All living things that evolved on Earth evolved with the force of gravity pulling them against the Earth's surface. Our sense of linear motion comes from acceleration due to gravity; as we move, our body is pulled back to Earth by this acceleration. Humans and other higher animals detect gravity by means of tiny hair cells in the inner ear. These hair cells are topped with microscopic calcium carbonate stones. When the head moves, linear acceleration bends the stones to one side, triggering an electrical response. Proprioceptors in the joints also detect the body's location with respect to gravity.
Plants exhibit gravity-oriented growth in the roots and shoots (the roots growing with gravity and the shoots growing against gravity), termed gravitropism. In the roots, specialized cells called statocytes have been implicated in the gravitropic response. These cells contain starch grains that settle in the direction of gravitational force. Scientists believe that this settling triggers a hormonal reaction that causes the roots to bend and grow downwards.
As more is known about gravity sensing on a system-wide level, questions arise about what level of physiological complexity is needed for gravity sensing. Scientists are now investigating if single cells isolated from systemic effects are capable of exhibiting sensitivity to gravity or microgravity, and what the second-messenger pathways and molecules may be.