Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.mso.anu.edu.au/~charley/papers/MarsLife.doc Дата изменения: Fri Feb 20 09:51:14 2004 Дата индексирования: Tue Oct 2 03:19:19 2012 Кодировка: Поисковые слова: stellar evolution

Martian life: stuck somewhere between
inevitable biochemistry and quirky biology

Charles H. Lineweaver
School of Physics, University of New South Wales, Australian Centre for
Astrobiology, Sydney, email: charley@bat.phys.unsw.edu.au


If traces of life are found on Mars the question that needs to be asked is:
How independent is this life from life on Earth? A paradigm shift is
needed from "Was there a second genesis?" to "How much of one was there?"
This abandonment of a picture in black and white to a more nuanced grey is
based on the idea that the boundary between life and non-life was not sharp
and that the origin of life was an extended process of molecular tinkering.

Evidence for the idea that the origin of life was a long continuous process
comes from the long list of transitions required for molecules to become
microbes. This starts with the chemistry of molecular clouds and star
forming regions; and is followed by the formation and fractionation of
protoplanetary accretion disks near circumstellar habitable zones; and then
by the deposition of water and volatile-rich and carbon-rich material of
carbonaceous chondrites during the epoch of heavy bombardment (4.5 to 3.8
billion years ago). We have the molecular evolution of a diverse range of
organic molecules: amino acids, sugars, nucleotide bases and alcohol.
These monomers are so abundant in the universe that we expect that their
synthesis took place on or near any terrestrial planet in the universe
(Chyba and Sagan 1992, Raymond, Quinn and Lunine 2004).

The formation of closed lipid bilayers to produce spheroidal membrane-
bounded protobionts was the beginning of cellular life and follows directly
from the physical chemistry of a class of amphiphilic molecules found in
carbonaceous chondrites (Deamer & Pashley 1989). The transmembrane
potential was then exploited as a source of energy. Dehydration
condensation can be invoked to form polypeptides, link sugars to nucleotide
bases and also to store the chemical energy in the conversion between ADP
and ATP. Prebiotic chemical selection based on an ability to form self-
organizing chemical systems probably resulted in the transition from
racemic mixtures of amino acids and sugars to homochirality, and resulted
in primitive porphyrins, fermentation and the primitive photosynthesis
required for the transition from heterotroph to autotroph. With the
development of a reproducible macromolecule, life evolved from a pre-
genetic form to a genetic form using a genetic code. Then extensive
lateral gene transfer (or the "annealing" of Woese 2000) subsided to form
identifiable strains of bacteria and archaea.

In this sketch of the origin of life (Figure 1) an implicit assumption is
the physical determinism and inevitability of the first steps, followed by
progressively less determinism and more contingency as life's
idiosyncracies emerge. For example, the formation of atoms everywhere in
the universe is inevitable given the expanding cooling universe. The
formation of heavy elements by stellar processes and the approximate
relative abundances of these elements is inevitable given nuclear binding
energies. The formation of roughly terrestrial rocky planets near the
habitable zones of stars is probably inevitable for a wide range of stellar
metallicities (Lineweaver 2001). As we get closer to the origin of life
things may be less inevitable. Biochemical pathways become more
complicated, auto-catalytic, self-organized and self-referential.

Physics and chemistry are deterministic sciences. If you study them here on
Earth, you will be qualified to practise on the planets orbitting Proxima
Centauri. Biologist can make no such claims. Rules for the development of
proto-life anywhere in the universe are just the laws of physical chemistry
constrained by the terrestrial planet boundary conditions. Based on this
idea, Weber and Miller (1981) wrote: "If life were to arise on another
planet, we would expect that....75% of the amino acids would be the same as
on the earth." However, after the introduction of genetic information
processing, new more self-referential rules apply.

Evidence for increased quirkiness in metazoan evolution comes from the
sexual selection of extravagant colouration of face and genital regions.
Features that have nothing to do with adaptation to a physical environment
(peacock's tail) are selected as adaptations to the quirky behaviour of big-
brained sex partners, enemies and allies. The complex feedback loops of
ecosystems dominate the simple exigencies of chemistry and physics.

This transition from inevitable physics and biochemistry to the quirks of
history and biological
evolution (Smith and Morowitz 1982) has special relevance for evaluating
whatever signs of life we find on Mars (see Conway Morris 2003 for a
dissenting opinion on this progression from the deterministic to the
quirky).

What, if anything, does "a second genesis on Mars" mean? What could we
conclude from the discovery of a fossil on Mars? "If the biochemistry made
clear that Martian life derived from a separate and independent origin, it
would surely suggest that the universe is teeming with the stuff..." (Daley
2003). Finding evidence for a second genesis on Mars would be strong
evidence in favor of the idea that life is common in the universe (McKay
2001). With such evidence we might be justified to call life a convergent
feature of molecular evolution or a cosmic imperative (DeDuve 1995).
However, the strength of this evidence for convergence on life depends on
the degree of independence of Martian and terrestrial evolution. You can
not have convergence unless you first have divergence. The "independent"
evolution of the eye dozens of times is often cited as an example of
convergence, but the basic biochemistry and retinol in these "independent"
examples are the same and result from more than 3 billion years of shared
ancestry.

It may be the case that there is no identifiable event called the origin of
life anymore than there was an identifiable event called the origin of
France. Looking for DNA as a shibboleth among the earliest traces of life
on Earth or on Mars may be overestimating the past importance of something
which is only currently a universal feature of life -- analogous to looking
for buried French passports to determine when France originated.
[pic]
The divergence of Earth and Mars. As we look into the past, the closer we
get to the origin of the Earth and the origin of life on Earth, the less-
independence we have of what was happening on Mars. Mars and Earth have a
common ancestor: the inner Solar System. They emerged next to each other
out of the same protoplanetary disk, with a large, ever-decreasing exchange
of material between them during the formative period when life evolved, 4.5
to 3.8 billion years ago (plot on left). Only when the exchange rate
subsided did some degree of independent evolution become possible. The
stage in the origin of life at which the two planets became independent is
the important question to be answered.


Are we related to the guy next door? Of course we are, that is not an
interesting question. The question is how closely are we related? Are we
related to trees? Yes, but a long time ago (~ 2 Gyr). Are we related to E.
coli? Yes but even longer ago (~ 3 billion years). Are we related to
the organic material deposited on the Earth and Mars four billion years
ago? Yes, in a way. The threads of relatedness can be twined from
compositional similarity, not just genetics.
Determining the degree of independence is what we should focus on, not on
the naive question of complete independence or complete dependence. Will
whatever traces we find of life on Mars be related to life on Earth. Of
course it will be. Mars and Earth are not independent. They are both
terrestrial rocky planets orbiting the same star containing the same
elements, illuminated by the same temperature black body with the same
energy photons. The interesting question is how closely related...where in
the long process called the origin of life did the lives of Mars and of
Earth start to diverge?

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