A better comprehension of the Earth's atmosphere may aid in the detection of evidence of life further than our solar system.
McGill University researchers have found that the evolution and expansion of complex, eukaryotic ecosystems coincided with an increase in oxygen levels. Their conclusions provide the most compelling evidence yet that extremely low oxygen levels stifled evolution for billions of years.
“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,” says Maxwell Lechte, a postdoctoral researcher in the Department of Earth and Planetary Sciences under the supervision of Galen Halverson at McGill University.
The study observed iron-rich sedimentary rocks formed in ancient coastal environments from all over the world to gain answers. The team was able to quantify the amount of presence of oxygen when the rocks developed, as well as the influence it would have had on formative years such as eukaryotic microorganisms—the precursors to modern animals—by examining the chemistry of the iron in these rocks.
“These 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,” 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, 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,” says Lechte.
The Earth is still the only known location in the universe where life can be found. The Earth's atmosphere and oceans are now oxygen-rich, however it was not always the case. The oxygenation of the Earth's oceans and atmosphere was caused by photosynthesis, a method used by plants and other organisms to transform light energy into chemical energy, generating oxygen into the air and establishing the preconditions for respiration and animal life.
According to the researchers, the new findings suggests that Earth’s atmosphere was capable of maintaining low levels of atmospheric oxygen for billions of years. This has significant relevance for the search for evidence of life further than our solar system, because looking for residues of atmospheric oxygen is one method to look for proof of past or present extraterrestrial life – or what scientists refer to as a biosignature.
Scientists are using Earth's history to determine the oxygen levels that terrestrial planets can tolerate. The researchers believe that if terrestrial planets can stabilize at low atmospheric oxygen levels, the likelihood for oxygen detection will be to look for its photochemical byproduct ozone.
“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,” 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.
Per the researchers, more geochemical analyses of rocks in this era will enable scientists to paint a clearer picture of the development of oxygen levels through this time period, as well as comprehend the feedbacks on the global oxygen cycle.