Bipedalism
Evolution of human bipedalism

There are at least twelve distinct hypotheses as to how and why bipedalism evolved in humans, and also some debate as to when. Bipedalism evolved well before the large human brain or the development of stone tools. Bipedal specializations are found in Australopithecus fossils from 4.2 to 3.9 million years ago and recent studies have suggested that obligate bipedal hominid species were present as early as 7 million years ago. Nonetheless, the evolution of bipedalism was accompanied by significant evolutions in the spine including the forward movement in position of the foramen magnum, where the spinal cord leaves the cranium. Recent evidence regarding modern human sexual dimorphism (physical differences between male and female) in the lumbar spine has been seen in pre-modern primates such as Australopithecus africanus. This dimorphism has been seen as an evolutionary adaptation of females to bear lumbar load better during pregnancy, an adaptation that non-bipedal primates would not need to make. Adapting bipedalism would have required less shoulder stability, which allowed the shoulder and other limbs to become more independent of each other and adapt for specific suspensory behaviors. In addition to the change in shoulder stability, changing locomotion would have increased the demand for shoulder mobility, which would have propelled the evolution of bipedalism forward. The different hypotheses are not necessarily mutually exclusive and a number of selective forces may have acted together to lead to human bipedalism. It is important to distinguish between adaptations for bipedalism and adaptations for running, which came later still.

The form and function of modern-day humans' upper bodies appear to have evolved from living in a more forested setting. Living in this kind of environment would have made it so that being able to travel arboreally would have been advantageous at the time. Although different to human walking, bipedal locomotion in trees was thought to be advantageous. It has also been proposed that, like some modern-day apes, early hominins had undergone a knuckle-walking stage prior to adapting the back limbs for bipedality while retaining forearms capable of grasping. Numerous causes for the evolution of human bipedalism involve freeing the hands for carrying and using tools, sexual dimorphism in provisioning, changes in climate and environment (from jungle to savanna) that favored a more elevated eye-position, and to reduce the amount of skin exposed to the tropical sun. It is possible that bipedalism provided a variety of benefits to the hominin species, and scientists have suggested multiple reasons for evolution of human bipedalism. There is also not only the question of why the earliest hominins were partially bipedal but also why hominins became more bipedal over time. For example, the postural feeding hypothesis describes how the earliest hominins became bipedal for the benefit of reaching food in trees while the savanna-based theory describes how the late hominins that started to settle on the ground became increasingly bipedal.
startend:quotation, punctuation:no_comma