Mitochondria are often described as the powerhouses of the cell because they generate the majority of the cell's supply of adenosine triphosphate, used as a source of chemical energy. Unlike other organelles, mitochondria contain their own independent genome, which is circular and closely resembles the DNA found in bacteria. This observation forms the basis of the endosymbiotic theory, which proposes that mitochondria originated as free-living prokaryotes that were engulfed by a larger host cell billions of years ago. Instead of being digested, the prokaryote entered into a mutually beneficial relationship with the host, eventually evolving into a permanent organelle. Mitochondrial DNA is inherited almost exclusively from the mother, as the mitochondria in a sperm cell are typically destroyed after fertilization. This maternal inheritance allows scientists to trace human ancestry back through the female line, a technique that led to the identification of a common female ancestor known as Mitochondrial Eve. Despite their essential role, mitochondria are also a primary source of reactive oxygen species, which can damage cellular components and contribute to the aging process and various degenerative diseases.