Some genes that are important early on in development have a conserved sequence known as the homeobox. The first homeobox genes were discovered in the fruit fly Drosophila melanogaster, but genes in amphibians and mammals also turn out to have similar, conserved sequences, raising the possibility that these are ancient genes, inherited from a common ancestor millions of years ago.
Homeobox genes control cascades of other genes. Because of this, mutations in homeobox genes can have an enormous effect. Perhaps many of the larger differences between species will eventually be traced to subtle changes in which other genes the homeobox genes control.
