Scientists have for the first time shown that when simple chemicals are exposed to the harsh conditions of deep space, the molecules spontaneously arrange themselves into the hollow structures that look like the cell membranes found in all living things.
The work shows that early chemical steps considered important for the
origin of life can form in space, the researchers said. It lends
weight to arguments that life on Earth might have been
kick-started billions of years ago when organic compounds such
as these, born in cold interstellar clouds, landed on this planet
aboard comets, meteorites and interplanetary dust.
Scientists believe the molecules needed to make a cell's
membrane, and thus for the origin of life, are all over space,
said Louis Allamandola of NASA's Ames Research Center in
California's Silicon Valley, who led the team.
implies that life could be everywhere in the universe.
The findings provide an intriguing new clue to one of science's biggest and most complex mysteries: How did life arise? The leading theory of the origin of life on Earth proposes that the early planet provided the rich, vast soup of chemical resources within which, somewhere, conditions emerged that favored the formation of chemical compounds and processes that led to the first living organisms. Instead, the researchers said, crucial early processes appear to take place in space long before planet formation occurs, with the implication that if the resulting compounds land in any favorable environment, they can easily trigger life.
John Hayes, a biogeochemist at the Marine Biological Laboratory at
Woods Hole, Mass., who was not on the discovery team, said the work is
significant in that it provides a mechanism
in the right place at
the right time to deliver a lot of complicated organic material to
early planetary surfaces.
But he cautioned that there are
a lot of banana peels between
there and the rise of living things, and that
a lot more study
needs to be done on the nature of these structures.
No one knows how life began on Earth, whether it was through naked genetic material drifting in a primordial sea or genetic material already encapsulated in membranes. But at some point, the researchers said, membranes became important.
All life as we know it on Earth uses membrane structures to
separate and protect the chemistry involved in the life process from
the outside, said Jason Dworkin, of the SETI Institute, lead
author of the team's paper published in today's issue of the
Proceedings of the National Academy of Sciences.
All known biology
uses membranes to capture and generate cellular energy.
Dworkin compared membranes (thin, two-layered sheets made up mostly of
special fatty molecules) to a kind of housing.
molecules were just the raw lumber lying around that allowed
origin-of-life chemicals to move in and set up housekeeping or
construct their own houses.
Bruce Runnegar, head of UCLA's Center for Astrobiology and not a
member of the Allamandola team, said that, with the new evidence,
It's getting to the point where you can at least argue that
cell membranes might have been a very early step on the pathway toward
life on Earth. These hollow containers
are permeable and
eventually have electrical properties, and so if you can sort of
expect that they'd be available anyway, delivered to the primitive
Earth from comets, then it might make sense to have them as an early
At Ames's Astrochemistry Laboratory, the team created an
environment similar to that found in
empty space, with
temperatures close to absolute zero (minus 441 degrees Fahrenheit) in
an extreme vacuum. They froze a mixture of common, familiar chemicals
such as water, methanol (wood alcohol), ammonia and carbon
monoxide—the same ingredients known to make up the ice particles
in the dense clouds between the stars.
The researchers then zapped these simple ices with the harsh,
high-energy ultraviolet radiation that a nearby star in space would
emit. When they put the resulting yellowish residue in water under a
microscope at the University of California, Santa Cruz, they could see
the solids spontaneously organizing themselves into the
soap-bubble-like membranous structures, with
outside layers. Some of the compounds in the self-formed
vesicles are so complex they glow, Dworkin said. That is, they are
able to convert energy from the ultraviolet light to the visible
These structures themselves are not
life, Dworkin said: They
lack the genetic information they need to evolve, as required under
the accepted definition.
We're just starting to understand how
these things work, he said.
Scientists have long known that ultraviolet irradiation of icy solids produces chemicals more complex than those originally present in the ice. There was speculation that some of them might have played an important role in early Earth chemistry.
In the Ames laboratory, this team has routinely made copies of the extremely cold ice particles that make up the interstellar clouds—the birthplaces of stars and star systems, planets and smaller bodies.
Their goal had been merely to identify compounds that might be found
on comets and other icy bodies, to guide planning for space
missions. They were so surprised by the results that, Dworkin said,
they spent months checking the experiment for error.
I was sure it
was a contamination problem, he said.
But I couldn't get it
not to work.
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, Ames space scientist
Scott Sandford said.
The structures formed from the interstellar ices are similar to those formed from compounds found in a well-studied space rock—the primitive Murchison meteorite that landed in Australia—in work done earlier by chemist Dave Deamer of the University of California at Santa Cruz, a member of the Allamandola team. This suggests that interstellar ices might be the source of compounds delivered to Earth in the heavy bombardment by space rubble that occurred in its infancy. Today, more than a hundred tons of space stuff rains on Earth annually, much of it in the form of organic material (carbon-based compounds, some of which might form the building blocks of life).
We are just now beginning to realize that we are only seeing the
tip of the iceberg in terms of extraterrestrial molecular
complexity, Allamandola said.
Very complex organic molecules
that might be important for the origin of life could well be falling
on the surfaces of newly formed planets everywhere.