When the Earth was young and it had no ozone layer protecting it, life was only possible deep in the oceans. Only when the first life form on Earth, prokaryotic cells, had produced enough oxygen molecules - which later turned to three-atom ozone molecules creating slowly the ozone layer - life could move on land. The first vascular plants emerged on land some 410 million years ago.
The Sun's activity varies over an 11-year period. The number of sunspots, flares, and the radiation output changes dramatically over time. The next peak, or the sunspot maximum will occur in 2000. Two ESA missions in the Solar-Terrestrial Science Programme, SOHO and Cluster-II, will help scientists to understand more about how the Sun works and especially, how it affects the Earth.
