Oxygen levels key to animal evolution

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.

Chromium is usually bound up in rocks as chromium III. However, weathering will convert chromium III to chromium VI if there is at least 0.1 per cent oxygen in the atmosphere. This results in a small change in the ratios of chromium isotopes 52 and 53.

"If there's less than 0.1 per cent oxygen in the air, the chromium isotopes don't change," says McGoldrick.

"Our study shows chromium in rocks older than 800 million years hadn't changed, which indicates that atmospheric oxygen levels were less than 0.1 per cent until that time."

Multicellular organisms only diversified after atmospheric oxygen levels increased above 0.1 per cent around 800 million years ago.!