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Gravity
Scientists express gravity in terms of acceleration. An object falling toward Earth does so at a velocity of 9.81 m/s2. While the unit of measurement -- meters per second per second -- may sound odd, it takes into account the increase in speed that an object undergoes as it falls. During the first second that a skydiver falls to Earth, his body travels at 9.81 mps. During the next second, he travels at 19.62 mps. Humans walking on Earth's surface may not be aware of this constant downward acceleration, but they must overcome it with every step they take. Muscles hold terrestrial organisms erect when they would otherwise collapse toward Earth's surface at a rate of 9.81 mps.
Evolution
Every known creature that walks, swims or flies does so within Earth's gravitational pull. Evolution describes the process by which a species becomes adapted to its environment. As every environment on Earth feels the planet's gravitational pull, all life on Earth has evolved to withstand that pull. Just as deep-sea creatures have evolved to withstand the tremendous pressures of the water column above them, humans have evolved to move easily at the bottom of a gravitational well.
Skeleton
Although humans lack the extreme adaptations of taller animals such as giraffes, they do have subtle skeletal structures that reveal the evidence of evolving within Earth's gravitational pull. The upright, bipedal gait of human beings relies on a sturdy pelvic structure and strong leg bones. Walking upright also requires a good sense of balance. The human inner ear is exquisitely attuned and allows people to keep their balance while walking or running, activities that some scientists describe as a series of controlled falls.
Musculature
The human musculature provides the motive force to rise from a bed, stand up from a seat and walk. The largest muscle masses in the human body are designed to move the body against gravity: the muscles of the back, buttocks and thighs. In microgravity environments, moving the mass of the human body requires less force. Astronauts who have spent long periods in space find that without Earth's gravity, their muscles become atrophied. They must maintain rigorous workouts just to retain some of their muscle mass. On Earth, simply going about normal daily tasks is sufficient to keep muscles from atrophying. Not only are humans adapted to Earth's gravity, they require it for optimal muscle health.