For long periods during the past 2.4 billion years, Earth’s atmosphere may have been less inviting to life than researchers previously believed, according to a study published in the scientific journal Royal Society Open Science (1). State-of-the-art computer simulations show that the level of UV radiation on the Earth’s surface was probably ten times higher than expected. This high radiation is not conducive to life, as it can destroy critical biological compounds, such as proteins.
Researchers already knew there were massive changes during the past 2.4 billion years, namely increasing oxygen levels from almost zero to the amount we have today, which stabilised around 400 million years ago. Although it’s believed these changes allowed more complex multicellular organisms to start developing, these new simulations raise some further questions about the impact of UV light, which is known to affect many life forms.
“We know that UV radiation can have disastrous effects if life is exposed to too much. For example, it can cause skin cancer in humans. Some organisms have effective defence mechanisms, and many can repair some of the damage UV radiation causes,” said Gregory Cooke, a Ph.D. researcher at the University of Leeds. “Whilst elevated amounts of UV radiation would not prevent life’s emergence or evolution, it could have acted as a selection pressure, with organisms better able to cope with greater amounts of UV radiation receiving an advantage.”
The amount of UV radiation that reaches the surface of our planet is controlled by the ozone layer. In chemical terms, this layer forms due to sunlight, and its concentration depends on the amount of oxygen in the Earth’s atmosphere.
For the past 40 years, scientists assumed that the ozone layer started protecting life on Earth as soon as the oxygen level in the atmosphere reached about 1% (relative to the present day). However, the new model suggests that the oxygen level may have been much higher, probably around 5-10%.
As a consequence, there were many periods before that level was reached when UV radiation reaching the surface was much higher. “If our modelling is indicative of atmospheric scenarios during Earth’s oxygenated history, then for over a billion years, the Earth could have been bathed in UV radiation that was much more intense than previously believed,” said Cooke. “This may have had fascinating consequences for life’s evolution. It is not precisely known when animals emerged or what conditions they encountered in the oceans or on land. However, depending on oxygen concentrations, animals and plants could have faced much harsher conditions than today’s world. We hope that the full evolutionary impact of our results can be explored in the future.”
(1) Cooke G, Marsh D, Walsh C, Black B and Lamarque J (2022) A revised lower estimate of ozone columns during Earth’s oxygenated history. R. Soc. Open sci.9211165211165 http://doi.org/10.1098/rsos.211165