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IN THE near vacuum of interstellar space, temperatures hover just above
absolute zero, where even the wobbling of atoms grinds to a halt. Dotting this
empty, frigid world are huge globs of gas and dust grains, so numerous they
block out nearly all light. And outside the shelter of these interstellar
clouds, bombarding cosmic and ultraviolet rays slash most molecules to
shreds.

Millions of light years away, under the sunshine and palm trees of Mountain
View, California, in a lab tucked away in the grounds of NASA鈥檚 Ames Research
Center, Lou Allamandola and his colleagues have been recreating that world of
extremes. And in doing so, they have uncovered tantalising hints that life may
have emerged not from some warm primordial slime on Earth, but on a dust grain
in the icy heart of space.

It鈥檚 long been suspected that comets made the Earth habitable by delivering
water and gases, and that they perhaps even provided some of the simpler
chemical building blocks of life. But what the NASA Ames team has found goes far
beyond that. When they recreate the harsh conditions of space in their lab, not
only do they generate astonishingly complex organic compounds similar to those
vital for life on Earth today, but also curious cell-like structures that may
have housed our planet鈥檚 earliest life forms.

The origin of these intriguing findings is coincidentally enough also the
birthplace of stars and planets. With 10 000 atoms per cubic centimetre,
interstellar clouds of gas and dust are sufficiently dense that individual blobs
within them collapse under their own gravity, forming stars and, eventually,
swirling systems of planets. (While this density is crushing by space standards,
the air on Earth is about 25 000 trillion times thicker.) In space, the density
of these clouds also offers another advantage that may have helped generate life
on Earth. Like people in a crowded dance club, molecules of gases, such as
methane, carbon monoxide, water vapour and ammonia, are continuously banging and
bumping into dust 鈥渟eedlings鈥濃�攇rains of silicate the size of smoke
particles that have been ejected from old stars.

While most astronomers have concentrated on the gases, Allamandola has always
had his eyes on the grains. Just as water vapour molecules rising from bubbling
soup will hit a cold kitchen window on a frosty night, stick and freeze, says
Allamandola, the same thing happens on the silicate seedlings. When gas
molecules smash into the cold grains they stick, creating an icy skin of frozen
gases. Infrared telescopes first detected the gas particles and silicate grains
in the 1970s. But no detection device was, or is to this day, powerful enough to
see how they interact in space. So Allamandola, who trained as a cryogenics
chemist, and physicist Mayo Greenberg of Leiden University in the Netherlands,
set out to do what most people thought impossible: build bits of interstellar
space here on Earth.

Nowadays, a deafening buzz permeates Allamandola鈥檚 lab. It鈥檚 the sound of
cryocoolers鈥攈igh-powered refrigerators鈥攌eeping the simulated
interstellar clouds at a cool 10 degrees above absolute zero. The 鈥渃louds鈥� are
actually vacuum-sealed chambers the size of shoe boxes. A gaseous mixture of
water, methane, ammonia and carbon monoxide flows through a metal tube into each
chamber, freezing onto what in the lab counts as a dust grain鈥攁 few square
centimetres of aluminium or caesium iodide. As the molecules pile up and freeze,
a thin white layer of ice forms.

And since grains in space receive occasional doses of ultraviolet light from
stars, the simulated grain and its accumulated chemicals are also bathed in
ultraviolet radiation. Each hour of radiation in the lab is reckoned to equal
what a grain in space would receive in a thousand years, a mere blip in the
Universe鈥檚 15-billion-year history. But that鈥檚 enough to get the action started.
As soon as those rays hit the molecules on the grain, they start breaking
chemical bonds, producing highly reactive radicals such as .OH and .NH₂.
The icy temperatures provide the ideal mixer. Frozen to the spot, the radicals
are forced to rejoin with their neighbours in ways that would never occur if
they were still part of a gas free to fly off and join more suitable partners.
The result is a profusion of complex, organic compounds.

The work, however, is a little like cooking with a recipe that doesn鈥檛 tell
you what the dish will be. Allamandola and his colleagues know what they start
out with鈥攕imple, water-soluble gases鈥攂ut analysing the chemically
complex end result is trickier. Their latest chamber is fitted with both a mass
spectrometer and an infrared spectrometer which together give a rough idea of
the type of molecules that form on the simulated dust grain. So far, the NASA
Ames team has found a slew of alcohols, ketones, aldehydes, alkanes, a giant
molecule called hexamethylenetetramine or HMT, and other organic molecules, some
with as many as 40 carbon bonds. But apart from that broadbrush description,
Allamandola and his team have yet to put names on most of the chemicals they
have created in this simulation of an interstellar cloud.

