Step-by-step explanation:
The evolution of bipedalism (walking upright) signals the origin of hominid evolution, although no one is positively sure why bipedalism developed. The reason why bipedalism evolved in hominids remains a matter of discussion. No tools emerged until 2.5 million years ago, so toolmaking seems an improbable cause. Alternative ideas suggest that walking upright is more agile and uses less energy than walking on four legs; an upright posture allows hominids to pick fruit from trees and see over tall grass; upright reduces the body surface exposed to the sun’s rays; an upright stance aided the wading of aquatic hominids, and that bipedalism frees the forelimbs of males to carry food back to females, encouraging pair-bonding. All of these suggestions have their proponents, and none are universally accepted.
Fossils demonstrate that bipedalism extended back 4 million years ago; knee joints, pelvis, and leg bones all exhibit the signs of an upright stance. Remarkable evidence that early hominids were bipedal is a set of some 69 hominid footprints found at Laetoli, East Africa. Their footprints preserved in 3.7-million-year-old volcanic ash.