Extraterrestrial microbes might have brought life to Earth after travelling through space for millions of years, scientists have said.
The theory is based on calculations showing a high likelihood of rock fragments from planets in other star systems landing on Earth long ago.
Some of them could have carried embedded micro-organisms, according to experts writing in the journal Astrobiology.
The research suggests the dormant bugs could easily have survived the long journey through space, despite high levels of cosmic radiation.
Simple life may equally well have travelled from Earth to planets outside the Solar System, the scientists believe. The process, known as lithopanspermia, could mean the universe is teeming with Earth-like life.
"Our work ... says that lithopanspermia might have been very likely, and it may be the first paper to demonstrate that," said lead researcher Dr Edward Belbruno, from Princeton University in the United States.
"If this mechanism is true, it has implications for life in the universe as a whole. This could have happened anywhere."
Large volcanic eruptions, meteorite impacts and collisions with other bodies can cause rocky fragments of planets to fly into space.
When the Solar System was young, and the Sun much closer to its neighbours than it is today, some of this debris could have been exchanged between planets orbiting different stars, say the scientists.
The research was presented at the 2012 European Planetary Science Congress in Madrid.