When biologist David Deamer of the University of California in Santa Cruz
heard what was going on at the astrochemistry lab at Ames, he was soon knocking
at Allamandola鈥檚 door. Deamer had been studying the Murchison meteorite, which
landed in Australia in 1969 and has since kept numerous researchers busy
identifying its rich array of organic molecules. Deamer had found something
intriguing deep within the loose sandstone-like rock: hundreds of microscopic
globules. Grinding the stone into powder and flushing it with a solvent to rinse
out organic molecules revealed what looked like tiny two-layered vesicles
swimming in the liquid. When he published his findings in Nature in
1985 (vol 317, p 792) it caused a considerable fuss, not least because no one
had any idea what these vesicles were, nor what sort of chemistry created them,
and where.

Deamer suspected that clues to his 鈥渇ossil鈥� vesicles lay in the residue in
Allamandola鈥檚 space chambers. After all, the Murchison meteorite is believed to
be a remnant of a spent comet, and comets are little more than billions of ice
grains piled together in mountain-sized chunks. When Deamer warmed the chamber
residue in water and peered at it through the microscope he discovered tiny
droplets, each between 10 and 40 micrometres across, up to the size of red blood
cells. Their similarity with the Murchison droplets was a sure sign that the
meteorite vesicles had had their origins far off in space, not here on Earth.
Deamer also discovered that the vesicles fluoresced under ultraviolet light, one
more indication that they were made of complex organic molecules.

鈥淚t was a remarkable transformation of a few simple, water-soluble
chemicals,鈥� says Allamandola. 鈥淚t would have been considered science fiction a
few years 产别蹿辞谤别.鈥� It was time to call in a biochemist.

Last year, Jason Dworkin, who had worked with Stanley Miller, famous for
creating amino acids in the 1950s with little more than a spark of electricity
and a few hot gases, joined the group.

A closer look

After ruling out contamination, Dworkin has been working on creating enough
of the dust grain ice to get a closer look at the molecules that make up the
vesicles. It鈥檚 an onerous task. Running the space chamber for weeks on end
yields only about a milligram of organic residue. And it contains dozens of
different chemicals. Still, so far he has created enough residue to show that
the molecules that make up the outer layer of the vesicles behave like lipids,
which are the major component of cell membranes.

Just like soap molecules, one end of each vesicle molecule is attracted to
water, while the other end avoids it like the plague. This allows the molecules
to self-organise into spheres, sticking their water-loving heads towards the
outside, and keeping their water-hating tails tucked away inside.

Even mainstream research into the origin of life, which relies on mixing
chemicals under conditions thought to have existed at the start of the Earth鈥檚
history, hasn鈥檛 had any success in making lipids or lipid-like structures, says
Dworkin. Now Dworkin plans to find out whether the lipid-like molecules from the
space chamber can form bilayers similar to the membranes of all modern Earthly
cells.

But why get so excited about what looks at best like a very rudimentary
membrane that just happens to glow under ultraviolet light? Put simply, the
finding matters because many experts believe that life, or at least life as we
know it today, could not exist without boundaries, especially on a waterlogged
planet like our own
(鈥淟ife is鈥︹��, New Scientist, 13 June, p 38).

Without barriers, goes one argument, biologically important molecules would
be so diluted in the ocean that no chemistry could ever happen. Modern cell
membranes, with the help of proteins embedded in them, also act like a
home-security and climate-control system all rolled in one, regulating what
leaves and enters, maintaining the correct pH, and providing a means of
separating charges so that the cell can, for example, create the energy-carrying
molecule ATP. Membranes may even be essential for stabilising peptides, the
precursors of proteins.

A crazy idea

鈥淭he [Dworkin] results are very exciting,鈥� says geneticist and astrophysicist
Pascale Ehrenfreund of the Leiden Observatory in the Netherlands, who studies
the role of interstellar ice grains in the origin of life. 鈥淢embrane formation
is a crucial step in the first forms of life.鈥� Earth鈥檚 first life forms would
probably have been far too simple to make their own membranes. But whether they
were forced to make use of ready-made reaction chambers, or whether they were
merely strands of nucleic acids wriggling their way unprotected through the
primordial sludge (sticking to a clay surface or happening upon a drying puddle
could have concentrated the chemicals) is an open question. If they did need to
seek shelter, the Deamer-Allamandola vesicles could be just the thing.

鈥淚f you can form bubbles,鈥� says Tom Wdowiak, an astronomer at the University
of Alabama in Birmingham, 鈥渢hen you鈥檝e got something that can serve as a
capsule.鈥� But the Allamandola team has done more than recreate what might have
been our planet鈥檚 first mobile homes. Their reenactment of what happens in an
interstellar cloud has also shown that space can generate the right sorts of
life-giving chemicals to go inside. In one experiment, Allamandola and Bernstein
added water to the giant HMT molecules created in the space chamber, yielding
formaldehyde, ammonia and even small amounts of amino acids.

