The Early Earth was Really Horrible for Life

Earth has had a long and complex history since its formation roughly 4.5 billion years ago. Initially, it was a molten ball, but eventually, it cooled and became differentiated. The Moon formed from a collision between Earth and a protoplanet named Theia (probably), the oceans formed, and at some point in time, about 4 billion years ago, simple life appeared.

Those are the broad strokes, and scientists have worked hard to fill in a detailed timeline of Earth’s history. But there are a host of significant and poorly-understood periods in the timeline. One of them concerns UV radiation and its effects on early life.

A new study probes the effects of UV radiation on Earth’s early life-forms and how it might have shaped our world.

The Sun bombards Earth with ultraviolet radiation, which is hazardous to life. UV radiation makes up about ten percent of the radiation coming from the Sun. Most of it isn’t ionizing radiation, but it can still damage DNA, cause sunburn, and cause skin cancer.

Fortunately for us, the Earth’s ozone layer (O3 layer) is a protective shield against UV radiation. The O3 layer provides varying levels of UV protection across the Earth’s surface which varies with season and latitude. Overall, it prevents between 97% and 99% of the Sun’s medium-frequency UV light from striking the Earth’s surface. But the ozone layer’s effectiveness is dependent on the oxygen content in Earth’s atmosphere, and that content has fluctuated over time. The most pronounced change in Earth’s atmospheric oxygen content occurred in the Great Oxygenation Event (GOE,) and that event and its effects on ozone is a focus of the new study.

The title of the new study is “A revised lower estimate of ozone columns during Earth’s oxygenated history.” The lead author is Gregory Cooke, a Ph.D. researcher in the School of Physics and Astronomy at the University of Leeds. The paper is published in the journal Royal Society Open Science.

The GOE raised Earth’s atmospheric oxygen levels from near-zero to about what it is today, about 21%. That took place between about 2.4 billion and 2 billion years ago. Scientists attribute the rise to the appearance of cyanobacteria, also called blue-green algae. Cyanobacteria appeared about 2.7 billion years ago and used photosynthesis to produce energy. The by-product of all that photosynthesis was oxygen.

Scientists say that the Great Oxygenation Event (GOE), a period that scientists believe marked the beginning of oxygen’s permanent presence in the atmosphere, started as early as 2.33 billion years ago. Credit: MIT

Earth’s oxygen level has fluctuated over time, but the GOE is the most significant event in the history of Earth’s oxygen levels. Another event named the Neoproterozoic Oxygenation Event may have also played an essential role in raising Earth’s oxygen levels. Still, it’s not as well-understood—or even agreed-upon—as the GOE. But in any case, as oxygen levels go, so goes the ozone.

Previous research shows that ozone shielded Earth from harmful radiation when the oxygen level was as low as one percent. But this new study arrived at a different conclusion. It shows that the atmosphere needs between five to ten percent of modern Earth’s oxygen to shield life from UV radiation.

What does this mean for early life on Earth?

“This may have had fascinating consequences for life’s evolution.”

“We know that UV radiation can have disastrous effects if life is exposed to too much,” lead author Cooke said in a press release. “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.”

“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,” said Cooke. 

Models of Earth’s climate history are the basis of this study. The models show that previous estimates of surface UV levels could have underestimated UV exposure. Instead, Earth might have been subjected to ten times more UV than we thought.

Levels of UV radiation reaching the Earth’s surface may have changed over the last 2.4 billion years. Credit: Greg Cooke/ Royal Society Open Science

The modelling in this study is different than earlier modelling efforts. Previous efforts relied on one-dimensional modelling of Earth’s temporal oxygen. But this study uses more complexity. The study uses “… a whole atmosphere chemistry-climate model to simulate three-dimensional O3 variations with changing O2 concentrations under Proterozoic and Phanerozoic conditions applicable to the Earth,” the authors explain in their paper. “We demonstrate oxygen’s three-dimensional influence on the O3 <ozone> layer (its magnitude and spatial variation) and discuss how this affects habitability (the ability for life to survive on the surface) estimates.” 

The study uses the Whole Atmosphere Community Climate Model—WACCM6. WACCM6 combines atmosphere, land, land-ice, ocean and sea-ice sub-models. The team ran 12 different simulations in their study. 

This image is a schematic of the WACCM6 Earth System Model. In this work, WACCM6 used a fully interactive ocean model and land-ice, sea-ice, land and atmosphere models. WACCM6 has fully coupled chemistry and physics, a state-of-the-art moist physics scheme, and simulates up to roughly 140 km in altitude in the pre-industrial atmosphere. Image Credit: Cooke et al. 2022/University of Leeds

Scientists use Dobson Units to measure atmospheric ozone levels. One Dobson Unit is the number of molecules of ozone that would be required to create a layer of pure ozone 0.01 millimetres thick at a temperature of 0 degrees Celsius and a pressure of 1 atmosphere (the air pressure at the surface of the Earth).

This image from the study shows the density of Earth’s ozone column in Dobson Units. Each part of the image shows the DU for a different level of atmospheric oxygen shown as a percentage of PAL, or Present Atmospheric Level. Yellow represents more ozone, and purple represents less ozone. Black represents holes in the ozone layer. Note that the scale is different for each part. Image Credit: Cooke et al. 2022/University of Leeds

If life on Earth was exposed to much more UV than previously thought, it would’ve acted as a component of natural selection. Organisms that adapted somehow—by hiding underground, repairing damage, etc.— would have out-competed organisms without those adaptations.

“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,” Cooke said.

“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.”

Everything about the Earth is more complicated than one or two simple factors. The strength of the ozone layer and how well it protects life from UV radiation is highly dependent on the oxygen level. But there are many other factors like atmospheric mixing and the strength of the Sun’s output that come into play. Our industrial activities and other biological processes also affect the ozone layer. Interested readers can explore the paper in detail.

There are a couple of interesting takeaways from this work. The first is that we wouldn’t be here without being sufficiently protected from UV, and we also might not be here if earlier life wasn’t subjected to more UV. Can we call it the UV dichotomy?

The other exciting takeaway concerns exoplanets and our growing interest in them. Thanks to the launch of the James Webb Space Telescope—Hooray!—scientists will be able to study exoplanet atmospheres in greater detail. The presence and quantities of gases like oxygen and ozone in those atmospheres will help us understand potential exoplanet habitability. The modelling in this study might be one piece of the puzzle in interpreting JWST’s results and understanding exoplanets and the possibility of life.