A research team from McGill University has discovered that a deeper understanding of the Earth’s atmosphere could aid in identifying life on other planets.
Scientists detect a rise in oxygen levels
The scientists’ original aim was to investigate exactly when the Earth’s oxygen (O2) levels were able to sufficiently support animal life. It was discovered that a rise in O2 occurred in step with the evolution and expansion of ecosystems that were both complex and eukaryotic. The revelation of how extremely low O2 levels exerted an important limitation on evolution for billions of years, is considered the strongest evidence in existence.
“Until 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—possibly even well before animals had evolved,” added Maxwell Lechte, a postdoctoral researcher in the Department of Earth and Planetary Sciences under the supervision of Galen Halverson at McGill University.
What did scientists examine to discover these results?
Researchers examined iron-rich sedimentary rocks from around the world that had been deposited in ancient coastal environments. By analysing the chemistry of the iron in these rocks, scientists were able to estimate the amount of O2 present when the rocks formed, and the impact that this would have had on early life like eukaryotic microorganisms—the precursors to modern animals.
“These ironstones offer insights into the O2 levels of shallow marine environments, where life was evolving. The ancient ironstone record indicates around less than 1% of modern O2 levels, which would have had an immense impact on ecological complexity,” explained Changle Wang, co-leader of the study, and a researcher at the Chinese Academy of Sciences.
“These 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,” said Lechte.
Earth remains the only place in the universe known to harbour life. Today, Earth’s atmosphere and oceans are rich with O2, but this was not always the case. The oxygenation of the Earth’s ocean and atmosphere was the result of photosynthesis, a process used by plants and other organisms to convert light into energy, which released O2 into the atmosphere and created the necessary conditions for respiration and animal life.
How did these results go on to provide an insight into life on other planets?
Scientists believe that these results reveal that the Earth’s atmosphere was capable of maintaining low levels of atmospheric O2 for billions of years. This has important implications for exploration of signs of life on other planets, because searching for traces of atmospheric O2 is a method that can be utilised for detecting evidence of past or present life on another planet; or what scientists call a ‘biosignature’.
Scientists have considered Earth’s history to gauge the O2 levels under which terrestrial planets can be stabilised. If terrestrial planets can stabilise at low atmospheric O2 levels, as suggested by the findings, researchers stress that the best chance for O2 detection will be searching for its photochemical by-product ozone.
The research team recognised that more geochemical studies of rocks from this time period are required, as this will allow scientists to paint a clearer picture of the evolution of O2 levels during this time, and better understand the feedbacks on the global O2 cycle.
“Ozone 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,” concluded Noah Planavsky, a biogeochemist at Yale University.
