Life on Earth may have announced its arrival billions of years ago with a whistle and a thump, according to planetary scientists.
Experiments by an international team of researchers back a controversial theory that life flourished on Earth after primitive organisms arrived aboard a meteorite, itself gouged from Mars by a giant impact.

The theory supposes that life was able to gain a tentative foothold on the red planet as it cooled down and became more hospitable several billion years ago. At the time, the planet's surface was regularly bombarded with rocky detritus from the asteroid belt, knocking clumps of rock and the microbes living on them into space, where the gravity of the sun brought them hurtling towards Earth.

Charles Cockell, at the Open University, who studies microbes in extreme environments, joined a team of German and Russian scientists to test whether microbes could survive the enormous shock of being blasted into space and crash landing on another planet.

They gathered colonies of micro-organisms including cyanobacteria, which live in rocky fissures, lichen, which smother their surfaces, and spores of the hardy bacterium Bacillus subtilis, and sandwiched them between slices of gabbro, a coarse-grained rock similar to that known to make up Martian meteorites.

The researchers then used high explosives to fire a steel plate at the sandwiched organisms and after each shot transferred the microbes to a dish to see if any had survived. The shocks were equivalent to those suffered by Martian meteorites that have been found on Earth, with pressures of up to 50 billion pascals. One pascal is equivalent to the pressure exerted by a £5 note resting on a surface. The pressure in a car tyre is equivalent to 200,000 pascals.

To their surprise, the scientists found the lichen and bacterial spores survived all but the most cataclysmic impacts up to 45 billion pascals. The cyanobacteria survived shocks of up to 10 billion pascals.

The findings support the theory of "lithopanspermia", which suggests life may be spread from one planet to another aboard lumps of rock that are knocked off the surface.

Writing in the journal Icarus, the scientists state: "These results strongly confirm the possibility of a 'direct transfer' scenario of 'lithopanspermia' for the route from Mars to Earth, or from any Mars-like planet to other habitable planets in the same stellar system."