The influence of gravity on structure and function of animals.

Gravity is the only environmental parameter that has remained constant during the period of evolution of living matter on Earth. Thus, it must have been a major force in shaping living things. The influence of gravitational loading in evolution of the vertebrate skeleton is well recognized, and scale effects have been studied. This paper, however, considers in addition four pivotal events in early evolution that would seem to have been significant for the later success and diversification of animal life. These are evolution of the cytoskeleton, cell motility (flagellae and cilia), gravity detecting devices (accelerometers), and biomineralization. All are functionally calcium dependent in eukaryotes and all occurred or were foreshadowed in prokaryotes. A major question is why calcium was selected as an ion of great importance to the structure and function of living matter; another is whether gravity played a role in its selection.