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.