I earlier quoted Darwin's last sentence in the "Origin":

> "There is grandeur in this view of life, with its several powers, having
been
> originally breathed by the Creator into a few forms or into one; and that,
> whilst this planet has gone cycling on according to the fixed law of
gravity,
> from so simple a beginning endless forms most beautiful and most wonderful
> have been, and are being evolved."

To Darwin, the evidence that he saw everywhere in the life that surrounded
him, indicated to him -- without any knowledge of genetics, and certainly none
of DNA -- rather conclusively that all life was descended from one or at most a
few common ancestors.

I am always greatly impressed with individuals who will draw the heroic
intellectual conclusion. Mendeleyev leaving holes in "periodic table" for elements
that no one yet knew of, but which he felt had to be there if his
understanding was to be correct, is one of those people. Darwin and his conclusions is
another.

But Darwin wasn't sure that life originated just once on this planet, or
whether it arose polyphyletically ("many family trees"), thus his last sentence.
The question of the polyphyletic origin of life on this planet arose again to
prominence in the 1960's and early 1970's, but it's an idea that didn't persist
for a very good reason: we see absolutely no evidence of it all today. All
life on this planet that we know of is made of the same machinery of life, and
we are brothers to every bat, beetle and tree on the planet. We are all made of
the same stuff. The upshot of this evidence is that life apparently arose
only once on this planet, at only one specific point in time.

Nevertheless, the polyphyletic origin of life on Earth still isn't without
its possibilities, and in today's NY Times, Paul Davies, an Australian
astrobiologist writes the following:

=======================================

April 10, 2005
OP-ED CONTRIBUTOR
Goodbye Mars, Hello Earth
By PAUL DAVIES

Sydney

WHEN I was a student in the 1960's, anyone who believed that there might be
life on other planets was considered a crackpot. Now all that has changed. To
claim that life is widespread in the universe is not only respectable, it also
underpins NASA's ambitious astrobiology program. Find another Earth-like
planet, astrobiologists say, and life should have happened there too.

NASA is spending billions of dollars to search for life on Mars, the most
Earth-like of our sister planets. But we may not need to go all the way to Mars
to find another sample of life. It could be lurking under our very noses. No
planet is more Earth-like than Earth itself, so if life started here once, it
could actually have started many times over.

Geologists believe life established itself on Earth about four billion years
ago. Australian rocks dated at 3.5 billion years contain fossilized traces
suggesting that microbes were already well ensconced by then. But the ancient
Earth was no Garden of Eden. Huge asteroids and comets mercilessly pounded the
planet, creating conditions more reminiscent of hell. The biggest impacts would
have swathed our globe in incandescent rock vapor, boiling the oceans dry and
sterilizing the surface worldwide.

How did life emerge amid this mayhem? Quite probably it was a stop-and-go
affair, with life first forming during a lull in the bombardment, only to be
annihilated by the next big impact. Then the process was repeated, over and over.
As the bombardment began to abate and the impacts diminished in severity, so
isolated colonies of primitive microbes sheltering deep underground managed to
cling on. One of these colonies was destined to become life as we know it.

What about the preceding life forms? Were they all completely destroyed? It's
possible that pockets of microbes could have survived in obscure niches until
the next genesis, opening up the tantalizing prospect of two or more
different forms of life co-existing on the same planet. Although they would compete
for resources, one type of life is not necessarily bound to eliminate the rest.
After all, within the microbial realm of "life as we know it," many different
species make a living side by side.

Thus, microbes from another genesis - alien bugs, if you will - could
conceivably have survived on Earth until today. The chances are that we wouldn't have
noticed. Under a microscope, many microbes appear similar even if they are as
genetically distinct as humans are from starfish. So you probably couldn't
tell just by looking whether a micro-organism is "our" life or alien life.
Genetic sequencing is used to position unknown microbes on the tree of life, but
this technique employs known biochemistry. It wouldn't work for organisms on a
different tree using different biochemical machinery. If such organisms exist,
they would be eliminated from the analysis and ignored. Our planet could be
seething with alien bugs without anyone suspecting it.

How could we go about identifying "life as we don't know it"? One idea is to
look in exotic environments. The range of conditions in which life can thrive
has been enormously extended in recent years, with the discovery of microbes
dwelling near scalding volcanic vents, in radioactive pools and in pitch
darkness far underground. Yet there will be limits beyond which our form of life
cannot survive; for example, temperatures above about 270 degrees Fahrenheit. If
anything is found living in even harsher environments, we could scrutinize its
innards to see whether what makes it tick is so novel that it cannot have
evolved from known life.

Identifying alien organisms in more equable settings would be a much harder
challenge, especially if they use the same basic molecules as familiar life -
nucleic acids and proteins. But there is one sure-fire giveaway. The building
blocks of proteins, called amino acids, are all lopsided in the same
distinctive way. Viewed in a mirror, these "left-handed" amino acids would appear
right-handed. Such mirror-image molecules exist, but the life forms we are familiar
with don't use them. Most biochemists think it is just an accident that "life
as we know it" selected the left-handed version. If this supposition is
correct, then there is a 50-50 chance that alien life would have picked the opposite
handedness. Such "anti-life" would eat "anti-food": right-handed amino acids
and other mirror molecules. This offers a simple way to filter out known life
from anything alien. Prepare a culture medium of anti-food and see if anything
flourishes. Of course it's a long shot, but it is easy to try, and scientists
at the Marshall Space Flight Center are now testing the response of microbes
from various extreme environments to a bowl of anti-soup.

Even if alien life has not endured to the present day, it may still have left
its mark. Geochemists have identified organic detritus from ancient microbes
in rocks as old as 2.7 billion years. Alien organisms might have left remnants
containing peculiar suites of molecules or produced distinctive geochemical
alterations like unusual mineral deposits.

These remnants would still give us a genuine second sample, a form of biology
that is unrelated to familiar life. By comparing the way evolution works in
both cases, we could identify which features of life follow from general
principles and which are just accidents of history.

But there is a more profound dimension to this research. Nobody knows how
life began. Somehow a mixture of lifeless chemicals assembled itself into a
primitive organism, presumably through a long and complex sequence of chemical
reactions. Our ignorance of this process is so great that scientists can't even
agree on whether it was a gigantic, one-time fluke, or the expected and frequent
outcome of intrinsically bio-friendly natural laws, as the astrobiologists
hope. Jacques Monod, a Nobel Prize-winning biologist, was adamant that life is a
bizarre accident confined to Earth. On the other hand Christian de Duve,
another Nobel laureate, declares life to be "a cosmic imperative," bound to occur
wherever Earth-like conditions prevail.

The discovery of a second sample of life on Earth would confirm that
bio-genesis was not a unique event and bolster the belief that life is written into
the laws of the cosmos. It is hard to imagine a more significant scientific
discovery. Our view of the universe and our place within it would be forever
transformed, and we would at last have the answer to the biggest of the big
questions of existence: Are we alone?


Paul Davies, a professor at the Australian Center for Astrobiology at
Macquarie University, is the author of "The Fifth Miracle: The Search for the Origin
and Meaning of Life."

========================================

Wirt Atmar

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