Extraterrestrial microbes might have brought life to Earth after travelling through space for millions of years, according to scientists.

The theory is based on calculations showing a high likelihood of rock fragments from planets in other star systems landing on Earth long ago.

Experts writing in the journalAstrobiology suggest some of these pieces of rock could have carried embedded micro-organisms.

The research suggests the dormant bugs could easily have survived the long journey through space, despite high levels of cosmic radiation.

The scientists believe that simple life may equally well have travelled from Earth to planets outside the Solar System.

The process, known as lithopanspermia, could mean the universe is teeming with Earth-like life.