Seeds of life may have reached Earth on a comet

Powerful evidence that life on Earth originated in outer space is published today by scientists who have created the biological building blocks of living organisms in a laboratory designed to mimic interstellar dust clouds.

Powerful evidence that life on Earth originated in outer space is published today by scientists who have created the biological building blocks of living organisms in a laboratory designed to mimic interstellar dust clouds.

The findings support the belief that life originated with the help of complex organic molecules that rained down on Earth from comets and other cosmic debris.

Scientists from the Ames Research Center near San Francisco, part of the US National Aeronautics and Space Administration, and the University of California at Santa Cruz claim that they not only generated complex molecules, but that the compounds organised themselves into cell-like "vesicles" on contact with water.

The researchers' equipment was designed to replicate the extremely harsh conditions of interstellar dust clouds such as the Eagle Nebula famously photographed by the Hubble space telescope, where temperatures can plummet to near absolute zero (minus 273C).

The researchers added simple molecules such as ammonia, carbon monoxide and dioxide, and methanol to a mixture of fine ice particles trapped in a vacuum. When they irradiated the mixture with ultraviolet light, they found to their surprise that complex organic molecules were created. The molecules "self assembled" as aggregates of circular vesicles, reminiscent of a living cell's outer membrane.

Lou Allamondola, the team's leader, said that the aim of the study was to find out what sort of compounds Nasa might expect to find in comets and other planetary bodies, which would help the agency in future space missions.

"We expected ultraviolet radiation would make a few molecules that might have some biological interest, but nothing major," Dr Allamondola said. "Instead, we found that this process transforms some of the simple chemicals that are very common in space into larger molecules which behave in far more complex ways, which many people think are critical to the origin of life."

The findings, published in the Proceedings of the National Academy of Sciences, surprised the scientists in the degree to which the environment of an interstellar dust cloud complex was hospitable to the creation of organic material. Scott Sandford, a member of the research team, said: "Instead of finding a handful of molecules only slightly more complicated than the starting compounds, hundreds of new compounds are produced in every mixed ice we have studied. We are finding that the types of compounds produced in these ices are strikingly similar to many of those brought to Earth today by falling meteor-ites and their smaller cousins, the interstellar dust particles."

Equally surprising was the finding that some of those complex molecules possessed properties that were important to life, such as the ability to form a membrane enclosing a "bag" of biological chemicals.

Dave Deamer, professor of chemistry at the University of California at Santa Cruz, said the microscopic vesicles created by the molecules in the presence of water resembled living cells with membranes.

"All life today is cellular, and cells are defined by membranes that separate the [inside] cytoplasm from the outside world. When life began, at some point it became compartmented in the form of cells. But where did the first cell membranes come from?" Professor Deamer said.

Several lines of evidence point towards space being an important generator of life's complex building blocks. Scientists have found that the three-dimensional structures of organic molecules in comets tend to be a "left-handed" form similar to those on Earth.

Otherscientists have found the window of opportunity for life to begin has narrowed. Research has shown that the early Earth was bombarded with life-destroying comets much later in its history than was realised. Yet the earliest signs of life are being pushed further back towards the planet's origins some 4.5 billion years ago.

This suggests that help may have arrived from space, supporting the view of Sir Fred Hoyle, the pioneering British cosmologist who proposed in the 1960s that life on Earth could have been "seeded" by biological molecules from outer space. His ideas were ridiculed at the time.