Final answer:
Bipedalism is the term used for standing erect and walking on two feet, an adaptation seen in hominins. Significant anatomical changes, such as the valgus angle of the femur and the evolution of spinal curves and an arched foot, were necessary for this form of locomotion. Other traits that classify a species under the genus Homo include brain size and hand dexterity.
Step-by-step explanation:
The ability of an individual to stand erect and walk on two feet is called bipedalism. This adaptation involved significant anatomical changes in early hominins and is a defining characteristic of the group known as hominids or hominins. One of the most important anatomical changes was the evolution of the valgus angle of the femur, which positions the knees and feet directly under the pelvis, supporting upright posture. Additionally, the evolution of spinal curves, an arched foot, and the alignment of the toes aided in successful bipedal locomotion.
Early hominin species such as Australopithecus show evidence of walking upright, but it was around 2.8 million years ago, with species like Homo erectus, that bipedal locomotion became nearly as efficient as in modern humans. These species had longer legs, a broader pelvis, and arched feet, necessitating other adaptations to their diet and lifestyle due to the shift from arboreal living to a fully terrestrial, bipedal lifestyle. Hands were freed for tool use, leading to further evolutionary benefits.
While bipedalism is a key trait, other criteria used to classify a species under the genus Homo includes brain size, hand dexterity for tool-making, such as observed in Homo habilis, and dietary changes as seen with Homo erectus.