We could have alien origins, say scientists who sent fossilized microscopic life-forms into space and back inside an artificial meteorite.
The researchers attached the baseball-size rock to the outside of the European Space Agency's Foton M3 spacecraft to test whether biological material could survive the round-trip journey.
Sculpted from stone from the Orkney Islands in northern Scotland, the rock contained fossilized microbes and the molecular signatures of microbes.
The unmanned spacecraft was launched by rocket from Kazakhstan's Baikonur Cosmodrome carrying 43 experiments. The craft landed in Kazakhstan on September 26 after orbiting the planet for 12 days.
"In the bit of rock we got back, some biological compounds have survived," said project leader John Parnell from the University of Aberdeen in Scotland.
Preliminary findings suggest that it's possible simple organisms could arrive via meteorites, he said.
The research also suggests that living microbes would likely have survived in a slightly bigger rock, he added.
"This study of organic material is completely new," he said.
Previous artificial meteorite experiments have examined only the degree to which rocks melt upon entering the atmosphere.
The new experiment is part of European Space Agency's STONE program, which tests effects of reentry on artificial meteorites.
A rock measuring 2.8 inches (7 centimeters) across was fitted to the exterior of Foton M3.
"It was shielded when it went up into space but exposed when it came back," Parnell said.
The rock had similar properties to a type of meteorite known as a carbonaceous chondrite. Such meteorites contain water and carbon compounds, both essential to life.
"We wanted to see if a rock that was rich in carbon and water would suffer a lot of mass loss," Parnell said. "That was certainly the case. About three-quarters of the mass of our sample disappeared."
Living microbes probably wouldn't have survived in a meteorite this size because it reached temperatures of about 392 degrees Fahrenheit (200 degrees Celsius), the project leader said.
But "if our rock was bigger, say 20 centimeters (about 8 inches) across, then we can be quite confident that [the] temperature would not penetrate to the middle, so that if anything had been living there, it would have survived."
A much larger meteorite, however, would completely melt and vaporize on impact, according to Parnell.
"There's a sort of window of opportunity in terms of size, between being too small and too big," he added.
Microbes are known to live deep inside rocks, and are found several kilometers down in Earth's crust, Parnell noted.
The theory that says interplanetary organisms seeded life on different planets, such as Earth, is known as panspermia.
If panspermia explains the origins of life on Earth, astrobiologists believe that Mars is the most likely source.
For instance, studies suggest about 5 percent of meteorites from Mars eventually end up hitting Earth.
"That journey can take anything up to 15 million years, but there are a few that will make it very quickly," Parnell said.
"A very few will make it in a year or so. Those are the ones which could conceivably bring something interesting with them."
"The surface of Mars is quite inhospitable, due to dryness and low temperature, but one could conceive of subsurface life still being on Mars," he added.
In the experiment, microbes were also dried onto the undersides of several artificial meteorites.
"This biological material didn't survive, but it may have been preserved, or its signatures may have been preserved," said STONE scientist Charles Cockell of the Open University in the United Kingdom.
The rocks are still being analyzed, Cockell added.
"We know that life can make it from continent to continent, but what about from planet to planet?" he said.
"Of course, at the moment we don't know of life on another planet, but this experiment is an intriguing test of an interplanetary version of an old ecological question."
David Morrison is a senior scientist at the NASA Astrobiology Institute in Moffett Field, California.
Parnell's project lends credibility to the idea that meteors from outer space can give rides to hitchhiking microbes, he told National Geographic News by email.
Whether exchange of life has ever occurred following the meteorites' impact is a more complex question, but "we should be open to the possibility that there is microbial life on Mars that shares a common ancestor with Earth life," he said.
"It may not be likely, but we cannot exclude the possibility that we are, in effect, all Martians."