How and When Did Complex Life Begin?
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2016-01-11. Ancient Start of Animal Evolution Wasn't Delayed by Low Oxygen.
By Cody Sullivan, Earth & Space Science News (EoS; AGU).
2010 July 26. Translating Stories of Life Forms Etched in Stone.
By Sean B. Carroll, The New York Times. Excerpt:
…The difficulty posed by the Cambrian Explosion was
that in Darwin’s day (and for many years after), no fossils were known
in the enormous, older rock formations below those of the Cambrian. This
was an extremely unsettling fact for his theory of evolution because
complex animals should have been preceded in the fossil record by
simpler forms… but we now have fossil records from the time immediately
preceding the Cambrian. The rocks reveal a world whose oceans were
teeming with a variety of life forms, including primitive animals, which
is certainly good news for Darwin.
…This once-worrisome gap in the fossil record is a
period of intense interest to geologists as well as paleontologists. The
former have even given it its own division in the geological timescale.
The Ediacaran Period, from 635 to 542 million years ago, is the first
new geological period to be named in more than a century. Moreover,
geologists have developed some intriguing theories about how dramatic
changes in the Earth’s climate and chemistry during the Ediacaran may
have allowed for the evolution of animals
…But finding these fossils has posed many new
mysteries. Many of the creatures are so unlike modern forms that
deciphering what they are and how they lived continues to challenge
paleontologists. Prof. Andrew Knoll of Harvard University has likened
the Ediacaran forms to a paleontological “Rorschach” test because
different scientists often interpret the same fossil very differently.
…The kinds of animals that paleontologists have been
especially eager to identify in the Ediacaran are those with bilateral
body symmetry, the feature characteristic of the majority of modern
animal groups, including ourselves.
…Recent chemical analyses of Ediacaran sediments reveal
that the deep ocean lacked oxygen before and during that ice age, then
became much richer in oxygen and stayed that way after the glaciers
melted. That sharp rise in oxygen may have been the catalyst to the
evolution of animals, including our ancestors.
2008 October 17. Evolution
Center for Science Education - articles