"irr"  wrote in
news:bv8vf1$15dr$1{at}darwin.ediacara.org: 

>> So tell me how a modern procaryote is different from its 2 billion
>> year 
> old
>> ancestor.  How has the niche changed?  I could easily say that a 2
>> billion year old cyanobacterium is no different from today.
>> Do you have a 2 billion year old gene map?

> Of course no one has a 2 billion year old gene map, though inferring
> this "2 billion year old gene map" is not intractable and is exactly
> the point of evolutionary genomics.  So I could tell you with great
> certainty that a 2 billion year old cyanobacterium would have been, in
> most respects, quite unlike anything that we've so far discovered on
> the modern Earth.  This can be argued from a genetic basis: for
> example, differences in extant chloroplasts (which had either not yet
> emerged, or just barely come about circa 2 BYA) and cyanobacteria
> indicate that the photosynthetic apparatus was still several proteins
> shy of its modern composition -- but also from a "niche" perspective.
> This can also be argued from a "changing niche" basis, for example
> scientists know that even though oxygen had begun to accumulate by 2
> billion years ago, the partial pressure was probably less than 1/10
> what it is on the modern Earth.  Enzymes that are inactivated at high
> O2 levels, such as nitrogenase, or whose function has been largely
> attenuated since the oxidation of the atmosphere, such as RuBisCO,
> were undoubtedly controlled by different regulatory mechanisms and
> there is good evidence that their active sites have indeed changed
> since that time. 


But is this true of chemoautotrophs? I realize that, for example,  
Nitrosomonas and Nitrobacter need relatively high E0 levels, so that 1/10 
of the oxygen level would not be enough for them, but 1/5 would be - and 
_their_ niche really hasn't changed that much. Similarly for some of the 
methanogens, which don't even need (in fact can't tolerate) high oxygen 
levels. Obviously we would expect considerable substituting at non-active 
sites, and some increase in overall efficiency, but perhaps not all that 
much - AFAIK hemoglobin is remarkably similar at the active site 
throughout the vertebrates, despite many millions of years of divergent 
evolution.  

Yours,

Bill Morse
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