The impact of gravity on life

Gravity has been constant throughout the history of Earth. This simple fact masks the complexity of gravity as an evolutionary force. Gravity is a vector, i.e. a force that has magnitude and direction at each point in space. Gravitational loading is directional toward the center of the Earth. Gravitational loading acts on all masses at the Earth’s surface and defines the weight of each object. Weight is the product of the object’s mass times the force of gravity, which on Earth is equal to 1 G. Weight drives many chemical, biological, and ecological processes on Earth. Altering weight changes these processes. Given these facts, one should not be surprised that changes in gravity could alter life, as we know it. If gravity causes changes to biology, then gravity, per se, must be a major physical environmental force shaping life on Earth. Life evolved from the sea. Neutrally buoyant aquatic species still have gravity acting upon them, but the uniform pressure around them and internal organs, such as the swim bladder, tend to counterbalance the intensity of the gravitational signal. However, some aquatic species appear to use gravity as a directional cue. When life evolved from the sea, it likely experienced gravitational loading for the first time. Land species changing their orientation with respect to the gravity vector or increasing in height probably began to develop adaptive mechanisms for coping with directional changes and for moving fluids and structures against this load. Due to gravity, the force necessary to lift an object above the surface of the Earth increases with the distance that an object is lifted. Birds had to solve the lift/ drag problem related to air density and gravity before they could fly and had to evolve a musculoskeletal system that could provide adequate thrust. As species on land increased in