Changes during the development of the biosphere

The Earth is approximately 5 milliard (5×109) years old and the development on Earth can be described by a ‘biogeological clock’ (Figure 11.1). The first atmosphere around the Earth is assumed to have been formed when the planet started to cool off. Earlier it was too hot for gases to be retained around the Earth. The first atmosphere was formed from volcanic outgassing from the Earth's interior and consisted probably of helium, hydrogen, ammonia and methane. Later, water vapour, carbon dioxide, nitrogen and sulfur gases became the dominant gases. The water vapour together with particles (condensation nuclei) led to cloud formation and rain, forming rivers, lakes and filling the oceans. The water bodies became sinks for carbon dioxide, as they are today, and through biological, chemical and physical processes sedimentary rocks were formed. As nitrogen is not very chemically active it accumulated in the atmosphere. The new atmosphere with clouds and water vapour regulated the energy balance of the Earth and thus became an important climatic regulator.

Life, which appeared around 3.8 milliard years ago or perhaps even earlier (Des Marais 2000), has played an important role in the development of the atmosphere. Here life means that the first living organisms capable of performing processes leading to energy fixation in organic matter appeared, in other words photosynthesis. The earliest photosynthesis occurred without the production of oxygen.