During long portions of the past 2.4 billion years, the Earth may have been more inhospitable to life than scientists previously thought, according to new computer simulations.
Using a state-of-the-art climate model, researchers now believe the level of ultraviolet (UV) radiation reaching the Earth’s surface could have been underestimated, with UV levels being up to ten times higher.
UV radiation is emitted by the sun and can damage and destroy biologically important molecules such as proteins.
The last 2.4 billion years represents an important chapter in the development of the biosphere. Oxygen levels rose from almost zero to significant amounts in the atmosphere, with concentrations fluctuating but eventually reaching modern day concentrations approximately 400 million years ago.
During this time, more complex multicellular organisms and animals began to colonize land.
Gregory Cooke, a PhD researcher at the University of Leeds who led the study, said the findings raise new questions about the evolutionary impact of UV radiation as many forms of life are known to be negatively affected by intense doses of UV radiation.
He said: “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 defense 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.”
The research “A revised lower estimate of ozone columns during Earth’s oxygenated history” was published on January 5, 2022, in the scientific journal Royal Society Open Science.
The amount of UV radiation reaching the Earth is limited by the ozone in the atmosphere, described by the researchers as “…one of the most important molecules for life” because of its role in absorbing UV radiation as it passes into the Earth’s atmosphere.
Ozone forms as a result of sunlight and chemical reactions – and its concentration is dependent on the level of oxygen in the atmosphere.
For the last 40 years, scientists have believed that the ozone layer was able to shield life from harmful UV radiation when the level of oxygen in the atmosphere reached about one percent relative to the present atmospheric level.
The new modeling challenges that assumption. It suggests the level of oxygen needed may have been much higher, perhaps 5% to 10% of present atmospheric levels.
As a result, there were periods when UV radiation levels at the Earth’s surface were much greater, and this could have been the case for most of the Earth’s history.
Mr. Cooke said: “If our modeling 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.
“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.”
The results will also lead to new predictions for exoplanet atmospheres. Exoplanets are planets that orbit other stars. The presence of certain gases, including oxygen and ozone, may indicate the possibility of extra-terrestrial life, and the results of this study will aid in the scientific understanding of surface conditions on other worlds.
Reference: “A revised lower estimate of ozone columns during Earth’s oxygenated history” by G. J. Cooke, D. R. Marsh, C. Walsh, B. Black and J.-F. Lamarque, 5 January 2022, Royal Society Open Science.
The study was funded by UK Science and Technology Facilities Council and involved collaboration with scientists at the National Centre for Atmospheric Research, Rutgers University, and the City University of New York, all of which are in the US.
It appears that there are a couple of important things that the modelers overlooked. Until the Devonian (~400 My BP) life was constrained to the oceans. Water is a strong absorber of UV. Hence, the atmospheric machinations of UV and ozone are really irrelevant with respect to the life in the oceans, particularly for bottom dwellers like trilobites. The Great Oxygenation Event at ~2.4 BY BP strongly suggests that the photosynthetic organisms that had to have access to light weren’t troubled much by the amount of UV that reached them.
When the first hardy plants made it onto dry land, there was already oxygen in the atmosphere, hence ozone in the atmosphere!
Something else that the modelers appear to have overlooked is Rayleigh Scattering. The reason our sky looks blue is because light is scattered inversely with the 4th power of the wavelength of light. That means, UV is scattered even more strongly than blue light. Thus, their model gives an upper bound on the amount of UV reaching the surface. That is, they have over-estimated the amount of UV reaching the surface before ozone reached its current levels.
What has been ignored, dismissed. is that the early atmosphere had oxygen delivered to it by the UV photodissociation of water vapor in the stratosphere with the loss of light hydrogen to space. This allowed the oxygen to create a minimum ozone screen without which life could never have evolved. No models are needed to see that. Peptide and nucleotide bonds require energy to remove water. Not possible IN water. Certainly not possible to allow the evolution of photosynthesis.
“Not possible IN water. Certainly not possible to allow the evolution of photosynthesis.”
And yet, the evidence is unequivocal that photosynthesis evolved first IN water, and remained there for 2 billion years!