Upright we are, and we experience ourselves in this specific relation to the world. Erwin Straus, “The Upright Posture”
Many of the medical complaints that prompt routine radiologic imaging can be traced to the fact that human beings are not perfectly adapted to one of the most distinctive features of our species, upright posture. We walk upright on our hind limbs—a feature not found in any other nonhuman mammal in evolutionary history. Evolutionary biologists have been investigating this property for decades, but many of their more intriguing findings remain largely unknown to radiologists.
Bipedalism raises questions for radiologists. How did it evolve? What advantages does it afford, and what anatomic and physiologic challenges does it give rise to, particularly in the spine, pelvis, and lower limbs? Gaining a deeper understanding of these questions can help radiologists better understand the genesis of a diverse range of conditions we encounter on a daily basis, such as traumatic and insufficiency fractures, osteoarthritis, spondylolisthesis, and even disorders with no apparent relationship to upright posture.
Bipedalism
An inspection of the fossil record suggests that bipedalism probably appeared in human ancestors approximately 7 million years ago. The musculoskeletal modifications that have permitted upright posture involve the entire skeleton from head to toe, and make such adaptations easy to identify in the paleontological record. For example, the species Sahelanthropus, thought to have appeared at about the same time that the chimpanzee and human branches started to diverge, exhibits a relatively anteriorly placed foramen magnum, a requirement for the transition to an upright spine.
More certain is that by 4 million years ago, species such as Australopithecus afarensis had emerged. They appear to have lived in open areas near dense woods, where upright posture increased their height and visual range, enabling them to recognize predators from a greater distance. Their forelimbs, now freed of locomotion, could be used for carrying, climbing, and manipulating objects. By about 2.5 million years ago, human ancestors had evolved a lumbar lordosis and thoracic kyphosis, and by 1.9 million years ago, greater hip support had appeared, permitting full bipedalism.
Interestingly, bipedalism appears to have predated the development of the expanded human brain by a substantial period of time. This suggests that the development of a larger brain was not the cause but the effect of freeing the forelimbs for more complex activities such as making and using tools. Instead of developing the excess capacity of a larger brain and then finding uses for it, it appears that human progenitors began to place increased demands on their brains, which grew over time to meet the challenge.
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Incomplete Adaptations
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Future Prospects
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