[tt] Are Aliens Among Us?

[Eugen Leitl]

(one of the rarer catches from reddit, which is now officially dead)

http://www.sciam.com/article.cfm?id=are-aliens-among-us&print=true

Are Aliens Among Us?

In pursuit of evidence that life arose on Earth more than once, scientists
are searching for microbes that are radically different from all known
organisms

By Paul Davies

The origin of life is one of the great unsolved problems of science. Nobody
knows how, where or when life originated. About all that is known for certain
is that microbial life had established itself on Earth by about three and a
half billion years ago. In the absence of hard evidence of what came before,
there is plenty of scope for disagreement.

Thirty years ago the prevailing view among biologists was that life resulted
from a chemical fluke so improbable it would be unlikely to have happened
twice in the observable universe. That conservative position was exemplified
by Nobel Prize–winning French biologist Jacques Monod, who wrote in 1970:
“Man at last knows that he is alone in the unfeeling immensity of the
universe, out of which he emerged only by chance.” In recent years, however,
the mood has shifted dramatically. In 1995 renowned Belgian biochemist
Christian de Duve called life “a cosmic imperative” and declared “it is
almost bound to arise” on any Earth-like planet. De Duve’s statement
reinforced the belief among astrobiologists that the universe is teeming with
life. Dubbed biological determinism by Robert Shapiro of New York University,
this theory is sometimes expressed by saying that “life is written into the
laws of nature.”

How can scientists determine which view is correct? The most direct way is to
seek evidence for life on another planet, such as Mars. If life originated
from scratch on two planets in a single solar system, it would decisively
confirm the hypothesis of biological determinism. Unfortunately, it may be a
long time before missions to the Red Planet are sophisticated enough to hunt
for Martian life-forms and, if they indeed exist, to study such
extraterrestrial biota in detail.

An easier test of biological determinism may be possible, however. No planet
is more Earth-like than Earth itself, so if life does emerge readily under
terrestrial conditions, then perhaps it formed many times on our home planet.
To pursue this tantalizing possibility, scientists have begun searching
deserts, lakes and caverns for evidence of “alien” life-forms—organisms that
would differ fundamentally from all known living creatures because they arose
independently. Most likely, such organisms would be microscopic, so
researchers are devising tests to identify exotic microbes that could be
living among us.

Scientists have yet to reach a consensus on a strict definition of life, but
most would agree that two of its hallmarks are an ability to metabolize (to
draw nutrients from the environment, convert those nutrients into energy and
excrete waste products) and an ability to reproduce. The orthodox view of
biogenesis holds that if life on Earth originated more than once, one form
would have swiftly predominated and eliminated all the others. This
extermination might have happened, for example, if one form quickly
appropriated all the available resources or “ganged up” on a weaker form of
life by swapping successful genes exclusively with its own kind. But this
argument is weak. Bacteria and archaea, two very different types of
microorganisms that descended from a common ancestor more than three billion
years ago, have peacefully coexisted ever since, without one eliminating the
other. Moreover, alternative forms of life might not have directly competed
with known organisms, either because the aliens occupied extreme environments
where familiar microbes could not survive or because the two forms of life
required different resources.

The Argument for Aliens

Even if alternative life does not exist now, it might have flourished in the
distant past before dying out for some reason. In that case, scientists might
still be able to find markers of their extinct biology in the geologic
record. If alternative life had a distinctively different metabolism, say, it
might have altered rocks or created mineral deposits in a way that cannot be
explained by the activities of known organisms. Biomarkers in the form of
distinctive organic molecules that could not have been created by familiar
life might even be hiding in ancient microfossils, such as those found in
rocks dating from the Archean era (more than 2.5 billion years ago).

A more exciting but also more speculative possibility is that alternative
life-forms have survived and are still present in the environment,
constituting a kind of shadow biosphere, a term coined by Carol Cleland and
Shelley Cop­ley of the University of Colorado at Boulder. At first this idea
might seem preposterous; if alien organisms thrived right under our noses (or
even in our noses), would not scientists have discovered them already? It
turns out that the answer is no. The vast majority of organisms are microbes,
and it is almost impossible to tell what they are simply by looking at them
through a microscope. Microbiologists must analyze the genetic sequences of
an organism to determine its location on the tree of life—the phylogenetic
grouping of all known creatures—and researchers have classified only a tiny
fraction of all observed microbes.

