Geology Reference
In-Depth Information
outlined in a lecture at a meeting of geologists in Denver, Colorado, is in support of a con-
troversial idea by British biologist Donald Williamson. In 2009 Williamson proposed that
butterflies represent the merging of the genetic material of two very different animals—the
wormlike caterpillar and the winged butterfly. The controversy intensified when Margulis
used her privilege as a member of the National Academy of Sciences to shortcut the peer-
review process and sponsor Williamson's publication in the Academy's prestigious journ-
al,
Proceedings
. Some members were incensed, calling the hypothesis “nonsense,” more
suited to the
National Enquirer
than a scientific periodical. Margulis countered that Willi-
amson's article is worthy of serious scrutiny and debate. “We don't ask anyone to accept
Williamson's ideas,” she said, “only to evaluate them on the basis of science and scholar-
ship, not knee-jerk prejudice.”
Whatever the eventual outcome of that debate, Margulis's theory of endosymbiosis is
now conventional thought. By the Neoproterozoic Era, complex cells with nuclei and oth-
er internal structures were well established and were poised to cross a new symbiotic
threshold.Morethansixhundredmillionyearsago,single-celledorganismslearnedhowto
cooperate, to congregate, to specialize, and to grow and move in a collective. They learned
to become animals.
The earliest fossil evidence for an animal-dominated ecosystem comes from the so-
called Ediacaran Period, which began about 635 million years ago, shortly after the second
of the three great snowball Earth events. The first distinctly patterned fossils were re-
cognized from 580-million-year-old rocks from Ediacara in southern Australia (hence the
name).Thesesoft-bodiedanimals,possiblerelativesofjellyfishandworms,leftpleasingly
symmetrical impressions, like ornately lined pancakes or fancifully striated leaves up to
two feet across. Similar fossils have subsequently been found all over the world in rocks
between about 610 and 545 million years old. Most remarkably, the 633-million-year-old
phosphate-richDoushantuoformationofsouthernChinaholdsclumpsofmicroscopiccells
interpreted as animal eggs and embryos. These structures, which grew in shallow seas just
after the Marinoan glaciation, appear identical in every respect to modern animal embryos.
Soitappearsthattheseveresnowball-hothousecycleultimatelyplayedacentralroleinthe
evolution of the modern world. It would even be fair to say that we multicelled organisms
owe our existence to that moment eight hundred million years ago when Earth reached a
climatictippingpointaftermorethanabillionyearsinwhichsteadysunlightandheat-trap-
pingCO
2
hadkeptitwarm.Whenthatcarbondioxidewasrapidlyconsumedbytheweath-
ering of new equatorial continents, and reflective ice spread from both poles to the Equat-
or, Earth's temperatures plunged for millions of years—until steady CO
2
buildup, perhaps
amplified by the rapid release of methane from the ocean floors, triggered an equally rapid
runaway greenhouse effect.
