A team of Australian scientists has recreated the first formation of mitochondria, which led to the evolution of the complex cells that made up life.
Their work provides concrete evidence to support one of the competing theories on how mitochondria evolved.
Mitochondria are present in all eukaryotes - organisms whose cells contain complex structures.
They produce energy and regulate cell metabolism in nearly all eukaryotic cells and even small defects in mitochondria can lead to serious defects in humans.
In a paper appearing in today's issue of Science, Professor Trevor Lithgow and colleagues from Monash University in Melbourne, argue their work validates the view that primitive cells developed the machinery to create mitochondria from within.
An alternative theory had suggested mitochondria evolved from the transfer of proteins from an outside host cell.
Evolving the first mitochondria more than a billion years ago, Lithgow says the first eukaryotes needed to establish protein import machinery in the membranes of what was a bacterial endosymbiont.
In tests on a modern alpha-proteobacteria, which is the closest living relative to the original bacterium that gave rise to mitochondria, Lithgow and colleagues found the component parts for the protein delivery machine were contained within the bacterium.
"All evolution had to do is find a way for the parts to come together," says Lithgow.
His team discovered that just one small change in one of the component parts was enough to activate it, resulting in the formation of mitochondria.
Lithgow says the changes made within the laboratory experiment would have been easily made in the ancient bacterium.
The component parts in the bacterium would have already been interacting with each other, bumping into each other but not staying together.
However anything that could make them stick together for even a short period of time would have been sufficient to begin the process.
Lithgow says their experiments show Darwin's theory of evolution at work.
"Evolution simply selects a feature because it provides an advantageous phenotype. Through time, very complex phenotypes can evolve. Even at the molecular level, the rules of the game are the same," he says.
Lithgow says while the finding does not break new ground, understanding how mitochondria developed is a biological equivalent of astronomy's Big Bang theory.
"It is part of the explanation for how life came into being," says Lithgow.
Professor Jim Whelan, at the ARC Centre of Excellence in Plant Energy Biology at the University of Western Australia, agrees the evolution of mitochondria is a fundamental question.
"Mitochondria in all organisms come from this event, one billion years ago, that led to the formation of mitochondria," he says.
Whelan says mitochondria are involved in many diseases and have in particular been linked to ageing neurodegenerative diseases.
"Understanding the evolution of mitochondria may provide insights to treatments," he says.
"However, the evolution of mitochondria has diverged with various lineages. [For example], plant mitochondria differ somewhat to animal.
"This can be important as then can have different targets for drugs. For instance, some parasites, such as malaria, resemble plant lineages. So understanding their evolution allows design of drugs that should not affect humans."
Whelan says a big scientific question maybe who is the master - the mitochondrion or the cell.
"Are all eukaryotic cell just incubation chambers for the 'ultimate parasite' which is the mitochondria?" he asks.