Billions of years ago when the first living organisms started harnessing the energy of the sun through photosynthesis, they began producing a waste product which was oxygen. When oxygen rises high up into the atmosphere, it transforms into ozone which is why the Earth has a thin protective layer of ozone which alongside the magnetic field, helps protect the Earth from the violent ultraviolet radiation spewed out by the sun. As plants, microbes, and fungi migrated from the water to land, even more oxygen was produced, extending the ozone layer to become much more formidable. This was the catalyst for life migrating onto dry land which occurred 450,000,000 – 500,000,000 (450 million – 500 million) years ago during the Paleozoic Era, specifically during the Ordovician and Silurian periods