Scientific American on life originating elsewhere

Scientific American
October 24, 2005

Did Life Come from Another World?

New research indicates that microorganisms could have survived a
journey from Mars to Earth

By David Warmflash and Benjamin Weiss

Most scientists have long assumed that life on Earth is a homegrown
phenomenon. According to the conventional hypothesis, the earliest
living cells emerged as a result of chemical evolution on our planet
billions of years ago in a process called abiogenesis. The alternative
possibility--that living cells or their precursors arrived from
space--strikes many people as science fiction. Developments over the
past decade, however, have given new credibility to the idea that
Earth's biosphere could have arisen from an extraterrestrial seed.
Planetary scientists have learned that early in its history our solar
system could have included many worlds with liquid water, the essential
ingredient for life as we know it. Recent data from NASA's Mars
Exploration Rovers corroborate previous suspicions that water has at
least intermittently flowed on the Red Planet in the past. It is not
unreasonable to hypothesize that life existed on Mars long ago and
perhaps continues there. Life may have also evolved on Europa,
Jupiter's fourth-largest moon, which appears to possess liquid water
under its icy surface. Saturn's biggest satellite, Titan, is rich in
organic compounds; given the moon's frigid temperatures, it would be
highly surprising to find living forms there, but they cannot be ruled
out. Life may have even gained a toehold on torrid Venus. The Venusian
surface is probably too hot and under too much atmospheric pressure to
be habitable, but the planet could conceivably support microbial life
high in its atmosphere. And, most likely, the surface conditions on
Venus were not always so harsh. Venus may have once been similar to
early Earth.

Moreover, the expanses of interplanetary space are not the forbidding
barrier they once seemed. Over the past 20 years scientists have
determined that more than 30 meteorites found on Earth originally came
from the Martian crust, based on the composition of gases trapped
within some of the rocks. Meanwhile biologists have discovered
organisms durable enough to survive at least a short journey inside
such meteorites. Although no one is suggesting that these particular
organisms actually made the trip, they serve as a proof of principle.
It is not implausible that life could have arisen on Mars and then come
to Earth, or the reverse. Researchers are now intently studying the
transport of biological materials between planets to get a better sense
of whether it ever occurred. This effort may shed light on some of
modern science's most compelling questions: Where and how did life
originate? Are radically different forms of life possible? And how
common is life in the universe?

>>From Philosophy to the Laboratory
To the ancient philosophers, the creation of life from nonliving matter
seemed so magical, so much the realm of the gods, that some actually
preferred the idea that ready-made living forms had come to Earth from
elsewhere. Anaxagoras, a Greek philosopher who lived 2,500 years ago,
proposed a hypothesis called "panspermia" (Greek for "all seeds"),
which posited that all life, and indeed all things, originated from the
combination of tiny seeds pervading the cosmos. In modern times,
several leading scientists--including British physicist Lord Kelvin,
Swedish chemist Svante Arrhenius and Francis Crick, co-discoverer of
the structure of DNA--have advocated various conceptions of panspermia.
To be sure, the idea has also had less reputable proponents, but they
should not detract from the fact that panspermia is a serious
hypothesis, a potential phenomenon that we should not ignore when
considering the distribution and evolution of life in the universe and
how life came to exist specifically on Earth.



--------------------------------------------------------------------------------
Earth's biosphere could have arisen from an extraterrestrial seed.
--------------------------------------------------------------------------------

In its modern form, the panspermia hypothesis addresses how biological
material might have arrived on our planet but not how life originated
in the first place. No matter where it started, life had to arise from
nonliving matter. Abiogenesis moved from the realm of philosophy to
that of experimentation in the 1950s, when chemists Stanley L. Miller
and Harold C. Urey of the University of Chicago demonstrated that amino
acids and other molecules important to life could be generated from
simple compounds believed to exist on early Earth. It is now thought
that molecules of ribonucleic acid (RNA) could have also assembled from
smaller compounds and played a vital role in the development of life.
ADVERTISEMENT





In present-day cells, specialized RNA molecules help to build proteins.
Some RNAs act as messengers between the genes, which are made of
deoxyribonucleic acid (DNA), and the ribosomes, the protein factories
of the cell. Other RNAs bring amino acids--the building blocks of
proteins--to the ribosomes, which in turn contain yet another type of
RNA. The RNAs work in concert with protein enzymes that aid in linking
the amino acids together, but researchers have found that the RNAs in
the ribosome can perform the crucial step of protein synthesis alone.
In the early stages of life's evolution, all the enzymes may have been
RNAs, not proteins. Because RNA enzymes could have manufactured the
first proteins without the need for preexisting protein enzymes to
initiate the process, abiogenesis is not the chicken-and-egg problem
that it was once thought to be. A prebiotic system of RNAs and proteins
could have gradually developed the ability to replicate its molecular
parts, crudely at first but then ever more efficiently.

