How are we able to balance?
In order for humans to gracefully move around on two legs, they must have a greater sense of balance than those creatures moving on four legs. When a dog or cat walks on all fours, the center of balance is within the four points of contact on the ground. Contrast this with the center of balance on two legs; this center of balance is between the two feet and represents a relatively smaller area. Humans achieve this balance primarily through inner ears.
Our inner ears have 3 semicircular fluid filled canals that are sensitive to movement and gravity. Fine hairs in the canals send out signals to indicate the direction and speed of head movements. The canals are arranged in three planes including the horizontal, vertical anterior, and vertical posterior and are at right angles to each other. This arrangement gives three-dimensional sensing for balance.
The semicircular canals in humans and apes differ from each other. Humans have two large vertical planes and one small horizontal plane, while apes have three small canals of similar size. The three ape semicircular canals are of similar size, allowing apes to be successful at climbing trees. In contrast, humans are designed for walking upright and need balance sensors specifically designed for walking upright. Thus, they have larger canals in the vertical planes. When the fossil record is examined, no transitional fossils of semicircular canals have ever been found. In conclusion, man was made to walk upright, and apes were made to walk on all fours. This is just one of a myriad of differences between the two.
(Source: Inspired Evidence – Von Vett & Malone)