Life on Earth may have been kick-started by a meteorite and comet bombardment of molecular "food", believe some scientists.
The theory has now received support from experiments that show that the complex chemical compounds life needs can be made in large quantities in the conditions found in space.
"This means that out there in interstellar space, we have these molecules being made which are the same as those involved in our biochemistry," Dr Max Bernstein of the Nasa Ames Research Center in California, told the BBC.
Dr Bernstein led the research and says: "We know these molecules are brought to Earth all the time on meteorites and tiny flakes of dust. So on the very early Earth, when life was just getting started, these organic molecules were available to the first organisms.
"You can see why these molecules still feature in our biochemistry today."
Space simulation
The scientists from the Nasa-Ames Research Center in California took some of the simple chemicals that are found in space.
They simulated space conditions by creating a vacuum, lowering the temperature and shining intense ultra-violet light on them. They found complex carbon compounds were formed.
Scientists believe enough of these compounds could be created in space to play a role in the start of life on Earth.
How the first organisms arose on Earth remains a mystery, but raw materials were needed and space may be the only place where sufficiently large enough amounts could have been produced.
"It is possible that significant amounts of extra-terrestrial organic matter were delivered to the early Earth this way 4.4 billion years ago," says Dr Pascale Ehrenfreund of Leiden Observatory in a commentary on the research.
Vast clouds
Scattered throughout our galaxy are vast molecular clouds, tens of light years in size. These clouds of gas and dust may be the chemical factories which produced the basic building blocks of life.
Astronomical observations show that these clouds contain complex molecules, especially at their cores. Here, the murky gas can shield the molecules from the heat of stars' radiation. This allows reactions to take place that would not be possible outside the cloud's core.
However, other scientists are not so sure that life got started with a meteorite delivery from space. Dr Mike Russell, of the University of Glasgow, Scotland, thinks the relative safety of the ocean floor is a more likely site.
Scorched Earth
"Just think what the Earth was like when life got started," he told BBC News Online.
"The Sun was vicious, producing 40 times more harmful ultra-violet radiation than it does today. At first there was a magma ocean and when water oceans formed they were extremely hot. There were typhoons, enormous tides and lightning. It was a different planet, not the Earth as we know it."
Dr Russell believes the violent early Earth would have destroyed the PAH molecules from comets before they could help start life.
"Life needs protection," he says. "That's why I think life began on the sea floor, next to hot water vents and with no outside help from space."
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