Theory:

Humans started walking upright:
The first humans marched on the field and climbed trees. This adaptability allowed them to travel about in a variety of environments and cope with changing climates.
Important!
To learn how humans learned to walk upright, watch the video below:
 
humans upright.jpg
Silhouette of Sahelanthropus tchandensis.
 
Early humans mixed apelike and humanlike ways of getting around from at least 6 to 3 million years ago. Fossil bones record the gradual transition from climbing trees to walking upright.
7–6 million years ago:
Sahelanthropus may have walked on two legs.
About 6 million years ago:
Sahelanthropus, one of the first humans discovered, provides the earliest evidence for walking on two legs. Walking upright may have helped this species thrive in various habitats near its home, including forests and grassland.
Straight knees:
Every time early men took a move, \(4.1\) million years ago, he momentarily stood on one leg, putting stress on his leg bones. The stress has resulted in a large area of bone just below the knee joint in Australopithecus anamensis. This individual walked in a straight line.
 
A \(4\) million-year-old fossil has been identified, suggesting that early human species existed near open areas and dense forests. Their bodies had developed in such a way that they could walk upright for the most part while still climbing trees. As a result, they'd be able to use all habitats.
Hip support:
\(1.95\) million years ago, Homo Erectus hip bones were the same size and shape as modern humans, indicating that this early human species had abandoned climbing in favour of walking.
  
The curve of your lower back absorbed shock as you walked \(2.5\) million years ago. It's one of a kind. The spine of Australopithecus africanus, an early human who walked upright like modern humans, has a similar curve.
Bipedal:
Homo Erectus pelvis and thigh bones were almost identical to modern humans \(1.9\) million years ago, suggesting that this early human was capable of long-distance travel. Getting that talent was a big help during this period. East Africa's climates alternated between rainy and dry, and open grasslands were starting to emerge.
Long legs:
The long thigh bones of Homo erectus permitted its owner to take long strides and thus walk farther and faster than earlier humans \(1.89\) million years ago.
Compare and contrast a chimp with a primitive and modern human:
Modern chimps rarely stand up, but their skeletons are not designed for normal two-legged walking. The skeletons of early humans evolved to hold their bodies in an upright position. Modern humans have developed bodies that allow them to walk and run long distances on two legs.
 
Important!
These are the muscles that protect the body when walking. They attach to the inward-curving areas above the hip socket.