User:Pancakes2022/Lumbar vertebrae

Lumbar Vertebrae in Modern Human Evolution
Lumbar vertebrae were a key factor in the emergence of bipedalism, which is an evolutionary development that marks the beginning of modern human evolution. For a mammal to maintain upright posture, the trunk must be atop the pelvis, which must be directly over the feet. In modern humans the lumbar spine can bend, which creates flexible hips and strong back muscles that allows humans to walk upright. However, there is still a significant amount of pressure at the lumbosacral joint, caused by the increased caudal load in the lumbar spine, combined with the wedged morphology of L5 and the angulation of the lumbosacral joint.

Adaptations that contributed to bipedalism include a difference in the zygapophyseal joint orientation as well as a progressively increasing distance craniocaudally between the zygapophyseal joints in the lumbar spine. It the early 1970's Robinson used the zygapophyseal orientation to define lumbar joints. He later discovered that Australopithecus Africanus had six lumbar vertebrae. The additional lumbar vertebra is thought to increase column strength, upper body flexibility, as well as the ability to absorb more shearing force, which occurs when one the vertebrae is sliding back and forth, or in the opposite direction of the vertebrae below it. The modern homo sapien spine is sinusoidal and is fundamentally strong and stable, despite the increased load that is typically associated with a bipedal posture and movement. Furthermore, humans have the highest placement of the sacrum within the pelvis, which increases the hip stabilization and makes bipedalism easier.

Last Common Ancestor
There is still a question of how many lumbar vertebrae were present in the last common ancestor between homo sapiens and chimpanzees. Humans have five lumbar vertebrae, one less than ancestors such as Australopithecus Africanus. There are two main theories about the last common ancestor. There is the long-back theory, which suggests that the last common ancestor had six lumbar vertebrae. The other prevailing theory is that the last common ancestor had less than the five lumbar vertebrae that humans have. Unfortunately the fossil record isn't clear enough at the moment to determine which theory is accurate. When considering everything that is currently known, the lumbar stiffening of modern humans is most compatible with the last common ancestor having 5-6 lumbar vertebrae(long-backed). However, the short-backed model still requires fewer evolutionary transitions in morphology.