Informe

B IOLOGY

ALIENS
among Us?

A LIEN MICROBES may be hiding in plain sight. Al-

though they might look like ordinary bacteria,
their biochemistry could involve exotic amino
acids or different elemental building blocks.

62

SCIENTIFIC AMERICAN

December 2007

KENN BROWN Mondolithic Studios

Are

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

that two of its hallmarks are an ability to metabhe origin of life is one
olize (to draw nutrients from the environment,
of the great unsolved
convert those nutrients into energy and excrete
problems of science. Nobody
waste products) and an ability to reproduce. The
knows how, where or whenlife originated. About all that is known for certain is that orthodox view of biogenesis holds that if life on
microbial life had established itself on Earth by Earth originated more than once, one form
about three and a half billion years ago. In the would have swiftly predominated and eliminatabsence of hard evidence of what came before, ed all the others. This extermination might havehappened, for example, if one form quickly apthere is plenty of scope for disagreement.
Thirty years ago the prevailing view among propriated all the available resources or “ganged
biologists was that life resulted from a chemical up” on a weaker form of life by swapping sucfluke so improbable it would be unlikely to have cessful genes exclusively with its own kind. But
happened twice in theobservable universe. That this argument is weak. Bacteria and archaea,
conservative position was exemplified by Nobel two very different types of microorganisms that
Prize–winning French biologist Jacques Monod, descended from a common ancestor more than
who wrote in 1970: “Man at last knows that he three billion years ago, have peacefully coexistis alone in the unfeeling immensity of the uni- ed eversince, without one eliminating the other.
verse, out of which he emerged only by chance.” Moreover, alternative forms of life might not
In recent years, however, the mood has shifted have directly competed with known organisms,
dramatically. In 1995 renowned Belgian bio- either because the aliens occupied extreme envichemist Christian de Duve called life “a cosmic ronments where familiar microbescould not
imperative” and declared “it is almost bound to survive or because the two forms of life required
arise” on any Earth-like planet. De Duve’s state- different resources.
ment reinforced the belief among astrobiologists that the universe is teeming with life. T he Argument for Aliens
Dubbed biological determinism by Robert Even if alternative life does not exist now, it
Shapiro ofNew York University, this theory might have flourished in the distant past before
is sometimes expressed by saying that “life is dying out for some reason. In that case, scienwritten into the laws of nature.”
tists might still be able to fi nd markers of their
How can scientists determine which view is extinct biology in the geologic record. If alternacorrect? The most direct way is to seekevidence tive life had a distinctively different metabolism,
for life on another planet, such as Mars. If life say, it might have altered rocks or created minoriginated from scratch on two planets in a sin- eral deposits in a way that cannot be explained
gle solar system, it would decisively confirm the by the activities of known organisms. Biomarkhypothesis of biological determinism. Unfortu- ers inthe form of distinctive organic molecules
nately, it may be a long time before missions to that could not have been created by familiar life
the Red Planet are sophisticated enough to hunt might even be hiding in ancient microfossils,
for Martian life-forms and, if they indeed exist, such as those found in rocks dating from the
to study such extraterrestrial biota in detail.
Archean era (more...