Living things have histories—they have descended with modification from common ancestors. As such, every species reveals evidence of its past in its morphology, its genetics, its biochemistry, and its behavior. The first chapter of Darwin’s The Descent of Man (1871) is, while dated, still a good source of examples in the case of humans, as is Neil Shubin’s Your Inner Fish (2008). And of course there are examples at the genetic as well as the morphological level (e.g., Gerstein and Zheng 2006).
The diseases that an organism exhibits similarly reflect its evolutionary history. For example, humans are unique in being bipedal primates: no other primate habitually walks erect. Bipedalism has produced distinctive changes in the human skeleton that have significant consequences for muscular and skeletal conditions. There are reasons why humans have so many complaints about lower back and knee pain. The weight of a quadrupedal animal—a “walking bridge”—is borne by both the shoulder and pelvic girdles, whereas all of the weight of a bipedal primate has to pass through the pelvis to the legs. To keep the upper body weight balanced, the spine has to curve back upon itself (forming an “S” shape). The lumbar vertebral region of humans, therefore, is subject to stresses not encountered by quadrupeds like apes and monkeys, too often resulting in lower back pain. This is not an inconsequential illness: in the U.S., lower back pain is said to be suffered by 80% of adults at some time or another. And many young students are warned—rightly—not to carry heavy backpacks across one shoulder, risking muscle strain and spinal injury.
Similarly, in a quadruped, the weight of the viscera more or less rests evenly along the abdominal muscles, supported by mesenteries running from the dorsal side of the abdominal cavity. In a biped, the viscera are tipped, which means there is more opportunity for them to bulge through the muscles of the abdominal wall, forming painful hernias.
But perhaps the most serious medical consequence of our bipedal evolution is that changes in the pelvis required for bipedalism make childbirth very difficult. The shape of the pelvis in humans reflects the biomechanical needs of bipedalism (Rosenberg and Trevathan 2005). The upper bone of the pelvis, the ilium, is shortened and broadened, and other pelvic bones are modified to transfer the full weight of the upper body down the spine and the sacrum to the femur, and to bring muscles into position to move the legs efficiently during walking and running. All of these postural and locomotor adaptations result in changes to the size and shape of the birth canal, which has obvious effects on the delivery of the fetus. In a normal birth, the fetus has to rotate twice—first to get the head and second the shoulders into position to pass through the birth canal. That’s if everything goes well, and of course, it doesn’t always. Archeological records regularly show that for most pre-industrial populations, the highest mortality for women is in the prime child-bearing years of 15–35. Combine a tight birth canal with a large-brained fetus, and these are not surprising statistics.
These are examples of how just one feature of human evolution—bipedalism—affects health and disease. Other examples exist as well, of course: the nutritional requirements of humans are a result of our evolutionary heritage, much of which was spent as hunter-gatherers. The advent of agriculture changed those selection pressures, causing new diseases, and post-industrial dietary changes have created yet newer ones. A discussion can be found in Trevathan (2007) and references therein.