Oxygen levels in the Earth’s atmosphere are likely to have “fluctuated wildly” one billion years ago, creating conditions that could have acceleratedтАпthe development of early animals, say researchers.
Scientists believe atmospheric oxygenтАпdeveloped in three stages, starting with what is known as the Great Oxidation EventтАпaround two billion years ago, when oxygen first appeared in theтАпatmosphere. The third stage, around 400 million yearsтАпago, saw atmospheric oxygen rise to levelsтАпthat exist today.тАп
What is uncertain is what happenedтАпduring the second stage, in a time known as the Neoproterozoic Era, which started about one billion years ago and lasted for around 500 million years, during which time early forms of animal life emerged.тАптАп
The question scientists have tried to answer isтАп- was there anything extraordinary about the changes to oxygen levels in the Neoproterozoic Era thatтАпmay have played a pivotal roleтАпin the earlyтАпevolution of animalsтАп- did oxygen levels suddenly rise or was there a gradual increase?тАп
Fossilised tracesтАпof early animalsтАп-тАпknown as Ediacaran biota, multi-celled organisms that requiredтАпoxygenтАп- have been found in sedimentary rocks that areтАп541 to 635 million years old.тАп
To try to answer the question, a research team at the University of Leeds supported by the Universities of Lyon, Exeter and UCL, used measurements of theтАпdifferent forms of carbon, or carbon isotopes, found in limestone rocks taken from shallow seas.тАпBased on the isotope ratios of the different typesтАпof carbonтАпfound,тАпthe researchers were able to calculate photosynthesis levels that existed millions of years ago and infer atmospheric oxygen levels.тАп
As a result of the calculations, they have been able to produce a record of oxygen levels in the atmosphereтАпover the last 1.5 billion years, which tells us how much oxygen would have been diffusing into the ocean to support early marine life.
Dr Alex Krause, a biogeochemical modeller who completed his PhD in the School of Earth and Environment at LeedsтАпand was the lead scientist on the project, said the findings give a new perspective on the way oxygen levels were changing on Earth.тАп
He added: “The early Earth, for the first two billion years of its existence, was anoxic, devoid of atmospheric oxygen. Then oxygen levels started to rise, which is known as the Great Oxidation Event.тАптАп
“Up until now, scientists had thoughtтАпthat after the Great Oxidation Event, oxygen levels were either lowтАпandтАпthen shot upтАпjust before we see the first animals evolve, or thatтАпoxygen levels were high for many millions of years before the animals came along.
“But our study shows oxygen levels were far more dynamic. There was an oscillation betweenтАпhigh and low levels of oxygen for a long time before early forms of animal life emerged.тАпWe are seeing periods where the ocean environment, where early animals lived, would have had abundant oxygen — and then periods where it does not.”
Dr Benjamin Mills, who leads the Earth Evolution Modelling Group at Leeds and supervised the project, said: “This periodic change in environmental conditions would have produced evolutionary pressures where some life forms may have become extinct and new ones could emerge.”тАп
Dr Mills said the oxygenated periods expanded what are known as “habitable spaces” — parts of the ocean where oxygen levels would have beenтАпhighтАпenough to support early animal life forms.тАп
He said: “It has been proposed in ecological theory that when you have a habitable space that is expanding and contracting, this can support rapid changes to the diversity of biological life.тАп
“When oxygen levels decline,тАпthere is severe environmental pressure on some organisms which could drive extinctions.тАпAnd when the oxygen-rich waters expand, the new space allows the survivors to riseтАпto ecologicalтАпdominance.тАп
“These expanded habitable spaces would have lasted for millions of years, giving plenty of time for ecosystems to develop.”
The findingsтАп- “Extreme variability in atmospheric oxygen levels in the late Precambrian” тАп- are published in the journal Science Advances.