To be sure, all the organisms that have so far been studied in detail almost
certainly descended from a common origin. Known organisms share a similar
biochemistry and use an almost identical genetic code, which is why
biologists can sequence their genes and position them on a single tree. But
the procedures that researchers use to analyze newly discovered organisms are
deliberately customized to detect life as we know it. These techniques would
fail to respond correctly to a different biochemistry. If shadow life is
confined to the microbial realm, it is entirely possible that scientists have
overlooked it.

Ecologically Isolated Aliens Where might investigators look for alien
organisms on Earth today? Some scientists have focused on searching for
organisms occupying a niche that is ecologically isolated, lying beyond the
reach of ordinary known life. One of the surprising discoveries in recent
years is the ability of known life to endure extraordinarily harsh
conditions. Microbes have been found inhabiting extreme environments ranging
from scalding volcanic vents to the dry valleys of Antarctica. Other
so-called extremophiles can survive in salt-saturated lakes, highly acidic
mine tailings contaminated with metals, and the waste pools of nuclear
reactors.

Nevertheless, even the hardiest microorganisms have their limits. Life as we
know it depends crucially on the availability of liquid water. In the Atacama
Desert in northern Chile is a region that is so dry that all traces of
familiar life are absent. Furthermore, although certain microbes can thrive
in temperatures above the normal boiling point of water, scientists have not
yet found anything living above about 130 degrees Celsius (266 degrees
Fahrenheit). It is conceivable, though, that an exotic alternative form of
life could exist under more extreme conditions of dryness or temperature.

Thus, scientists might find evidence for alternative life by discovering
signs of biological activity, such as the cycling of carbon between the
ground and the atmosphere, in an ecologically isolated region. The obvious
places to look for such disconnected ecosystems are in the deep subsurface of
Earth’s crust, in the upper atmosphere, in Antarctica, in salt mines, and in
sites contaminated by metals and other pollutants. Alternatively, researchers
could vary parameters such as temperature and moisture in a laboratory
experiment until all known forms of life are extinguished; if some biological
activity persists, it could be a sign of shadow life at work. Scientists used
this technique to discover the radiation-resistant bacterium Deinococcus
radiodurans, which can withstand gamma-ray doses that are 1,000 times as
great as what would be lethal for humans. It turns out that D. radiodurans
and all the other so-called radiophiles that researchers have identified are
genetically linked to known life, so  The Integrated Ocean Drilling Program
has been sampling rocks from the seabed to a depth approaching one kilometer,
in part to explore their microbial content. Boreholes on land have revealed
signs of biological activity from even deeper locations. So far, however, the
research community has not conducted a systematic, large-scale program to
probe the deep subsurface of Earth’s crust for life.

Ecologically Integrated Aliens One might suppose it would be easier to find
alternative life-forms if they were not isolated but integrated into the
known biosphere existing all around us. But if shadow life is restricted to
alien microbes that are intermingled with familiar kinds, the exotic
creatures would be very hard to spot on casual inspection. Microbial
morphology is limited—most microorganisms are just little spheres or rods.
Aliens might stand out biochemically, though. One way to search for them is
to make a guess as to what alternative chemistry might be involved and then
look for its distinctive signature.

A simple example involves chirality. Large biological molecules possess a
definite handedness: although the atoms in a molecule can be configured into
two mirror-image orientations—left-handed or right-handed—molecules must
possess compatible chirality to assemble into more complex structures. In
known life-forms, the amino acids—the building blocks of proteins—are
left-handed, whereas the sugars are right-handed and DNA is a right-handed
double helix. The laws of chemistry, however, are blind to left and right, so
if life started again from scratch, there would be a 50–50 chance that its
building blocks would be molecules of the opposite handedness. Shadow life
could in principle be biochemically almost identical to known life but made
of mirror-image molecules. Such mirror life would not compete directly with
known life, nor could the two forms swap genes, because the relevant
molecules would not be interchangeable.

Fortunately, researchers could identify mirror life using a very simple
procedure. They could prepare a nutrient broth consistinigestible. The study,
however, looked at just a small fraction of the microbial realm.

