a pair of structures, or genes, in different species.A common example of homologous structures in evolutionary biology are the wings of bats and the arms of primates.Evolutionary theory explains the existence of homologous structures adapted to different purposes as the result of descent with modification from a common ancestor. In the context of sexual differentiation—the process of development of the differences between males and females from an undifferentiated fertilized egg—the male and female organs are homologous if they develop from the same embryonic tissue. A typical example is the ovaries of female humans and the testicles of male humans.