Using solar energy for photosynthesis, they consumed carbon dioxide, and pumped out oxygen. Eventually free oxygen began to accumulate in the atmosphere. The ozone layer, which shields life from damaging ultra-violet radiation, was formed. The blue-green algae altered the planet far more drastically than we have done so far. Because of the continuing presence of life, Earth's atmosphere is now quite different from that of the other planets.
Carbon dioxide makes up less than one two thousandth of one per cent.
Life even altered geology. Free oxygen combined with iron in the sea and on land, making iron oxides and other mineral deposits. The skeletal remains of sea creatures drifted down into sea-floor sediments and became chalk and limestone. Other organic remains became coal and oil. The creation of Gaia Life needed the right circumstances to get going - water, the right temperature range and so on.