New evidence has emerged that early evolution was triggered by a dramatic rise in atmospheric oxygen levels following major climate change.
Around 635 million years ago, the fossil record shows, there was a big increase in numbers of animals and algae. And, now, an analysis of organic-rich rocks from South China has shown that there was also a sudden spike in oceanic oxygen levels right around this time, following severe glaciation.
"This work provides the first real evidence for a long speculated change in oxygen levels in the aftermath of the most severe climatic event in Earth's history - one of the so-called 'Snowball Earth' glaciations," says professor Timothy Lyons of UC Riverside.
The research team analyzed concentrations of trace metals and sulfur isotopes - which are tracers of early oxygen levels - in mudstone collected from the Doushantuo Formation in South China.
"We found levels of molybdenum and vanadium in the Doushantuo Formation mudstones that necessitate that the global ocean was well ventilated," says Noah Planavsky, a former UCR graduate student now at CalTech.
"This well-oxygenated ocean was the environmental backdrop for early animal diversification."
The concentrations of trace metals that they found are comparable to those in our oceans today, and imply that there was a big increase in the amount of oxygen around that time. This, say the researchers, the oxygen rise is likely due to the way more nutrients would have been available after the glaciation.
Burial of organic carbon from photosynthetic organismsin ocean sediments would lead to the release of vast amounts of oxygen into the ocean-atmosphere system.
"We are delighted that the new metal data from the South China shale seem to be confirming these hypothesized events," says Lyons.