...And where does the buck finally stop? What finally bears the full weight of our upright body? Two ridiculously tiny platforms.
"The human foot has rightfully been called the most characteristic peculiarity in the human body," says Will Harcourt-Smith, a paleontologist at theAmericanMuseumof Natural History. "For one thing, it has no thumblike opposable toe. We're the only primate to give up the foot as a grasping organ."
This was a huge sacrifice. The chimp's foot is a brilliantly useful and versatile feature, essential to tree climbing and capable of as much motion and manipulation as its hand. The human foot, by contrast, is a hyper-specialized organ, designed to do just two things, propel the body forward and absorb the shock of doing so. Bipedality may have freed the hands, but it also yoked the feet.
Harcourt-Smith studies foot bones of early hominins with the new technique of geometric morphometrics—measuring objects in three dimensions. The variations in foot structure he has discovered in Australopithecus and Homo habilis (a species that lived 2.5 to 1.6 million years ago) suggest that these early hominins may have walked in different ways. "We have a desire to see the story of bipedalism as a linear, progressive thing," he says, "one model improving on another, all evolving toward perfection in Homo sapiens. But evolution doesn't evolve toward anything; it's a messy affair, full of diversity and dead ends. There were probably lots of ways of getting around on two feet."
Still, in all the fossil feet Harcourt-Smith studies, some type of basic human pattern is clearly present: a big toe aligned with the long axis of the foot, or a well-developed longitudinal arch, or in some cases a humanlike ankle joint—all ingenious adaptations but fraught with potential problems. "Because the foot is so specialized in its design," Harcourt-Smith says, "it has a very narrow window for working correctly. If it's a bit too flat or too arched, or if it turns in or out too much, you get the host of complications that has spurred the industry of podiatry." In people with a reduced arch, fatigue fractures often develop. In those with a pronounced arch, the ligaments that support the arch sometimes become inflamed, causing plantar fasciitis and heel spurs. When the carrying angle of the leg forces the big toe out of alignment, bunions may form—more of a problem for women than men because of their wider hips.
And that's not all.
"One of the really remarkable aspects of the human foot, compared with the chimp and other apes, is the relatively large size of its bones, particularly the heel bone," Bruce Latimer notes. "A 350-pound male gorilla has a smaller heel bone than does a 100-pound human female—however, the gorilla bone is a lot more dense." While the ape heel is solid with thick cortical bone, the human heel is puffed up and covered with only a paper-thin layer of cortical bone; the rest is thin latticelike cancellous bone. This enlargement of cancellous bone is pronounced not just in the heel, but in all the main joints of our lower limbs—hip, ankle, knee—and has likely marked the skeleton of our ancestors since they first got upright; it has been found in the joints of 3.5-million-year-old hominin fossils from Ethiopia.
"The greater volume of bone is an advantage for dissipating the stresses delivered by normal bipedal gait," Latimer says. However, it's not without cost: "The redistribution in our bones from cortical to cancellous means that humans have much more surface exposure of their skeletal tissue. This results in an accelerated rate of bone mineral loss—or osteopenia—as we age, which may eventually lead to osteoporosis and hip and vertebral fractures."
