Punctuated Equilibrium and the Validation of Macroevolutionary Theory 899
popular model for life's origins, from the old primordial soup of Haldane and
Oparin, to Cairns-Smith's clay templates (1971), to preferences for deep-sea vents
as a primary locale—life can hardly begin in any other morphological status than
just adjacent to what I have called (see Fig. 9-29) the "left wall" of minimal
conceivable preservable complexity, that is, effectively, as bacteria (at least in
terms of entities that might be preserved as fossils). I can hardly imagine a scenario
that could begin with the precipitation of a hippopotamus from the primordial
soup.
Once life originates, by physicochemical necessity, in a location adjacent to
this left wall (see Kauffman, 1993), the subsequent history of right-skewed
expansion arises predictably as a fundamental geometric constraint of this initial
condition combined with the principles of Darwinian evolution—that is, so long as
the most genuine trend of life's history then prevails: "success" measured
variationally, in true Darwinian fashion, as expansion in diversity and range
through time.
If life continues to add taxa and habitats, then structural constraints of the
system virtually guarantee that a right tail of complexity will develop and increase
in skew through time as a geometric inevitability, and not necessarily for any
overall advantage conferred by complexity. As noted above, life must begin, for
physicochemical reasons, next to the left wall of minimal complexity. Little or no
"space" therefore exists between the initial bacterial mode and this natural lower
limit; variation can expand only into the "open" domain of greater complexity. The
vaunted trend to life's increasing complexity must be reconceived, therefore, as a
drift of a small percentage of species from the constant mode of life's central
tendency towards the only open direction
9 - 30. In life's full genealogy, all three multicellular kingdoms grow as twigs at the terminus of
just one branch among the three great domains of life's history. The other two domains are
entirely prokaryotic. From work of Woese and colleagues, as presented in Gould, 1996a.