Rise of the Animal Kingdom and Epigenetic Mechanisms of Evolution - Building the Most Complex Structure on Earth - page 244

extinction of algae and extremely low temperatures plausibly caused the extinction
of the bulk of preglacial microorganisms and plants. During the deglaciation period
following the Marinoan glaciation, new conditions and new ocean chemistry and
niches might have favored Ediacaran fauna evolution and the Cambrian explosion.
This again fails to explain why a surge in animal diversification did not occur after
the three earlier glaciations (Erwin, 2005) and why the Cambrian occurred only tens
of millions of years after the preceding glaciation ( Conway Morris, 2003 ).
Oxygenation of Oceans and Earth Atmosphere
It is believed that the gradual transition from the anoxic Proterozoic oceans into the
oxygenated atmosphere and oceans increased the chances of success of evolutionary
novelties and inventions. However, it is not easy to prove that the oxygen enrich-
ment of the oceans and the atmosphere coincided with the advent of the Ediacaran
fauna (Erwin, 2005). It has been suggested that the oxygen concentration required
by active macrobenthic animals may have reached about 100 million years before
the Cambrian radiation occurred. Even if such a coincidence of these ecological and
geochemical changes with the Cambrian radiation existed, it does not prove a causal
relationship between them.
At best, these external factors might have represented no more than selection
pressures that indirectly (by providing selective advantages/disadvantages) may have
influenced the course of metazoan evolution, and they do not tell us anything about
the mechanisms that produce evolutionary innovations and novelties. These specific
environmental changes also fail to explain the long periods of evolutionary stasis
prior to the Cambrian explosion.
Before I present my hypothesis and relevant substantiating evidence, I will briefly
outline a few of the best known hypotheses for the evolution of the organic world.
Current Hypotheses 2 of Organic Evolution
Natural selection is generally accepted as the driver of biological evolution. Hence,
the basic criterion used here to classify the hypotheses will be the mechanism of evo-
lutionary change.
Almost all evolution hypotheses posit that the cause of the evolutionary change
or novelty lies within the evolving organism, rather than in its environment. The
essential difference between these hypotheses is in the nature of changes; are these
changes spontaneous and random or is there any directionality in their occurrence?
Based on this criterion, the hypotheses of evolution fall into one of the two groups,
genetic or epigenetic hypotheses.
Genetic hypotheses see the origin of evolutionary changes as spontaneous and
random changes in the genetic material, in genes, regulatory sequences, or other
2 Due to the large number of competing scientiic proposals for explaining organic evolution, I prefer to
use the general term hypotheses in describing each of them.