Rise of the Animal Kingdom and Epigenetic Mechanisms of Evolution - Building the Most Complex Structure on Earth - page 243

genera, and species), rejecting the basic bottom-up neoDarwinian tenet in favor of
the “top-down” hypothesis that predicts the origin of all the extinct and extant meta-
zoan taxa, from the class down to the species levels, from those early Bauplans of
the Cambrian phyla ( Erwin et al., 1987 ).
Like Darwin himself, we still wonder:
What might have caused this Big Bang in the diversification of animal forms?
There is no convincing explanation, let alone consensus, on factors that enabled
the unprecedented surge of evolutionary rates during the Cambrian explosion. The
early trend in evolutionary studies emphasized environmental factors as causes for
this eruptive diversification.
Presumed External Factors Involved in the Cambrian
Explosion
Several hypotheses have been presented positing that the Cambrian explosion was
perhaps triggered by external factors, by specific changes in the environment at the
time. The idea of a trigger per se suggests the evolution of a mechanism that may
be set in motion, but the bone of contention in understanding the evolution is the
intrinsic mechanism of the evolutionary change rather than the extrinsic factors that
may trigger or facilitate activation of the mechanism. Studying each of these factors
on its own, or all of them together, elevates the understanding of the possible selec-
tive forces that might have been in effect then, but it contributes little—if anything at
all—to an understanding of the mechanisms that generated the phenomenal evolu-
tionary diversification.
Ecological Factors
Some authors think that the expansion of phytoplankton during the transition from
Ediacaran to Cambrian may have contributed to the evolutionary innovations of
the Cambrian fauna ( Butterfield, 1997, 2001 ). Others believe that predation might
have been the main driving force of Cambrian diversification ( Conway Morris and
Bengtson, 1994 ). Growing predator pressure and competition for ecospace might
have forced shallow marine fauna to explore deeper settings by the end of the
Cambrian period ( Crimes, 2001 ).
Glaciation
According to the snowball Earth hypothesis, for a long period of time, extending
from 720 to about 630 Mya, the Neoproterozoic Earth experienced a freezing period
during which a thick layer of ice covered oceans and separated the Earth's atmos-
phere from the ocean. This snowball Earth, also known as Cryogenian, resulted from
a number of glaciations, the last of which was the Marinoan glaciation that lasted
from 660 to 630 Mya. The exhaustion of oxygen in the ocean might have led to the