Another possibility is that shadow life might share the same general
biochemistry with familiar life but employ a different suite of amino acids
or nucleotides (the building blocks of DNA). All known organisms use the same
set of nucleotides—designated A, C, G and T for their distinguishing bases
(adenine, cytosine, guanine and thymine)—to store information and, with rare
exceptions, the same 20 amino acids to construct proteins, the workhorses of
cells. The genetic code is based on triplets of nucleotides, with different
triplets spelling out the names of different amino acids. The sequence of
triplets in a gene dictates the sequence of amino acids that must be strung
together to build a particular protein. But chemists can synthesize many
other amino acids that are not present in known organisms. The Murchison
meteorite, a cometary remnant that fell in Australia in 1969, contained many
common amino acids but also some unusual ones, such as isovaline and
pseudoleucine. (Scientists are not sure how the amino acids formed in the
meteorite, but most researchers believe that the chemicals were not produced
by biological activity.) Some of these unfamiliar amino acids might make
suitable building blocks for alternative forms of life. To hunt for such
aliens, investigators would need to identify an amino acid that is not used
by any known organisms nor generated as a by-product of an organism’s
metabolism or decay, and to look for its presence in the environment, either
among living microbes or in the organic detritus that might be generated by a
shadow biosphere.

To help focus the search, scientists can glean clues from the burgeoning
field of synthetic, or artificial, life. Biochemists are currently attempting
to engineer completely novel organisms by inserting additional amino acids
into proteins. A pioneer of this research, Steve Benner of the Foundation for
Applied Molecular Evolution in Gainesville, Fla., has pointed out that a
class of molecules known as alpha-methyl amino acids look promising for ara
relatively simple matter to use standard tools for analyzing the composition
of proteins, such as mass spectrometry, to learn which amino acids the
organisms contain. Any glaring oddities in the inventory would signal that
the microbe could be a candidate for shadow life.

If such a strategy were successful, researchers would face the difficulty of
determining whether they were dealing with a genuine alternative form of life
descended from a separate origin or with merely a new domain of known life,
such as archaea, which were not identified until the late 1970s. In other
words, how can scientists be sure that what seems like a new tree of life is
not in fact an undiscovered branch of the known tree that split away a very
long time ago and has so far escaped our attention? In all likelihood, the
earliest life-forms were radically different from those that followed. For
example, the sophisticated triplet DNA code for specifying particular amino
acids shows evidence of being optimized in its efficiency by evolutionary
selection. This observation suggests the existence of a more rudimentary
precursor, such as a doublet code employing only 10, rather than 20, amino
acids. It is conceivable that some primitive organisms are still using the
old precursor code today. Such microbes would not be truly alien but more
like living fossils. Nevertheless, their discovery would still be of immense
scientific interest. Another possible holdover from an earlier biological
epoch would be microbes that use RNA in place of DNA.

The chance of confusing a separate tree of life with an undiscovered branch
of our own tree is diminished if one considers more radical alternatives to
known bio­chemistry. Astrobiologists have speculated about forms of life in
which some other solvent (such as ethane or methane) replaces water, although
it is hard to identify environments on Earth that would support any of the
suggested substances. (Ethane and methane are liquid only in very cold places
such as the surface of Titan, Saturn’s largest moon.) Another popular
conjecture concerns the basic chemical elements that make up the vitalematic
for life in some ways. It is relatively rare and would not have existed in
abundance in readily accessible, soluble form under the conditions that
prevailed during the early history of Earth. Felisa Wolfe-Simon, formerly at
Arizona State University and now at Harvard University, has hypothesized that
arsenic can successfully fill the role of phosphorus for living organisms and
would have offered distinct chemical advantages in ancient environments. For
example, in addition to doing all the things that phosphorus can do in the
way of structural bonding and energy storage, arsenic could provide a source
of energy to drive metabolism. (Arsenic is a poison for regular life
precisely because it mimics phosphorus so well. Similarly, phosphorus would
be poisonous to an arsenic-based organism.) Could it be that arseno-life
still lingers in phosphorus-poor and arsenic-rich pockets, such as ocean
vents and hot springs?

