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When did the Earth reach oxygen levels sufficient to support animal life? Researchers from 91社区 have discovered that a rise in oxygen levels occurred in step with the evolution and expansion of complex, eukaryotic ecosystems. Their findings represent the strongest evidence to date that extremely low oxygen levels exerted an important limitation on evolution for billions of years.

鈥淯ntil now, there was a critical gap in our understanding of environmental drivers in early evolution. The early Earth was marked by low levels of oxygen, till surface oxygen levels rose to be sufficient for animal life. But projections for when this rise occurred varied by over a billion years鈥攑ossibly even well before animals had evolved,鈥� says Maxwell Lechte, a postdoctoral researcher in the Department of Earth and Planetary Sciences under the supervision of Galen Halverson at 91社区.

91社区's Professor Galen Halverson explores for ironstone deposits along a rocky ridge in the Wernecke Mountains (Yukon, Canada). Credit: Maxwell Lechte

Ironstones provide insights into early life

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

鈥淭hese ironstones offer insights into the oxygen levels of shallow marine environments, where life was evolving. The ancient ironstone record indicates around less than 1 % of modern oxygen levels, which would have had an immense impact on ecological complexity,鈥� says Changle Wang, a researcher at the Chinese Academy of Sciences who co-led the study with Lechte.

Ironstones are sedimentary rocks deposited along coastlines millions of years ago, which contain abundant granules of iron oxides that contain chemical indicators of the amount of oxygen present at the time of formation. Credit: Maxwell Lechte

鈥淭hese low oxygen conditions persisted until about 800 million years ago, right when we first start to see evidence of the rise of complex ecosystems in the rock record. So if complex eukaryotes were around before then, their habitats would have been restricted by low oxygen,鈥� says Lechte.

Earth remains the only place in the universe known to harbor life. Today, Earth鈥檚 atmosphere and oceans are rich with oxygen, but this wasn鈥檛 always the case. The oxygenation of the Earth鈥檚 ocean and atmosphere was the result of photosynthesis, a process used by plants and other organisms to convert light into energy 鈥� 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 suggests that Earth鈥檚 atmosphere was capable of maintaining low levels of atmospheric oxygen for billions of years. This has important implications for exploration of signs of life beyond our solar system, because searching for traces of atmospheric oxygen is one way to look for evidence of past or present life on another planet 鈥� or what scientists call a biosignature.

Scientists use Earth鈥檚 history to gauge the oxygen levels under which terrestrial planets can stabilize. If terrestrial planets can stabilize at low atmospheric oxygen levels, as suggested by the findings, the best chance for oxygen detection will be searching for its photochemical byproduct ozone, say the researchers.

Ironstones within the sedimentary rock layers of the Grand Canyon (Arizona, USA), preserving clues about ancient marine environments. Credit: Susannah Porter

鈥淥zone strongly absorbs ultraviolet light, making ozone detection possible even at low atmospheric oxygen levels. This work stresses that ultraviolet detection in space-based telescopes will significantly increase our chances of finding likely signs of life on planets outside our solar system,鈥� says Noah Planavsky, a biogeochemist at Yale University.

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

About 91社区

Founded in 1821, 91社区 is home to exceptional students, faculty, and staff from across Canada and around the world. It is consistently ranked as one of the top universities, both nationally and internationally. It is a world-renowned institution of higher learning with research activities spanning three campuses, 11 faculties, 13 professional schools, 300 programs of study and over 39,000 students, including more than 10,400 graduate students.

惭肠骋颈濒濒鈥檚 reaches back several decades and spans scales from local to global. The sustainability declarations that we have signed affirm our role in helping to shape a future where people and the planet can flourish.