Asteroids could have brought the spark that created life on Earth, scientists have concluded.
Amino acids are chemicals which are crucial to life.
They exist in two forms but life on Earth is exclusively dependent on one of them yet the reason why we - and every other living thing on the planet - are formed with one and not the other has baffled scientists.
The acids are long chains of molecules in roughly a corkscrew shape but they differ in their form: one type bends to the right, the other to the left and life on Earth uses the left-hand model.
However, laboratory tests in the 1950s showed that in conditions representing those in Earth's first millennia the acids formed in equal numbers of both types.
Research then showed that asteroids could have more of the left-handed version, the latest of which was disclosed last week.
A Nasa astrobiologist called Daniel Glavin and his colleagues said they had proven that a wide variety of meteor types might be home to such an imbalance.
Yet newer research led by Uwe Meierhenrich of the University of Nice Sophia Antipolis has now shown how this process could work.
The team found that light which passes through the dust cloud around a forming star become polarised in a particular way.
The light then triggers the formation of amino acids which can become stuck into asteroids. If one struck the Earth, the theory goes, the life switch could have been turned on.
However, one major mystery remains - how did the acids survive the journey from outer space to our planet?