1. Gravity: The most fundamental factor is Earth's gravity. It pulls us towards the ground, providing the force that keeps our feet in contact with the surface.

2. Our Skeletal Structure: Our bones provide the rigid framework that supports our weight and allows us to stand upright. The structure of our legs, with the femur (thigh bone), tibia and fibula (shin bones), and the complex arrangement of joints, is specifically designed for walking.

3. Muscles: Our muscles are responsible for generating the force needed to move our limbs. Muscles in our legs, like the quadriceps, hamstrings, and calf muscles, work together to lift our legs, propel us forward, and control our balance.

4. Nervous System: Our brain and spinal cord control the intricate coordination of our muscles. They receive sensory information from our feet, which helps us adjust our balance and gait as we walk.

5. Balance and Proprioception: The inner ear and the proprioceptive system (the sense of our body's position in space) play a crucial role in maintaining our balance. This allows us to adjust our movements and prevent falls.

6. Friction: Friction between our shoes and the ground provides the necessary force to push off and move forward.

7. The Earth's Surface: While it seems obvious, the solid surface of the Earth is essential. Walking requires a firm base to push against.

8. Adaptation: Human evolution has led to adaptations that make walking easier and more efficient, including bipedalism, the development of strong leg muscles, and the unique structure of our feet.

It's important to note that walking is a complex process that involves the coordinated action of many different systems within our body. Each factor plays a vital role in enabling us to move around on Earth.