This new understanding of life's origins has transformed the scientific
debate over panspermia. It is no longer an either-or question of
whether the first microbes arose on Earth or arrived from space. In the
chaotic early history of the solar system, our planet was subject to
intense bombardment by meteorites containing simple organic compounds.
The young Earth could have also received more complex molecules with
enzymatic functions, molecules that were prebiotic but part of a system
that was already well on its way to biology. After landing in a
suitable habitat on our planet, these molecules could have continued
their evolution to living cells. In other words, an intermediate
scenario is possible: life could have roots both on Earth and in space.
But which steps in the development of life occurred where? And once
life took hold, how far did it spread?

Scientists who study panspermia used to concentrate only on assessing
the basic plausibility of the idea, but they have recently sought to
estimate the probability that biological materials made the journey to
Earth from other planets or moons. To begin their interplanetary trip,
the materials would have to be ejected from their planet of origin into
space by the impact of a comet or asteroid. While traveling through
space, the ejected rocks or dust particles would need to be captured by
the gravity of another planet or moon, then decelerated enough to fall
to the surface, passing through the atmosphere if one were present.
Such transfers happen frequently throughout the solar system, although
it is easier for ejected material to travel from bodies more distant
from the sun to those closer in and easier for materials to end up on a
more massive body. Indeed, dynamic simulations by University of British
Columbia astrophysicist Brett Gladman suggest that the mass transferred
from Earth to Mars is only a few percent of that delivered from Mars to
Earth. For this reason, the most commonly discussed panspermia scenario
involves the transport of microbes or their precursors from Mars to
Earth.

Simulations of asteroid or comet impacts on Mars indicate that
materials can be launched into a wide variety of orbits. Gladman and
his colleagues have estimated that every few million years Mars
undergoes an impact powerful enough to eject rocks that could
eventually reach Earth. The interplanetary journey is usually a long
one: most of the approximately one ton of Martian ejecta that lands on
Earth every year has spent several million years in space. But a tiny
percentage of the Martian rocks arriving on Earth's surface--about one
out of every 10 million--will have spent less than a year in space.
Within three years of the impact event, about 10 fist-size rocks
weighing more than 100 grams complete the voyage from Mars to Earth.
Smaller debris, such as pebble-size rocks and dust particles, are even
more likely to make a quick trip between planets; very large rocks do
so much less frequently.


Could biological entities survive this journey? First, let us consider
whether microorganisms could live through the ejection process from the
meteorite's parent body. Recent laboratory impact experiments have
found that certain strains of bacteria can survive the accelerations
and jerks (rates of changes of acceleration) that would be encountered
during a typical high-pressure ejection from Mars. It is crucial,
however, that the impact and ejection do not heat the meteorites enough
to destroy the biological materials within them.

Planetary geologists formerly believed that any impact ejecta with
speeds exceeding the Martian escape velocity would almost certainly be
vaporized or at least completely melted. This idea was later
discounted, though, following the discovery of unmelted, largely intact
meteorites from the moon and Mars. These findings led H. Jay Melosh of
the University of Arizona to calculate that a small percentage of
ejected rocks could indeed be catapulted from Mars via impact without
any heating at all. In short, Melosh proposed that when the
upward-propagating pressure wave resulting from an impact reaches the
planetary surface, it undergoes a 180-degree phase change that nearly
cancels the pressure within a thin layer of rock just below the
surface. Because this "spall zone" experiences very little compression
while the layers below are put under enormous pressure, rocks near the
surface can be ejected relatively undeformed at high speeds.

Next, let us consider survivability during the entry into Earth's
atmosphere. Edward Anders, formerly of the Enrico Fermi Institute at
the the University of Chicago, has shown that interplanetary dust
particles decelerate gently in Earth's upper atmosphere, thus avoiding
heating. Meteorites, in contrast, experience significant friction, so
their surfaces typically melt during atmospheric passage. The heat
pulse, however, has time to travel a few millimeters at most into the
meteorite's interior, so organisms buried deep in the rock would
certainly survive.

