Mitochondria are energy systems withing modern cells, but new research reveals the tiny structures were once parasitic bacteria. Biologists have long known mitochondria were once independent lifeforms, but this recent study sheds new light on the question of what they were like before learning to live within cells.

University of Virginia researchers carefully examined the DNA of 18 species of bacteria which are closely related to mitochondria.

Mitochondria are found in all eukaryotic cells, which form the building blocks of all pants and animals. These cells are characterized by having a distinct nucleus within their structure. These tiny structures began to form on Earth roughly two billion years ago, long before the rise of any complex life form. Little is known about the evolution or life cycle of these tiny organisms in their earliest days. Modern versions of mitochondria provide energy to cells by creating the molecule adenosine triphosphate (ATP), the energy storage molecule that powers the structures.

"We are saying that the current theories -- all claiming that the relationship between the bacteria and the host cell at the very beginning of the symbiosis was mutually beneficial -- are likely wrong. Instead, we believe the relationship likely was antagonistic -- that the bacteria were parasitic and only later became beneficial to the host cell by switching the direction of the ATP transport," Martin Wu from the University of Virginia and lead author of the study, said.

Mitochondria were likely first described in medical research during the 1840s, although they were not identified as a distinct organelle until 1894. Richard Altmann, a pathologist who first recognized mitochondria, called the structures bioblasts. Their modern name was provided in 1898 by Carl Brenda. The first high-resolution photographs of the organelles were produced in 1952, leading to a greater understanding of their structure.

Mitochondria can fail, leading to a range of diseases, from diabetes and Parkinson's to Alzheimer's and other disorders related to aging.

The tiny organelles are passed on to children solely from the mother, allowing them to be used to trace back the history of human descent. Such a study was used to identify remains discovered in England, proving they were once the body of the 15th century King Richard III.

"We reconstructed the gene content of mitochondrial ancestors, by sequencing DNAs of its close relatives, and we predict it to be a parasite that actually stole energy in the form of ATP from its host -- completely opposite to the current role of mitochondria," Wu told reporters.

Discovery of the role of mitochondria as a parasitic bacteria was profiled in the online journal PLOS ONE.