Origin of Animals
trilobite was one of the many multicellular animals to
evolve during the Cambrian Explosion
the weight of all this evidence, the Snowball Earth hypothesis
is gaining credibility. In fact, the geologic record indicates
the global climate flip-flopped not just once, but two to
five times. But what's most remarkable about the idea is not
that the Earth ever froze over, but when. The last thaw occurred
about 585 million years ago, just about the same time multicellular
animal life proliferated on this planet. Hoffman and Schrag
are convinced this is much more than coincidence.
would be remarkable," says Schrag, "if this catastrophic environmental
event were followed by this incredible radiation of life and
the two were not related."
could the Earth's alternate freezing and thawing contribute
to animal evolution? The freezing temperatures would have
killed off most living things on Earth, leaving only the hardiest
life forms behind. Small populations of these durable little
creatures likely huddled in volcanic hot springs or deep-sea
vents. Separated by miles of ice, these populations would
have diverged from each other over many generations. As with
Darwin's finches or African cichlids, "small populations and
physical separation," says Hoffman, "are the key ingredients
for creating new species."
propose that as the ice caps advanced, more sunlight was
reflected, and the temperature plummeted.
the ice receded, Hoffman and Schrag propose, the diverse species
were poised to exploit the novel ecological opportunities
the newly warm Earth presented - new habitats to colonize,
new resources to make use of. In the end, according to Schrag,
oxygen might have been the crucial resource that literally
fueled animal evolution.
the Earth's temperature rose, photosynthesis resumed full
throttle, as Hoffman demonstrated with the carbon-13 levels
in Namibia. Oxygen, a waste product of photosynthesis, must
have risen as well, providing an abundant energy source for
complex, multi-cellular life.
it's impossible to prove which variable truly triggered animal
evolution, for Hoffman and Schrag, the timing of the last
worldwide glacial retreat and the Cambrian explosion is simply
too much to over look.
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