A zebra has its stripes and a giraffe its neck, birds are known by their wings, and a fish by its gills. For humans, it's two feet and a unique locomotion—bipedalism.
A lot of theories try to explain why humans evolved to walk two-legged. Some say that bipedalism let early hominids rise up to see over the savannah grass and check for predators. Others say that having free hands to tend offspring and gather food was the advantage. Herman Pontzer, a biological anthropology major, supports a third idea: Bipedalism may have been the best way to travel.
"It's important to note in what ways our bones are stronger or weaker than those of other primates," says Pontzer. "The differences reflect our different activities." For example, the limb bones of a small, slowmoving, treehanging loris are stronger from end to end, or axially, because they are pulled at all day by the weight of the loris's body. A macaque, which is about the size of a terrier and runs on all fours, has limb bones that are stronger from front to back. Its legs are used for propulsion.
"Bones remodel all the time," Pontzer says. "Their shape reflects the different stresses on them, and therefore the activity that took place, whether it was walking on the ground or hanging from trees.
"Using bipedalism to move requires more propulsive strength." Specifically, the characteristic "striding gait" that humans use, rolling the feet and pushing off the ground, requires more strength in the tibia (the lower leg bone) from the shin back through the bone, rather than straight down from knee to ankle.
Using a collection of chimpanzee and human tibia bones, Pontzer is recording the differences between these two closely related species. He makes a mold of the outside of the bone, noting the oval shape of the chimp bone compared to the more triangular shape of the human tibia. From that mold, Pontzer determines the perimeter of each tibia and begins creating a digitized cross section of the bone on a computer. An xray of the bone then provides the cortical thickness, the amount of bone mass between the perimeter and the medullary cavity, or hollow center of the bone. From his various measurements, Pontzer is able to determine the maximum area of the cortical bone and the bone's bending, twisting, and compressive strength.
"The study shows that the human tibia is significantly stronger than the chimp's from front to back, but not significantly stronger either from side to side or compressively," says Pontzer. "Even though we made the switch from four legs to two legs, the only selective force on the strength of the tibia seems to be more propulsive strength." Breaking a tibia side to side, across the shin bone, is not difficult for humans or chimps. But selective adaptation has made our bones stronger front to back. Without the increased rigidity, our tibia would not be able to withstand the stress of constant walking and running. Our bones would break.
Pontzer's evidence seems to support the idea that humans became bipedal to travel, not just to occasionally stand up or carry something. But he wants to measure the cross sections of more bones, including those of other primates and even some hominid fossils. He hopes these additional specimens will show various stages in the evolution of bipedalism and show an increase in propulsive strength from chimp to early hominid to human.
Herman Pontzer is a biological/physical anthropology major in the College of the Liberal Arts and the Schreyer Honors College. His adviser is Alan Walker, Ph.D., distinguished professor of biology and anthropology in the College of the Liberal Arts and the Eberly College of Science, 322 Carpenter Building, University Park, PA 16802; 8148651531; axw8@psu.edu.