Melbourne: Scientists have contradicted the theory that mitochondria -- the vital energy-producing units -- were engulfed by eukaryotic cells, showing that they acted like parasites instead.

A team, led by the University of Sydney, investigated the bacterium Midichloria mitochondrii -- named after helpful Star Wars (flick) microbes, called Midi-chlorians, which live inside cells and grant the mystical power known as The Force.

It has revealed that mitochondria may have entered human cells though a parasitic bacterium that used a tail to swim and could survive with almost no oxygen, the latest edition of the 'Molecular Biology and Evolution' journal reported.

The research challenges traditional explanations of how the ancestors of mitochondria first entered human cells between one and a half and two billion years ago. It also sheds new light on a process recognised as one of the major transitions in the history of life on earth.

"Our results challenge the paradigm -- shown in every biology textbook -- that mitochondria were passive bacteria gobbled up by a primordial cell," said lead scientists Dr Nathan Lo.

"We have found instead that the mitochondrial ancestor most likely had a flagellum, so was able to move, and possibly acted as a parasite, rather than prey, on early eukaryotic cells," he added.

Eukaryotes include all forms of animal and plant life on earth that are more complex than bacteria. They differ from simpler life forms because their cells have both a nucleus and mitochondria, which are like little batteries that generate energy to power the cell.

"How eukaryotic cells evolved remains one of the most vexing problems in biology. Mitochondria are actually highly reduced bacteria, with their own set of DNA, that reside in our cells," said Dr Lo.

For clues, the team studied Midichloria mitochondrii -- a bacterium they discovered in 2004 and successfully obtained permission from director George Lucas to name after the Star Wars Midi-chlorians. M. mitochondrii is from the Rickettsiales family, considered to be the closest living relatives of the ancestor of mitochondria.

"We studied M. mitochondrii because its genome has never been analysed and because it is the only bacterium known to be able to enter into the mitochondria of living cells," said Lo.

After determining the DNA sequence of M. mitochondrii's entire genome, the scientists found the bacterium contained 26 genes coding for an entire flagellum -- including all the key components such as hook, filament and basal body.

They also found a second set of genes which coded for enzymes that would allow the bacterium to survive in low-oxygen environments. These genes have never been seen before in bacterial relatives of mitochondria.

Dr Lo added, "We found these two sets of genes were inherited from the common ancestor shared by M. mitochondrii and our own mitochondria. Mitochondria's ancestor most likely possessed a flagellum, which is a key characteristic of many parasitic bacteria.

"Our results show the ancestor of mitochondria probably played a much more active, even parasitic, role in the early interactions with its eukaryotic host than previously thought.

They also explain how the relationship could have evolved in the low-oxygen environments of two billion years ago."