Another important variable is size. All known organisms manufacture proteins
from amino acids using large molecular machines called ribosomes, which link
the amino acids together. The need to accommodate ribosomes requires that all
autonomous organisms on our tree of life must be at least a few hundred
nanometers (billionths of a meter) across. Viruses are much smaller—as tiny
as 20 nanometers wide—but these agents are not autonomous organisms because
they cannot reproduce without the help of the cells they infect. Because of
this dependence, viruses cannot be considered an alternative form of life,
nor is there any evidence that they stem from an independent origin. But over
the years several scientists have claimed that the biosphere is teeming with
cells that are too small to accommodate ribosomes. In 1990 Robert Folk of the
University of Texas at Austin drew attention to tiny spheroidal and ovoid
objects in sedimentary rocks found in hot springs in Viterbo, Italy. Folk
proposed that the objects were fossilized “nannobacteria” (a spelling he
preferred), the calcified remains of organisms as small as 30 nanometers
across. More recently, Philippa Uwins of the University of
Queensl€”they may be evidence of alternative life-forms that do not use
ribosomes to assemble their proteins and that thus evade the lower size limit
that applies to known life.

Perhaps the most intriguing possibility of all is that alien life-forms
inhabit our own bodies. While observing mammalian cells with an electron
microscope in 1988, Olavi Kajander and his colleagues at the University of
Kuopio in Finland observed ultrasmall particles inside many of the cells.
With dimensions as small as 50 nanometers, these particles were about
one-tenth the size of conventional small bacteria. Ten years later Kajander
and his co-workers proposed that the particles were living organisms that
thrive in urine and induce the formation of kidney stones by precipitating
calcium and other minerals around themselves. Although such claims remain
controversial, it is conceivable that at least some of these Lilliputian
forms are alien organisms employing a radically alternative biochemistry.

What Is Life, Anyway?

If a biochemically weird microorganism should be discovered, its status as
evidence for a second genesis, as opposed to a new branch on our own tree of
life, will depend on how fundamentally it differs from known life. In the
absence of an understanding of how life began, however, there are no
hard-and-fast criteria for this distinction. For instance, some
astrobiologists have speculated about the possibility of life arising from
silicon compounds instead of carbon compounds. Because carbon is so central
to our biochemistry, it is hard to imagine that silicon- and carbon-based
organisms could have emerged from a common origin. On the other hand, an
organism that employed the same suite of nucleotides and amino acids as known
life-forms but merely used a different genetic code for specifying amino
acids would not provide strong evidence for an independent origin, because
the differences could probably be explained by evolutionary drift.

A converse problem also exists: dissimilar organisms subjected to similar
environmental challenges will often gradually converge in their properties,
which will become optimized for thriving under existing conditions. If this
evolution to adopt the same set that familiar life-forms use.

The difficulty of determining whether a creature is alien is exacerbated by
the fact that there are two competing theories of biogenesis. The first is
that life begins with an abrupt and distinctive transformation, akin to a
phase transition in physics, perhaps triggered when a system reaches a
certain threshold of chemical complexity. The system need not be a single
cell. Biologists have proposed that primitive life emerged from a community
of cells that traded material and information and that cellular autonomy and
species individuation came later. The alternative view is that there is a
smooth, extended continuum from chemistry to biology, with no clear line of
demarcation that can be identified as the genesis of life.

If life, so famously problematic to define, is said to be a system having a
property—such as the ability to store and process certain kinds of
information—that marks a well-defined transition from the nonliving to the
living realm, it would be meaningful to talk about one or more origin-of-life
events. If, however, life is weakly defined as something like organized
complexity, the roots of life may meld seamlessly into the realm of general
complex chemistry. It would then be a formidable task to demonstrate
independent origins for different forms of life unless the two types of
organisms were so widely separated that they could not have come into contact
(for instance, if they were located on planets in different star systems).

It is clear that we have sampled only a tiny fraction of Earth’s microbial
population. Each discovery has brought surprises and forced us to expand our
notion of what is biologically possible. As more terrestrial environments are
explored, it seems very likely that new and ever more exotic forms of life
will be discovered. If this search were to uncover evidence for a second
genesis, it would strongly support the theory that life is a cosmic
phenomenon and lend credence to the belief that we are not alone in the
universe.