Over the past five years a series of papers by one of us (Weiss) and
his colleagues analyzed two types of Martian meteorites: the nakhlites,
a set of rocks blasted off Mars by an asteroid or comet impact 11
million years ago, and ALH84001, which left the Red Planet four million
years earlier. (ALH84001 became famous in 1996 when a group of
scientists led by David McKay of the NASA Johnson Space Center claimed
that the rock showed traces of fossilized microorganisms akin to
Earth's bacteria; a decade later researchers are still debating whether
the meteorite contains evidence of Martian life.) By studying the
magnetic properties of the meteorites and the composition of the gases
trapped within them, Weiss and his collaborators found that ALH84001
and at least two of the seven nakhlites discovered so far were not
heated more than a few hundred degrees Celsius since they were part of
the Martian surface. Furthermore, the fact that the nakhlites are
nearly pristine rocks, untouched by high-pressure shock waves, implies
that the Martian impact did not heat them above 100 degrees C.

Many, though not all, terrestrial prokaryotes (simple one-celled
organisms such as bacteria that lack a membrane-bound nucleus) and
eukaryotes (organisms with well-defined nuclei) could survive this
temperature range. This result was the first direct experimental
evidence that material could travel from planet to planet without being
thermally sterilized at any point from ejection to landing.

The Problem of Radiation
For panspermia to occur, however, microorganisms need to survive not
only ejection from the first planet and atmospheric entry to the second
but the interplanetary voyage itself. Life-bearing meteoroids and dust
particles would be exposed to the vacuum of space, extremes in
temperature and several different kinds of radiation. Of particular
concern is the sun's high-energy ultraviolet (UV) light, which breaks
the bonds that hold together the carbon atoms of organic molecules. It
is very easy to shield against UV, though; just a few millionths of a
meter of opaque material is enough to protect bacteria.


Indeed, a European study using NASA's Long Duration Exposure Facility
(LDEF), a satellite deployed by the space shuttle in 1984 and retrieved
from orbit by the shuttle six years later, showed that a thin aluminum
cover afforded adequate UV shielding to spores of the bacterial species
Bacillus subtilis. Of the spores protected by the aluminum but exposed
to the vacuum and temperature extremes of space, 80 percent remained
viable--researchers reanimated them into active bacterial cells at the
end of the mission. As for the spores not covered by aluminum and
therefore directly exposed to solar UV radiation, most were destroyed,
but not all. About one in 10,000 unshielded spores stayed viable, and
the presence of substances such as glucose and salts increased their
survival rates. Even within an object as small as a dust particle,
solar UV would not necessarily render an entire microbial colony
sterile. And if the colony were inside something as large as a pebble,
UV protection would be sharply increased.

Informative as it was, the LDEF study was conducted in low Earth orbit,
well within our planet's protective magnetic field. Thus, this research
could not say much about the effects of interplanetary charged
particles, which cannot penetrate Earth's magnetosphere. From time to
time, the sun produces bursts of energetic ions and electrons;
furthermore, charged particles are a major component of the galactic
cosmic radiation that constantly bombards our solar system. Protecting
living things from charged particles, as well as from high-energy
radiation such as gamma rays, is trickier than shielding against UV. A
layer of rock just a few microns thick blocks UV, but adding more
shielding actually increases the dose of other types of radiation. The
reason is that charged particles and high-energy photons interact with
the rocky shielding material, producing showers of secondary radiation
within the meteorite.

These showers could reach any microbes inside the rock unless it was
very big, about two meters or more in diameter. As we have noted above,
though, large rocks make fast interplanetary voyages very infrequently.
Consequently, in addition to UV protection, what really matters is how
resistant a microbe is to all components of space radiation and how
quickly the life-bearing meteorite moves from planet to planet. The
shorter the journey, the lower the total radiation dose and hence the
greater the chance of survival.

In fact, B. subtilis is fairly robust in terms of its radiation
resistance. Even more hardy is Deinococcus radiodurans, a bacterial
species that was discovered during the 1950s by agricultural scientist
Arthur W. Anderson. This organism survives radiation doses given to
sterilize food products and even thrives inside nuclear reactors. The
same cellular mechanisms that help D. radiodurans repair its DNA, build
extra-thick cell walls and otherwise protect itself from radiation also
mitigate damage from dehydration. Theoretically, if organisms with such
capabilities were embedded within material catapulted from Mars the way
that the nakhlites and ALH84001 apparently were (that is, without
excessive heating), some fraction of the organisms would still be
viable after many years, perhaps several decades, in interplanetary
space.

