Science > Physics > Lecture of the Week: Could We Tell Life if We Saw It?
| Topic: |
Science > Physics |
| User: |
"Wirt Atmar" |
| Date: |
21 Mar 2006 04:00:01 PM |
| Object: |
Lecture of the Week: Could We Tell Life if We Saw It? |
March 20, 2006
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March 20, 2006
Could We Tell Life If We Saw It?
ALH84001 in 2004
Joe Kirschvink
California Institute of Technology
34 min. (requires QCShow Player)
How can we hope to distinguish true biological microfossils from random
assemblages of crystalline mineral material — especially if the life
that those microfossils might represent were potentially an independent
origin of life, billions of years ago, on another planet, and is now
likely extinct? That's the question that has raged around the structures
found in the Allen Hills 84001 meteorite.
As UCLA's William Schopf has written, "There are fine lines between what
is known, guessed, and hoped for, and because science is done by real
people these lines are sometimes crossed. But science is not a guessing
game. The goal is to know. 'Possibly... perhaps... maybe' are not firm
answers and feel-good solutions do not count. With regard to the famed
Mars meteorite, for example, life either once existed on Mars or it
didn't. Meteorite ALH84001 either holds telling evidence or it doesn't.
Eventually, hard facts will sort it out."
Schopf has been among the harshest critics of the earliest
interpretations of life in ALH84001. "Probably the best way to avoid
being fooled by nonbiologic structures is to accept as bona fide fossils
only those of fairly complex form. This may seem an unreasonably
stringent rule for truly ancient fossils since the earliest kinds of
cellular life (here and presumably elsewhere) almost certainly were very
simple — probably individual, tiny, spheroidal cells. But until we have
a sounder base of knowledge and better rules to separate nonfossils from
true, it is best to err on the side of caution."
CalTech's Joe Kirschvink agrees, but comes to a different conclusion.
Microfossil paleontologies based on morphologies are undoubtedly flawed.
In morphology's place however, Kirschvink compellingly argues that the
fingerprint of natural selection can be detected by the very complexity
and purity of the results that selection produces. While there are no
biological processes that can not be reproduced in some manner by
non-life processes, the results of simple inorganic syntheses are
haphazard at best.
Magnetite exists in ALH84001, and Kirschvink argues that it was
biologically produced, primarily by subjecting it to a Venn diagram
analysis of seven different physical characteristics, each ranging from
hard to easy, and in the process pointing out that ALH84011's magnetite
is of an even higher quality than is capable of currently being
manufactured by human processes.
Magnetotactic bacteria were discovered on Earth only in the 1960's, but
we now know of south- and north-pole seeking bacteria. For an organism
evolved to exist in ponds within a narrow range of oxic-anoxic
conditions, where light doesn't penetrate and gravity is overwhelmed by
random Brownian motions, the evolution of magnetotaxis is an
exceptionally clever solution to the problem determining orientation.
Although Mars no longer has either a magnetic field or liquid water, it
is strongly presumed that Mars once had both, and the most parsimonious,
simplest explanation for the high-quality magnetite crystals that appear
in ALH84001 is that they were synthesized by organisms similar to
terrestrial bacteria.
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| User: "Hexenmeister" |
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| Title: Re: Lecture of the Week: Could We Tell Life if We Saw It? |
21 Mar 2006 06:08:09 PM |
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I can.
"Hello, life! Peek-a-boo. I saw you."
It is even possible to tell unintelligent life to construct clear
and unambiguous sentences, although probably pointless.
Androcles.
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