Stuart Gary
Atmospheric oxygen levels during the billion years or so
prior to the rise of animals were far too low for complex life forms to
develop, according to a new study.
The findings, reported in the journal Science, imply that
the appearance of diverse animal life on Earth about 800 million years ago, was
triggered by increases in oxygen levels - and not just genetic innovations in
individual organisms.
"No one really doubted that oxygen levels were low, but
how low is the real surprise," says one of the study's authors Dr Peter
McGoldrick of the University of Tasmania.
"Our work shows those levels were just 0.1 per cent of
present atmospheric levels, which is significant from an evolutionary point of
view because biologists believe that complex multicellular life forms require
much more oxygen than 0.1 per cent."
This is the first time anyone has been able to quantify the
levels of oxygen in the atmosphere during the mid-Proterozoic period between
0.8 and 1.8 billion years ago, he says.
McGoldrick describes this period in Earth's history as the
'boring billion', when life remained largely constant and unchanging between
the appearance of complex cells around 2 billion years ago, and the sudden
diversification of multicellular animals about 800 million years ago.
Scientists already knew that oxygen began to accumulate in
the atmosphere after cyanobacteria began using photosynthesis to produce oxygen
over three billion years ago.
So they wondered why animal species didn't flourish during
the boring billion year stretch leading up to the end of the Proterozoic, when
most researchers thought there was plenty of oxygen.
"We knew oxygen levels had gone up over all, but we
didn't know if it had gone up to 1, 10 or 40 per cent of present atmospheric
levels," says McGoldrick.
"This explains why complex animals don't appear in the
rock record until maybe 750 to 800 million years ago, there simply wasn't
enough oxygen for the metabolic things they need to do."
The increase in atmospheric oxygen levels wasn't constant -
spikes and drops were caused by fluctuations in bacterial activity and
geochemical events such as tectonic movements and weathering.
Chrome key
Oxygen levels in the atmosphere were determined by examining
chromium isotopes in ironstone samples. This provided information on oxygen
levels for the billion or so years leading up to the 'Cambrian explosion' -
when most major animal groups appeared on the planet.
The samples came from China, Canada, the United States, and
1.4 billion year old iron ore deposits from the Sherwin formation near
Borroloola in the Northern Territory.
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