More recently, the NASA Ames team turned to polycyclic aromatic hydrocarbons,
the largest reservoir of carbon in the universe. Carbon is an essential part of
all known life, and PAHs spewed out by the sloppy combustion of stars in the
process of being born, contain between 10 and 20 per cent of all the carbon in
the Universe. And although PAHs are never seen in normal, healthy cells, they
are highly biologically active鈥攆or example, PAHs in soot from car engines
and factories are carcinogenic simply because they can wiggle their way into
DNA.

The NASA Ames team decided to use their space chamber to find out what might
be happening to the PAHs deep within the interstellar clouds. The result was
totally unexpected. When they fired gaseous water and PAHs such as naphthalene
and anthracene one at a time on to the simulated 鈥渄ust grains鈥� in the space
chamber, and bombarded the mixture with ultraviolet light, it produced compounds
uncannily similar to those needed for life on Earth. 鈥淚t鈥檚 the UV light that
makes PAHs useful [for life],鈥� says team member Scott Sandford. The rays broke
the water molecules apart, and the separated hydrogens and oxygens鈥攍ocked
in place by the cold temperatures鈥攔eattached themselves to the PAH,
creating a huge array of complex chemicals.

鈥淭his change is antithetical to what everyone thought,鈥� says Wdowiak.
鈥淓veryone thought that PAHs would just break down [when exposed to UV light].
This is great. We didn鈥檛 think PAHs would turn into anything of use. And
suddenly we had this huge reservoir of carbon we never thought about
产别蹿辞谤别.鈥�

Just this July, space-in-a-lab simulations by NASA Ames team member Max
Bernstein produced compounds called quinones and alkaloids from PAHs. (It is far
easier to analyse the irradiated PAHs, than the residue made from squirting
methane, carbon monoxide, water vapour and ammonia into the space chambers.)
Alkaloids are ubiquitous in the plant world. Meanwhile, quinones help all cells
move electrons around, are crucial for photosynthesis in plants and are broken
down for energy in human muscle and brain cells. The NASA Ames team 鈥渋s showing
that carbon [from space] is coming in, in a form that is rich, that can be
utilised by life,鈥� says chemist Richard Zare of Stanford University in Palo
Alto.

Bernstein speculates that interstellar quinones raining down on Earth
provided a tasty treat for early organisms until, at some point, the primordial
forerunner of the plant took advantage of the quinones to harness sunlight. But
Allamandola envisages a more radical scenario. Sure, the compounds raining down
on Earth in cometary dust could have provided a nutritious meal for struggling
primordial life, he says. But what of those vesicles? Is it possible鈥攁nd
this, Allamandola knows, 鈥渋s a crazy idea鈥濃�攖hat they and the complex
organic molecules jump-started life before reaching Earth?

Consider the inside of a comet, he says. There, under layers of icy material,
molecules created by ultraviolet light falling on PAHs and other gases stuck to
silicate grains would be shielded from the worse radiation. Perhaps the comet
swoops by a star, warming the outside just enough to melt some ice, providing
water for the cell-like vesicles to form, just as they did when Deamer thawed
the residue from the space chamber.

Whizzing through space in the belly of the comet wouldn鈥檛 be a smooth ride.
Gradually, the amino acids, PAHs and other organics would jiggle their way into
the vesicles. And voil脿, you鈥檇 have reaction chambers
chock-a-block with complex organic molecules primed to generate the very first
cells. 鈥淢aybe they鈥檙e just sitting there waiting like seeds in a packet to hit
the right place,鈥� he says. Allamandola鈥檚 鈥渃razy idea鈥� could be checked out early
next century when the European Space Agency鈥檚 International Rosetta Mission is
scheduled to drill into a comet鈥檚 core and brings whole pieces back to
Earth.

鈥淚t鈥檚 a big stretch from making vesicles and encapsulating organic compounds
to promoting life,鈥� says John Cronin, a prebiotic chemist at Arizona State
University in Tempe. 鈥淚t鈥檚 hard to imagine how that can take you to anything as
complex as a nucleic acid that can store and reproduce information.鈥� But, he
adds, 鈥渋t鈥檚 hard to imagine that process anywhere, so maybe it鈥檚 not such a big
蝉迟别辫.鈥�

Now, Bernstein and Allamandola plan to mix PAHs in their space chamber with
all the gases found in an interstellar cloud鈥攚ater, plus methane, ammonia
and carbon monoxide. 鈥淚t鈥檚 going to be like the Wild West. Anything can happen,鈥�
says Allamandola.

鈥淎nything can happen鈥� could be space鈥檚 new motto. The space-chamber shows
that under extreme conditions simple organic ingredients can produce a rich
banquet of potentially life-spawning chemicals far more easily than anyone
expected. In short, it could happen just about anywhere.

鈥淭he most amazing thing is that we start with something really simple. And
then suddenly we鈥檙e making this enormous range of complex molecules,鈥� says
Allamandola. 鈥淲hen I see this kind of complexity forming under these exceedingly
extreme conditions, I begin to really believe that life is a cosmic imperative.鈥