High up in the atmosphere, oxygen atoms combined to form three-atom molecules of ozone, O3, that began to shield Earth’s surface from dangerous solar ultraviolet radiation and have continued to do so ever since. Protected by the ozone layer, some algae may have started colonizing the land for the first time. Until then, bathed in solar radiation that would have ripped apart any bacteria brave enough to venture onto land, the continents of planet Earth had been more or less sterile.
