Paul Hoffman and Dan
Schrag theorize that the rapid warming of a previously
ice-bound Earth triggered the Cambrian Explosion- the spasm
of evolution that produced the first multicellular animals.
Fossil evidence tells us this animal explosion occurred some
600 million years ago. But what evidence is there of such
a dramatic change in the Earth's climate?
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and Schrag propose that roughly 700 million years ago, the
Earth's climate cooled and the polar ice caps expanded. As
the ice advanced, more and more sunlight was reflected back
into space, causing the temperature to drop even further.
By the time the ice caps covered roughly half of the globe,
the cooling process was rapid and unstoppable. Soon the Earth
was completely entombed in ice one kilometer thick.
all Hell broke loose," says Schrag. "The global
temperature would have averaged higher than 90 degrees
"Snowball Earth" hypothesis is not a new one. Clues to this
episode in Earth's history have stumped geologists since the
1960's. Only recently have Hoffman and Schrag made sense of
it all. And the evidence they have pooled, geological, biological
and chemical, comes from all over the globe.
in 1964, Cambridge geologist Brain Harland discovered glacial
deposits near the equator. The hottest place on Earth covered
in glaciers? For years, scientists resisted the idea, assuming
the deposits were not native to the tropics, but had migrated
to the equator over time. Twenty-two years later, researcher
Joe Kirschvink discovered the perfect sample in Adelaide,
Australia- a glacial deposit with undeniable tropical origins.
The deposits' telltale sign- the "magnetic signature" found
only in rocks forged near the equator- is the central bit
of evidence that the Earth was once entirely locked in ice.
from cliffs like this one in Namibia first sparked Hoffman
and Schrag's theory
around the globe, in Namibia on the southwest coast of Africa,
Paul Hoffman fit together another piece of this geologic puzzle.
Hoffman measured the carbon levels in sediments found both
below and above known glacial deposits. In the sediments beneath
the glacial deposits, he discovered an excess of carbon-13,
a geologic indicator of photosynthesis. The finding showed
that the region, submerged at the time, had been temperate
and teeming with algae.
beneath the glacial deposits, however, the amount of carbon-13
drops off dramatically, indicating an abrupt halt in photosynthetic
activity. In the layers above the glacial deposits, the carbon
levels gradually rise. These findings suggest that a rapidly
cooling Earth brought plant life to a halt. Life remained
suspended during the long period of global glaciation and
then resumed and blossomed in the spring-like thaw that followed.
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