Mitochondria are tiny structures found in our cells that serve the essential function of producing energy for the cell. They are present in all complex organisms and are fundamental to the functioning of complex life on our planet. However, they have much more to offer. Mitochondria have their own DNA, with a small collection of genes that are separate from those in the cell nucleus. It is believed that they were once bacteria that lived independently. Their incorporation into the larger cell was a turning point in the evolution of life, enabling the development of complex organisms and the origin of the two sexes. Unlike the DNA in the cell nucleus, mitochondrial DNA is passed down almost exclusively through the female line. For this reason, it has been used by some researchers to trace human ancestry from daughter to mother.