Yet the actual long-term survival of active organisms, spores or
complex organic molecules beyond Earth's magne-tosphere has never been
tested. Such experiments, which would put the biological materials
within simulated meteoritic materials and expose them to the
environment of interplanetary space, could be conducted on the surface
of the moon. In fact, biological samples were carried onboard the
Apollo lunar missions as part of an early incarnation of the European
radiation study. The longest Apollo mission, though, lasted no more
than 12 days, and samples were kept within the Apollo spacecraft and
thus not exposed to the full space-radiation environment. In the
future, scientists could place experimental packages on the lunar
surface or on interplanetary trajectories for several years before
returning them to Earth for laboratory analysis. Researchers are
currently considering these approaches.


Meanwhile a long-term study known as the Martian Radiation Environment
Experiment (MARIE) is under way. Launched by NASA in 2001 as part of
the Mars Odyssey Orbiter, MARIE's instruments are measuring doses of
galactic cosmic rays and energetic solar particles as the spacecraft
circles the Red Planet. Although MARIE includes no biological material,
its sensors are designed to focus on the range of space radiation that
is most harmful to DNA.

Future Studies
As we have shown, panspermia is plausible theoretically. But in
addition, important aspects of the hypothesis have made the transition
from plausibility to quantitative science. Meteorite evidence shows
that material has been transferred between planets throughout the
history of the solar system and that this process still occurs at a
well-established rate. Furthermore, laboratory studies have
demonstrated that a sizable fraction of microorganisms within a piece
of planetary material ejected from a Mars-size planet could survive
ejection into space and entry through Earth's atmosphere. But other
parts of the panspermia hypothesis are harder to pin down.
Investigators need more data to determine whether radiation-resistant
organisms such as B. subtilis or D. radiodurans could live through an
interplanetary journey. And even this research would not reveal the
likelihood that it actually happened in the case of Earth's biosphere,
because the studies involve present-day terrestrial life-forms; the
organisms living billions of years ago could have fared much worse or
much better.

Moreover, scientists cannot quantify the likelihood that life exists or
once existed on planets other than Earth. Researchers simply do not
know enough about the origin of any system of life, including that of
Earth, to draw solid conclusions about the probability of abiogenesis
occurring on any particular world. Given suitable ingredients and
conditions, perhaps life needs hundreds of millions of years to get
started. Or perhaps five minutes is enough. All we can say with any
certainty is that by 2.7 billion years ago, or perhaps several hundred
million years earlier, life-forms were thriving on Earth.

Because it is not possible at this time to quantify all the steps of
the panspermia scenario, investigators cannot estimate how much
biological material or how many living cells most likely arrived at
Earth's surface in a given period. Moreover, the transfer of viable
organisms does not automatically imply the successful seeding of the
planet that receives them, particularly if the planet already has life.
If, for example, Martian microbes arrived on Earth after life
independently arose on our planet, the extraterrestrial organisms may
not have been able to replace or coexist with the homegrown species. It
is also conceivable that Martian life did find a suitable niche on
Earth but that scientists have simply not identified it yet.
Researchers have inventoried no more than a few percent of the total
number of bacterial species on this planet. Groups of organisms that
are genetically unrelated to the known life on Earth might exist
unrecognized right under our noses.

Ultimately, scientists may not be able to know whether and to what
extent panspermia has occurred until they discover life on another
planet or moon. For example, if future space missions find life on the
Red Planet and report that Martian biochemistry is very different from
our own, researchers would know immediately that life on Earth did not
come from Mars. If the biochemistries were similar, however, scientists
might begin to wonder if perhaps the two biospheres had a common
origin. Assuming that Martian life-forms used DNA to store genetic
information, investigators could study the nucleotide sequences to
settle the question. If the Martian DNA sequences did not follow the
same genetic code used by living cells on Earth to make proteins,
researchers would conclude that Mars-Earth panspermia is doubtful. But
many other scenarios are possible. Investigators might find that
Martian life uses RNA or something else entirely to guide its
replication. Indeed, yet-to-be-discovered organisms on Earth may fall
into this category as well, and the exotic terrestrial creatures might
turn out to be related to the Martian life-forms.


Whether terrestrial life emerged on Earth or through biological seeding
from space or as the result of some intermediate scenario, the answer
would be meaningful. The confirmation of Mars-Earth panspermia would
suggest that life, once started, could readily spread within a star
system. If, on the other hand, researchers find evidence of Martian
organisms that emerged independently of terrestrial life, it would
suggest that abiogenesis can occur with ease throughout the cosmos.
What is more, biologists would be able to compare Earth organisms with
alien forms and develop a more general definition of life. We would
finally begin to understand the laws of biology the way we understand
the laws of chemistry and physics--as fundamental properties of nature.



http://www.sciam.com/print_version.cfm?articleID=00073A97-5745-1359-94FF83414B7F0000

.