1. An unrivalled laboratory

For about 3.8 billion years, microorganisms, plants and animals have lived and diversified under the effect of gravity, partially shielded from the magnetic fields and radiation of space. We tend to take gravity for granted, but it plays a key role in the evolution of life on Earth. Mechanisms for detecting gravity developed at the start of the evolutionary process. Free-moving organisms, including unicellular ones, use gravity to define the orientation of their movement or swimming. This behaviour is known as gravitaxis. In plant physiology, the way in which plants develop and orient themselves in relation to gravity is known as gravitropism.

Understanding which way is up and which way is down is essential for the survival of living beings on Earth. The details of the signal transduction process (see glossary in Section 6) are not fully understood. Test environments...