Re: Fw: Most popular/common dinosaur misconceptions

[David Marjanovic <david.marjanovic@gmx.at>]

The ancestors mentioned in phylogenetic definitions are usually understood 
to be "species, populations, or organisms" (as the PhyloCode words it).

> All Earth life shares certain key genetic sequences, similar cell
> membrane proteins, etc., and that is nearly incontrovertable proof for
> monophyletic Earth life.  But inherent in this thesis is the assumption
> that polyphyletic life would be recognizable in the first place.  That's
> a big assumption.

I think it's a lot more likely that, once life arises, it annihilates the 
conditions necessary for a repeat. What happens today when a ribozyme swims 
around in the ocean? It gets eaten.

I don't think there's a reason to expect that life elsewhere should share a 
lot of characteristics with life on Earth. There is no good reason for why 
nucleic acids contain exactly those four bases, and there is none why a 
sugar-phosphate backbone is used instead of a protein one. Lots of very 
interesting variations are imaginable, and some have patent advantages --  
pyridines (as opposed to purines and pyrimidines) can bind metal ions and 
thus allow for lots of interesting chemistry, a protein backbone (PNA) is 
much more stable than a sugar-phosphate one, and so on. There is no good 
reason why exactly those 20 amino acids normally occur in proteins, nor is 
there a good reason for the number 20. Some 800 amino acids are known to 
occur in some organism or other (usually unspectacular plants, AFAIK). The 
genetic code does seem to have evolved for maximal error tolerance (assuming 
precisely those 20 amino acids!), but even this should allow for more 
variation than we see. A bit of digging should bring up lots more examples 
that work like the old Microsoft joke: "How many Microsoft programmers do 
you need to change a lightbulb? -- None. Microsoft declares darkness the 
standard."