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Message:

> Very early it seems protection was not good, according to new estimates:
> 
> https://www.sciencealert.com/conditions-on-ancient-earth-were-probably-more-hostile-than-previously-thought?utm_source=ScienceAlert+-+Daily+Email+Updates&utm_campaign=347bd22f4b-MAILCHIMP_EMAIL_CAMPAIGN&utm_medium=email&utm_term=0_fe5632fb09-347bd22f4b-366098385
> 
> "...researchers now think that we've underestimated the levels of ultraviolet (UV) radiation from the Sun that reached the surface of Earth – and that these levels could have been up to 10 times higher than previously thought during certain periods.
> 
> "The focus of the research is on the last 2.4 billion years of history, since the Great Oxidation Event (GOE) when oxygen levels in the atmosphere and oceans first started rising from virtually nothing. Not only do the new findings teach us more about Earth's history, they could improve our understanding of the atmospheres on other planets too.
> 
> "We know that UV radiation can have disastrous effects if life is exposed to too much," says astrophysicist Gregory Cooke, from the University of Leeds in the UK. "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 researchers think that greater levels of UV radiation could have hit Earth because of a weaker ozone layer, which absorbs the radiation. The amount of ozone in our atmosphere depends on a number of factors and chemical reactions, but oxygen levels play a major role in the formation of ozone.
> 
> Previously, it was thought that atmospheric oxygen levels of around 1 percent of today's levels would produce enough ozone to keep harmful UV radiation away. Now, using advanced computer climate simulations, the team suggests that the key oxygen level might be more like 5-10 percent.
> 
> ***
> 
> "Around 400 million years ago, oxygen levels in the atmosphere got up to modern-day standards, and more complex life forms began to evolve, leading to the wide biodiversity in evidence across the planet today."
> 
> Comment: this tells us why life developed only in the deep oceans and reached land much later.