2.7 billion years ago a tiny speck of iron entered the Earth’s atmosphere and fell to the surface where it became trapped in sediment, and was eventually fossilised.

Now this minute particle of cosmic dust, the oldest ever found, could help scientists to explain how oxygen-breathing life came to exist on Earth.

Previously scientists thought there was little or no oxygen in Earth’s ancient atmosphere.

But the little grain of iron was found to have transformed into iron oxide, or magnetite, by the time it hit the ground, which means it must have passed through a thick layer of oxygen on its journey to the surface.

Smaller than the width of a human hair, it is one of several ‘micrometeorites’ discovered in Western Australia to show the transformation.