ARTICLES
Long before Dinosaurs, virtually all life on Earth was wiped off the
Face of the Planet
By Malcolm Hodgskiss
New evidence suggests that starvation trumps an asteroid hit –
when it comes to wiping out life on Earth.
small beings, notably marine microbes known as cyanobacteria,
were instrumental in stabilising and enriching the Earth’s
atmosphere, such that more sophisticated forms of life were
eventually able to evolve.
The 10-kilometre wide rock that crashed into the Gulf of Mexico
66 million years ago, not only killed off the dinosaurs, it wiped out
more than 75 per cent of all animals and plants on Earth; a pretty
good effort.
The story goes like this: about 2.4 billion years ago, oxygen was
scarce, certainly as a free-floating gas – most of it was trapped
in minerals or water. Cyanobacteria had already existed for at
least a billion years, and may have had the ocean to themselves.
Then, climatic conditions allowed them to thrive – and, via
photosynthesis, they processed sunlight to liberate oxygen into
the then unfriendly atmosphere. The oxygen levels were further
boosted by the weathering of rocks.
However, now scientists from Stanford University have found
strong evidence that an even bigger loss occurred 2.05 billion
years ago – when up to 99.5% of all life died out after a sudden
collapse in oxygen levels in the atmosphere.
Fossil record of cyanobacteria – courtesy of the
University of Berkeley
This led to what is known as the Great Oxidation Event, when the
air grew rich, and life thrived and diversified. However, it may have
been an excellent case of too much too soon – a theory known
as the “oxygen overshoot”. According to this theory, the boon
in atmospheric oxygen eventually waned as the cyanobacteria
exhausted their nutrient supply, largely phosphorus, in the ocean
and became less abundant. Consequently, when the oxygen
collapsed, so did virtually all life.
A wistromatolite. Courtesy of the University of Berkeley, USA
The new evidence for this theory was found in rocks from the
Belcher Islands in Canada’s Hudson Bay, where Stanford
University doctoral candidate Malcolm Hodgskiss collected
barite samples dating 2.02 to 1.87 billion years old.
There are no fossils of tortured creatures to support this
theory – because the kind of life thriving back then was tiny,
microorganisms easily obliterated by geologic forces. Still, these
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SCIENCE EDUCATIONAL NEWS VOL 68 NO 4