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By Steve Connor, Science Editor
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 they not only generated complex
molecules, but the compounds organised themselves into cell-like
"vesicles" on contact with water.
The researchers'
equipment was designed to replicate the conditions of interstellar
dust clouds such asthe Eagle Nebula photographed by the Hubble
space telescope, where temperatures can reach 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 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," he 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,"
he said. "When life began, at some point it became compartmented
in the form of cells. But where did the first cell membranes come
from?"
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 supports
the view of Sir Fred Hoyle, the Britishcosmologist who proposed
in the 1960s that life on Earth could have been "seeded"
bybiological molecules fromouter space. His ideas were ridiculed
at the time.
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