The earliest hominins split their days between the trees and the ground, alternately adopting ape-like tree-swinging behaviors and human-like bipedalism, or walking upright on two feet—albeit in a crouched position. By the time Lucy and her Australopithecus afarensis relatives arrived on the scene some four million years ago, bipedalism had largely overtaken tree-dwelling, but according to a study published in the Proceedings of the National Academy of Sciences, these human ancestors likely lacked a key evolutionary adaptation: the rigid big toe.
BBC News’ Angus Davison reports that the new findings suggest the big toe, which enables humans to push off of the ground while walking and running, was one of the last parts of the foot to evolve.
“It might have been last because it was the hardest to change,” lead author Peter Fernandez, a biomedicist at Milwaukee’s Marquette University, tells Davison. “We also think there was a compromise. The big toe could still be used for grasping, as our ancestors spent a fair amount of their time in the trees before becoming fully committed to walking on the ground."
To trace the big toe’s evolution, Fernandez and colleagues created 3D scans of human relatives’ toe bone joints, relying on a combination of living creatures—including apes and monkeys—and fossilized samples. After juxtaposing these scans with ones made of modern humans and mapping the data onto an evolutionary tree, researchers realized that the big toe developed much later than the rest of the foot’s bones. Early hominins’ gait, therefore, had more in common with apes’ than the easy human stride seen today.
According to Live Science’s Jennifer Welsh, differences between human and non-human primate feet come down to purpose. Whereas most primates use their feet to grasp onto tree branches and other objects, humans rely on theirs to navigate life on two legs. For example, arches, which are located on the inside of the foot close to the big toe, make it harder for humans to nimbly climb trees but offer shock absorption when planting one’s feet on the ground.
The human big toe specifically carries 40 percent of the five toes’ collective weight, Corey Binns writes for Scientific American, and it is the last part of the foot to leave the ground when one walks or runs. Comparatively, apes’ big toes are opposable, built for grasping and functioning similarly to the versatile opposable thumb, which allows primates to deftly perform a wide range of motions.
Although early humans such as A. afarensis and the roughly 4.4-million-year-old Ardipithecus ramidus walked upright, BBC News’ Davison notes that the study confirms this bipedalism did not preclude the existence of an opposable, ape-like big toe.
“It was a bit of shock when hominins were found that have a grasping, or opposable, big toe, as this was thought to be incompatible with effective bipedalism,” anatomist Fred Spoor of London’s Natural History Museum tells Davison. “This work shows that different parts of the foot can have different functions. When a big toe is opposable, you can still function properly as a biped."