Re: Article: Are Aliens Among Us?
- From: ragland31@xxxxxxxxx
- Date: Sun, 9 Dec 2007 23:40:47 -0500 (EST)
On Nov 23, 10:51 am, "Robert Karl Stonjek" <rston...@xxxxxxxxxxxxxx>
wrote:
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 Cop-ley 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 they are not
candidate aliens, but that finding does not rule out the possibility of
discovering alternative life-forms in this way.
Investigators have already pinpointed a handful of ecosystems that appear to
be almost completely isolated from the rest of the biosphere. Located far
underground, these microbial communities are cut off from light, oxygen and
the organic products of other organisms. They are sustained by the ability
of some microbes to use carbon dioxide and hydrogen released by chemical
reactions or radioactivity to metabolize, grow and replicate. Although all
the organisms found to date in these ecosystems are closely related to
surface-dwelling microbes, the biological exploration of Earth's deep
subsurface is still in its infancy, and many surprises may lie in store. 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 pos
sess 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 consisting entirely of the
mirror images of the molecules usually included in a standard culture
medium; a mirror organism might be able to consume the concoction with
gusto, whereas a known life-form would find it unpalatable. Richard Hoover
and Elena Pikuta of the NASA Marshall Space Flight Center recently performed
a pilot experiment of this kind, putting a variety of newly discovered
extremophiles into a mirror broth and then looking for biological activity.
They found one microbe that grew in the broth, an organism dubbed
Anaerovirgula multivorans that had been isolated from the sediments of an
alkaline lake in California. Disappointingly, this organism did not turn out
to be an example of mirror life; rather it was a bacterium with the
surprising ability to chemically alter the amino acids and sugars of the
wrong handedness so as to make them digestible. 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 artificial life because they can fold properly. These
molecules, however, have not been found in any natural organism studied to
date. As investigators identify new microbes, it would be a 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 bio-chemistry. 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
vital parts of known organisms: carbon, hydrogen, oxygen, nitrogen and
phosphorus. Would life be possible if a different element were substituted
for one of these five?
Phosphorus is problematic 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 Queensland has
discovered similar structures in rock samples from a deep-ocean borehole off
the coast of Western Australia. If these structures indeed arise from
biological processes-and many scientists hotly dispute this contention-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
evolutionary convergence were strong enough, it could mask the evidence for
independent biogenesis events. For example, the choice of amino acids may
have been optimized by evolution. Alien life that began using a different
set of amino acids might then have evolved over time 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.
Source: Scientific Americanhttp://www.sciam.com/article.cfm?id=are-aliens-among-us&sc=WR_20071120
Posted by
Robert Karl Stonjek
I don't believe in extraterrestrial intelligence having built up life
on earth before. I believe in earth based
life having built up and then being destroyed by asteroids and this is
the first time on earth life has
been left to evolve long enough for complexity to occur. I do believe
there is extraterrestrial intelligence
in the universe and nor has it been necessary to have an earth centric
basis to it. I base this on the
vastness of the universe and the various complexites, not all carbon
based, which could have arisen.
I definitely believe there are much more advanced life in the universe
than us; however, I have no
scientific evidence to back up my beliefs.
Michael Ragland
.
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