Earth has had a long and complex history. Initially, it was molten, but eventually cooled and became differentiated. Simple life appeared about 4 billion years ago when the oceans formed after the collision of Earth and Theia.
Scientists have worked hard to fill in a detailed timeline of Earth's history. There are a number of periods lined up like targets for the scientific method. UV radiation and its effects on early life are one of them.
A new study looks at the effects of UV radiation on early life-forms and how it might have shaped our world.
The sun is harmful to life. The sun's UV radiation makes up 10% of the total. It can cause sunburn and cause skin cancer, but most of it isn't ionizing radiation.
The Earth's ozone layer is a protective shield against UV radiation. The O3 layer provides different levels of UV protection across the Earth's surface. It prevents between 98% and 99% of the Sun's light from hitting the Earth. Oxygen content in Earth's atmosphere has fluctuated over time, and that is a key factor in the effectiveness of the ozone layer. The Great Oxygenation Event, which took place in the summer of 2005, caused the most pronounced change in Earth's atmospheric oxygen content.
The new study is called a revised lower estimate of ozone columns. The lead author is a researcher at the University of Leeds. The paper was published in a journal.
The GOE raised Earth's atmospheric oxygen levels from zero to 21%. Between 2 billion and 2.4 billion years ago, that took place. The rise in the appearance of blue-green algae is attributed to scientists. The organisms appeared about 2 billion years ago. Oxygen was the by-product of all that photosynthesis.
The Great Oxygenation Event is believed to have started as early as 2.33 billion years ago. Credit: MIT.
The GOE is the most significant event in the history of Earth's oxygen levels. The Neoproterozoic Oxygenation event may have played a role in raising Earth's oxygen levels. It is not as well- understood as the GOE. As oxygen levels go, so goes the ozone.
When the oxygen level was less than one percent, ozone protected Earth from harmful radiation. The new study came up with a different conclusion. It shows that the atmosphere needs between five and ten percent of the Earth's oxygen to survive.
What does this mean for life on Earth?
This may have had interesting consequences for life.
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The lead author is Gregory Cooke.
The lead author said in a press release that UV radiation can have disastrous effects if life is exposed to too much. It can cause skin cancer in humans. Some organisms can repair damage caused by UV radiation.
It could have been a selection pressure, with organisms better able to cope with greater amounts of UV radiation receiving an advantage, if elevated amounts of UV radiation had not prevented life's emergence or evolution.
This study is based on models of Earth's climate history. The models show that previous estimates of surface UV levels could have been underestimated. Earth might have been exposed to more UV than we thought.
UV radiation reaching the Earth's surface may have changed over time. The Royal Society Open Science has a credit.
The modelling effort in this study is different than before. One-dimensional modelling of Earth's temporal oxygen was used before. The study uses more complexity. The study used a climate model to model three-dimensional O3 variations under Proterozoic and Phanerozoic conditions. Oxygen has a three-dimensional influence on the O3 layer, and we discuss how this affects habitability.
The Whole Atmosphere Community Climate Model is used in the study. The sub-models of atmosphere, land, land-ice, ocean and sea-ice are combined. The simulations were used in the study.
This is a schematic of the model. In this work, a fully interactive ocean model and land-ice, sea-ice, land and atmosphere models were used. Up to 140 km in altitude in the pre-industrial atmosphere can be mimicked with the fully coupled chemistry and physics of the program. The image is from the University of Leeds.
Dobson Units are used to measure ozone levels. A Dobson Unit is the number of ozone molecule that would be required to create a layer of pure ozone at a temperature of 0 degrees Celsius and a pressure of 1 atmosphere.
The density of Earth's ozone column is shown in the image. Each part of the image has a percentage of PAL shown as a level of atmospheric oxygen. Yellow and purple represent different amounts of ozone. There are holes in the ozone layer. The scale is different for each part. The image is from the University of Leeds.
Life on Earth would have acted as a component of natural selection if it was exposed to more UV than previously thought. Organisms that hid underground, repaired damage, etc., would have been able to out-compete other organisms.
If our modelling is indicative of atmospheric scenarios during Earth's oxygenated history, the Earth could have been bathed in UV radiation that was much more intense than previously believed.
This may have had interesting consequences for life. It is not known when animals emerged or what conditions they encountered. Animals and plants could have faced harsher conditions if oxygen concentrations were higher. We hope that the full evolutionary impact of our results can be explored in the future.
The Earth is more complicated than just one or two factors. Oxygen level is very important in determining the strength of the ozone layer. The strength of the Sun's output is one of the factors that come into play. The ozone layer is affected by industrial activities and biological processes. Readers can read the paper in detail.
There are a couple of things that come from this work. We might not be here if life wasn't exposed to more UV, and we wouldn't be protected from UV if we were. Is it the UV dichotomy?
Our growing interest in exoplanets is one of the exciting revelations. Scientists will be able to study exoplanet atmospheres in greater detail thanks to the launch of the James Webb Space Telescope. Oxygen and ozone are gases that can help us understand exoplanet habitability. The modelling in this study might be a piece of the puzzle in interpreting the results of the study.
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