When did Earth reach sufficient oxygen levels to support animal life? Researchers at McGill University have found that increased oxygen levels accompany the evolution and expansion of complex eukaryotic ecosystems. Their findings represent the strongest evidence to date that extremely low oxygen levels have exerted a significant limitation on evolution for billions of years.

“Until now, there was a critical gap in our understanding of environmental factors in early evolution. Early Earth was marked by low oxygen levels, until oxygen levels in surface area are increasing to be enough for animal life. But projections of when this increase occurred have varied by more than a billion years – possibly even long before animals evolved,” says Maxwell. Lechte, postdoctoral researcher in the Department of Earth and Planetary Sciences under the supervision of Galen Halverson at McGill University.

Ironstones provides insight into early life

To find answers, researchers examined iron-rich sedimentary rocks from around the world deposited in ancient coastal environments. By analyzing the iron chemistry in these rocks, the researchers were able to estimate the amount of oxygen present when the rocks were formed and the impact it would have had on early life like eukaryotic microorganisms – the precursors of modern animals.

“These ironstones offer insight into the oxygen levels of shallow marine environments, where life evolved. The ancient ironstone record indicates approximately less than 1% of modern oxygen levels, which would have had a immense impact on ecological complexity,” says Changle Wang, a researcher at the Chinese Academy of Sciences who co-led the study with Lechte.

“These low oxygen conditions persisted until about 800 million years ago, just when we are beginning to see evidence of the rise of complex ecosystems in the rock record. So if eukaryotes complexes existed before, their habitats would have been limited by low oxygen,” says Lechte.

Earth remains the only place in the universe known to support life. Today, the Earth’s atmosphere and oceans are rich in oxygen, but that wasn’t always the case. Oxygenation of the Earth’s ocean and atmosphere is the result of photosynthesis, a process used by plants and other organisms to convert light into energy, releasing oxygen into the atmosphere and creating the necessary conditions for respiration and animal life.

Searching for signs of life beyond our solar system

According to the researchers, the new findings suggest that Earth’s atmosphere was capable of sustaining low levels of atmospheric oxygen for billions of years. This has important implications for exploring signs of life beyond our solar system, as searching for traces of atmospheric oxygen is one way to search for evidence of past or present life on another planet – or what what scientists call a biosignature.

Scientists use Earth’s history to assess the oxygen levels below which terrestrial planets can stabilize. If terrestrial planets can stabilize at low levels of atmospheric oxygen, as the results suggest, the best chance of detecting oxygen will be to search for its photochemical byproduct, ozone, the researchers say.

“Ozone strongly absorbs ultraviolet light, making detection of ozone possible even at low levels of atmospheric oxygen. This work emphasizes that ultraviolet detection in space telescopes will greatly increase our chances of finding probable signs of life on planets outside our solar system,” says Noah Planavsky, a biogeochemist at Yale University.

Further geochemical studies of rocks from this period will allow scientists to paint a clearer picture of how oxygen levels changed during this period and better understand feedbacks on the global oxygen cycle, the researchers say.

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