Earth is to our knowledge the only life-bearing body in the Solar System. This extraordinary
characteristic dates back almost 4 billion years. How to explain that Earth is teeming with
organisms and that this has lasted for so long? What makes Earth different from its sister
planets Mars and Venus? The habitability of a planet is its capacity to allow the emergence of
organisms. What astronomical and geological conditions concurred to make Earth habitable 4
billion years ago and how has it remained habitable since? What have been the respective roles
of non-biological and biological characteristics in maintaining the habitability of Earth? This
unique book answers the above questions by considering the roles of organisms and ecosystems in
the Earth System which is made of the non-living and living components of the planet.
Organisms have progressively occupied all the habitats of the planet diversifying into
countless life forms and developing enormous biomassesover the past 3.6 billion years. In this
way organisms and ecosystems took over the Earth System and thus became major agents in its
regulation and global evolution. There was co-evolution of the different components of the
Earth System leading to a number of feedback mechanisms that regulated long-term Earth
conditions. For millennia and especially since the Industrial Revolution nearly 300 years ago
humans have gradually transformed the Earth System. Technological developments combined with
the large increase in human population have led in recent decades to major changes in the
Earth's climate soils biodiversity and quality of air and water. After some successes in the
20th century at preventing internationally environmental disasters human societies are now
facing major challenges arising from climate change. Some of these challenges are short-term
and others concern the thousand-year evolution of the Earth's climate. Humans should become the
stewards of Earth.
