Feb. 2 (UPI) -- Scientists at the University of Exeter have offered an explanation for Earth's low oxygen levels during the middle ages, the 2 billion years after the Great Oxygenation Event.
The Great Oxygenation Event, roughly 2.4 billion years ago, marked the initial introduction of biologically induced oxygen -- dioxygen, or O2 -- into Earth's atmosphere, proof of oxygenic photosynthesis.
Despite the development of photosynthesis, oxygen levels remained low and stable for 2 billion years, slowing the rate of evolution and delaying the development of complex life forms. But why?
In a new paper, published in the journal Nature Communications, researchers argue tiny, simple lifeforms, dead and buried in sediment, began to react with oxygen for the first time in the wake of the Great Oxidation.
As plate tectonics exposed ancient sediments to the atmosphere, chemical reactions destroyed oxygen at the same rate O2 was being created. The more oxygen emitted into the atmosphere, the greater the rate of reactions in the soil.
It was only after the development of plants that this regulatory mechanism failed, and the atmosphere began to accumulate more oxygen -- rising to levels comparable to today.
"This time in Earth's history was a bit of a catch-22 situation," Tim Lenton, a professor of geography and Earth sciences at Exeter, said in a news release. "It wasn't possible to evolve complex life forms because there was not enough oxygen in the atmosphere, and there wasn't enough oxygen because complex plants hadn't evolved. It was only when land plants came about did we see a more significant rise in atmospheric oxygen."
The rise in concentrations of oxygen, occurring some 600 million years ago, paved the way for complex, multicellular animals, first in the ocean and then on land.
"The history of life on Earth is closely intertwined with the physical and chemical mechanisms of our planet," Lenton added. "It is clear that life has had a profound role in creating the world we are used to, and the planet has similarly affected the trajectory of life. I think it's important people acknowledge the miracle of their own existence and recognise what an amazing planet this is."