[ExI] Chemical Origins of Life (was Re: Panbiogenesis)

[Eugen Leitl]

On Sun, Feb 05, 2012 at 10:26:02AM +0000, BillK wrote:
> On Sun, Feb 5, 2012 at 9:34 AM, Eugen Leitl wrote:
> > We haven't found a single instance of a shadow biosphere yet on
> > this planet, so probably any crosscontamination would wipe out
> > the weaker biosphere. As a corollary, anything we'll find on Europa
> > or elsewhere will be our close relative, biochemically.
> >
> 
> 
> Well, there is Archaea as well as Prokaryote and they appear to have
> evolved separately.

I'm referring to the advent (or external panspermic contamination) 
of first autocatalytic sets on the prebiotic Earth. As the first 
ones burned through the prebiotic chemistry soup within subgeological 
time (subsequent accretion by meteor infall would have been negligible) 
the window for the advent for another, unrelated one was very short 
(probably <<MYear). It would also have to be of about the same fitness
in order to survive, so the odds are overwhelming there was a single 
effective nucleation event (not a point, it was an autocatalytic runaway 
with a slow start, likely with subsequent incorporation of multiple chemistries
just as with chloroplasts and mitochondria precursors later -- our world's
first linear semilog) and potential loss of bootstrap auxiliaries (as 
silicates for template autoassembly enhancement, maybe iron-sulfur and 
other chemistries -- some maybe still present in living fossils).

This simple picture might have been muddied by multiple resterilization/reseeding
events (both local and extraterrestrial) during the heavy bombardement phase.
 
> It is still speculative, of course. Some claim a common precurser.

They're based on lipids, carbohydrates, DNA, RNA and proteins -- 
and even a common genetic code, so they definitely have a common origin.
Independently emerged life would be very unlikely to resemble us down
to minute details, despite emerging from virtually identical prebiotic
ursoup.
 
> How eukaryotes evolved is also speculative as they may have bits of
> both lines incorporated.
> 
> Still early days in finding how life evolved.

What we need is prebiotic molecular fossils, which will be hard to find, as
oldest rocks are only 4.04 GYears (Acasta gneiss, Canada). Zircons
are slightly older, but sterilized via tectonic subduction.

Our best chances would be to look on the outskirts of the solar system,
which is unlikely to be so completely contaminated by life as to erase
the original chemistry -- whatever infalling samples we have do seem to
corroborate that story. There might be also pristine, albeit degraded by
age, samples of early life preserved in space ices ejected by impacts 
floating around in the solar system.