At the beginning of the Archaean Eon, 3.8 billion years ago, Earth was still three times as hot as it is today, though finally no longer hot enough to boil water. Most of Earth was newly covered with oceans, with the first volcanic islands beginning to surface. The atmosphere was composed of volcanic gases, including nitrogen, hydrogen and carbon dioxide. The first living cells were just beginning to form.
Most rocks were igneous or metamorphic as a result of high volcanic activity, although sedimentary rocks were beginning to form on ocean floors. The oceans were much warmer and more acidic than they are today. The first living cells in these oceans evolved into prokaryote cells about 3.5 billion years ago. The vast majority of cells on earth are still prokaryotes. About 3 billion years ago, some cells evolved the ability to photosynthesize, converting sunlight, water and carbon dioxide into the energy needed to live, with oxygen as a by-product. However, most of the oxygen was still not in the atmosphere. Instead, the majority was mixed with iron and sulfur rocks and was responsible for the era's signature rusty red rocks and limestone, which are used by modern scientists to date excavation to the period.