On a genetic level, all of life on Earth speaks the same language. We've all got DNA and/or its close cousin, RNA. All of our genetic material is composed of the same batch of basic nucleotides, which are often called by the first letters of their full names—A, C, G and T for DNA; A, C, G and U for RNA. All cells on Earth even read those letters in much the same way, so cells from different species are often able to read each other's DNA. That's how pharmaceutical companies are able to put the human gene for insulin into bacteria and yeast to produce insulin medicine for diabetics. Want see a gross example? Early in the history of genetics, some biologists put the human gene for eyes onto a fruit fly's leg... and the fruit fly's cells made a fruit fly eye.