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Healthy adult humans have an anterior (forward) curvature of the lower (lumbar) vertebral column known as lumbar (or lumbosacral) "lordosis." A lordosis is any anterior-ward convexity in the vertebral column, and the normal adult vertebral column has two of them - one in the lumbar region and one in the cervical region (an anterior-ward concavity is known as a kyphosis, and in humans there are two of these as well - one in the thoracic region and one in the sacrum). The two lumbar joints that are most involved with lumbar lordosis are L4-L5 and L5-S1. This lordosis is believed to be functionally important for bipedal locomotion, in that it serves to move the center of gravity posteriorly and thus over the hip joints (reducing the amount of muscular effort required to balance body weight over the lower limbs in bipedal standing and walking). Lumbar lordosis is not found in great apes, likely because of their lack of habitual erect posture, and the extreme reduction in the length of the lumbar region in the great apes. It has conventionally been thought that lordosis develops in human children during the first year of life, in response to new biomechanical loads (which influence the growth of the vertebrae) as they begin to pull themselves up into standing postures prior to taking their first steps. However, recent research suggests that lordosis is evident in the lumbosacral region of as much as 60% of human fetuses, suggesting that there may be a genetic component to the morphology.
The pelvis of Ardipithecus ramidus suggests that early in hominid evolution the ilia shortened and broadened to establish permanent lumbar lordosis, while the African great apes had modified ilia for increased abdominal stiffness so that no lordosis was functionally necessary. It is proposed that within the hominid clade during the Late Miocene there was facultative lumbar lordosis and that the lordosis was fixed within the lineage by the Mid-Pliocene. However, other research suggests that lumbar lordosis did not occur until 1-2 million years ago associated with the repositioning of the posterior superior iliac spine (PSIS). The placement of the PSIS posterior to the spine increased the mechanical advantage of the lumbar musculature in effecting lumbar extension (Filler 2007). Early hominins often are reported to have six lumbar vertebra and it is possible that the longer vertebral column facilitated the adoption of lumbar lordosis. Modern humans and all the extant apes (except occasionally the gibbons) have five lumbar vertebrae, although 2-8% of modern human populations can have individuals with an additional lumbar vertebra.
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