Long ago, as our ancestors hopped up and started getting around on two feet, their musculoskeletal systems evolved to accommodate the new bipedal locomotion. The size of the vertebrae increased, the lumbar region of the spine elongated, and the number of vertebrae in the lumbar region of the lower back eventually decreased from six to five.
In the female, further adaptations helped her endure pregnancy and carry up to 30% more of her body weight. Scientists wrote in Nature1 last week about sexually dimorphic features in the lumbar spines of women, such as larger joints and greater curvature of the spine that appear to ease the biomechanical strain of pregnancy. The lumbar lordosis (curvature) in women's spines extends over three vertebrae rather than two, as it does in men.
The authors proposed an evolutionary benefit to this increased flexibility in women's spines. During pregnancy, they said, the greater lordosis probably reduces the pain and stress on the spine caused by spinal shearing forces which intensify with the increased load bearing. The scientists also compared modern human spines to chimpanzees and early hominins such as Australopithecus africanus. From their measurements of the different lumbar regions and fossil vertebrae the scientists concluded that these adaptations were present in some of the earliest hominins, but not in chimpanzees. Male and female spines both evolved to accommodate bipedal locomotion (fortunately), but then women's spines became distinguishable from men's, which helps them through childbearing.
Many evolutionary adaptations aid gestation and birthing. But some of these changes seem to simultaneously compromise other activities, like locomotion. The female pelvis is wider then males', to accommodate the birth of a baby, however scientists debate whether this adaptation increases the Q-(quadriceps) angle in women, which may in turn cause injury (mostly knee tracking issues) during running or walking. Hormones during pregnancy make the ligaments temporarily lax, which doesn't help overall stability. But the authors suggest that this newest finding about the sexual dimorphism in lumbar lordosis probably benefited women by decreasing pain during pregnancy. It therefore may have helped females "forage effectively or escape predators", as they put it.
Foraging, ok. But I was trying to visualize how the new and improved physique would help our ancestors escape predators as I flipped through the meretricious "Fine Times" section of the Financial Times last weekend. I kept coming back to the science while reading "The Game Generations". The author described how her safari group tracked a female cheetah as she unsuccessfully hunted with her cubs. After days of failed forays, the hungry female cheetah chose to attack two rutting impala. The female interrupted the two fighting males, brought one down, left it to her cubs to the kill, but they bungled it -- so she stepped in and finished the job.
Granted our ancestors were a heartier bunch, not yet reduced to reading about "wild Africa" in the glossy pages of an insert subtitled "How to Spend It". Still, I have trouble visualizing the flexible but ponderously gravid Australopithecus effectively escaping a cheetah or tiger or leopard. An impala is about 75kg, with horns, and can make 30 feet long jumps as high as 8 feet off the ground -- but is still prey to every other beast. Women may leap tall buildings but they're no match for the impala, never mind a wildcat in the bush.
The "Fine Times" author may indeed have experienced an "awesome lesson in the savage ways of nature", but her tour removed her from the action to a distance sufficiently safe for hominins. Which made me realize that despite the superb-ness of this newly discovered evolutionary feature, our pregnant female ancestors might not have chosen to run with the wolves at all. Instead, while four or five months pregnant, perhaps they recognized their inferior speed, smaller teeth and duller claws, and chose to forage around for some berries, sticks and stones, before retiring to a cave to fashion some proper weapons?
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1Whitcome et al., "Fetal load and the evolution of lumbar lordosis in bipedal hominins" Nature 450, 1075-1078 (13 December 2007) | doi:10.1038/nature06342.
2The Q angle is an acute angle found at the knee. It's formed by at the the intersection of one line drawn from the patellar midpoint to a point on the anterior lateral (outside front) of the pelvis called superialic spine (ASIS), and another line drawn through the tibial tuberosity (on the front part of the top of the leg bone called the tibia) (See Fig. 2 here)
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Acronym Required has written previous articles on biomechanics, such as "The Stalwartness of Nepalese Porters", and hominins, such as "The Hobbit Species of Indonesia"
