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RE: exobiology possibilities

On Thu, Mar 12th, 2015 at 10:55 PM, "Thomas R. Holtz, Jr." <tholtz@umd.edu> 

> On the contrary: we have every reason to think that all life on Earth studied 
> so far are
> descendants of a single instance of life, and every reason to dismiss the 
> possibility that any
> known terrestrial life is of independent origin. A good reason for this is 
> the fact that A, T, C
> & G are NOT the only possible nucleotides.  Furthermore people have 
> synthesized nucleic acid
> analogues in the lab that work essentially like DNA and RNA but which have 
> different backbones.
> The fact that all living things use only the one system when there are 
> numerous alternatives
> which are seemingly just as likely points to a common origin. Statistically, 
> we would expect
> alien life will  like have DNA/RNA analogues using other nucleotides and/or 
> backbones, if indeed
> they used these forms of molecules as the carrier of genetic information.

That's not to say that life only arose once on this planet though. There could 
have been any 
number of failed lineages that died out completely prior to the ancestor of 
modern lifeforms. I'd 
even suggest that it's highly unlikely that life arose just once on this 
planet, and happened to get 
things right the very first time.

Once any one type of life becomes established, any subsequent independently 
arising lifeforms 
might then have trouble out-competing the established lineage. A simple 
self-replicating protein 
may once have been able to float about relatively unmolested. Once one form of 
organic life 
established itself however, any less complex organic compounds would have been 
little more than 
potential food.


Dann Pigdon
Spatial Data Analyst               Australian Dinosaurs
Melbourne, Australia               http://home.alphalink.com.au/~dannj