> 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?
>
> Brent Wegher

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.
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