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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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