How Fluctuating Oxygen Levels May Have Accelerated Animal Evolution On Earth – Astrobiology – Astrobiology News

Fossil records of early animals from Mistaken Point Ecological Reserve in Canada. CREDIT Dr Emily. G. Mitchell University of Cambridge

Oxygen levels in the Earths atmosphere are likely to have fluctuated wildly one billion years ago, creating conditions that could have acceleratedthe development of early animal life, according to new research.

Scientists believe atmospheric oxygendeveloped in three stages, starting with what is known as the Great Oxidation Eventaround two billion years ago, when oxygen first appeared in theatmosphere. The third stage, around 400 million yearsago, saw atmospheric oxygen rise to levelsthat exist today.

What is uncertain is what happenedduring 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.

The question scientists have tried to answer is- was there anything extraordinary about the changes to oxygen levels in the Neoproterozoic Era thatmay have played a pivotal rolein the earlyevolution of animals did oxygen levels suddenly rise or was there a gradual increase?

Fossilised tracesof early animals-known as Ediacaran biota, multi-celled organisms that requiredoxygen- have been found in sedimentary rocks that are541 to 635 million years old.

To try and answer the question, a research team at the University of Leeds supported by the Universities of Lyon, Exeter and UCL, used measurements of thedifferent forms of carbon, or carbon isotopes, found in limestone rocks taken from shallow seas.Based on the isotope ratios of the different typesof carbonfound,the researchers were able to calculate photosynthesis levels that existed millions of years ago and infer atmospheric oxygen levels.

As a result of the calculations, they have been able to produce a record of oxygen levels in the atmosphereover 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 Leedsand was the lead scientist on the project, said the findings give a new perspective on the way oxygen levels were changing on Earth.

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.

Up until now, scientists had thoughtthat after the Great Oxidation Event, oxygen levels were either lowandthen shot upjust before we see the first animals evolve, or thatoxygen 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 betweenhigh and low levels of oxygen for a long time before early forms of animal life emerged.We 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.

Dr Mills said the oxygenated periods expanded what are known as habitable spaces parts of the ocean where oxygen levels would have beenhighenough to support early animal life forms.

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.

When oxygen levels decline,there is severe environmental pressure on some organisms which could drive extinctions.And when the oxygen-rich waters expand, the new space allows the survivors to riseto ecologicaldominance.

These expanded habitable spaces would have lasted for millions of years, giving plenty of time for ecosystems to develop.

Extreme variability in atmospheric oxygen levels in the late Precambrian, Science Advances

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