One of the key traits that separates hominins from apes is bipedalism, or walking upright on two legs, freeing the arms to throw spears, pick berries or carry children. That transition from stable quadruped to confident biped took a long time as new members of the human family tree evolved. Now, reports National Geographic, a new study shows that toddlers of one famous early bipedal species, Australopithecus afarensis, retained some ape-like foot structures that likely allowed them to climb trees or cling to their mothers more securely.
"Lucy," the first and most famous A. afarensis fossil, was discovered in Ethiopia in 1974. Over a quarter-century later, in the early 2000s, researchers uncovered the foot of another A. afarensis fossil, a 2.5-year-old girl in Ethiopia’s Dikika region, which researchers named Selam. And in a new study, published Wednesday in the journal Science Advances, Jeremy DeSilva of Dartmouth and his team analyzed Selam’s well-preserved foot bones, each about the size of a human thumb, and found that while the foot looks good for bipedalism, Selam probably had some qualities similar to baby apes as well.
“This foot is very human-like and indicates that the Dikika child was walking on two legs,” DeSilva tells George Dvorsky at Gizmodo. “However, the bone at the base of our big toe—called the medial cuneiform—has a connection for the big toe that is more curved and slightly more angled than what is found in humans today. Such a curved surface would allow motion of that big toe—which modern apes use for grasping. We conclude from this, and from previous studies on the shoulders of the Dikika child, that she would have been able to climb, and to also grasp onto her mother during travel.”
Dvorsky reports, however, that the foot bones of adults don’t seem quite as ape-like. The grasping toes, the researchers believe, helped young A. afarensis children to scurry up trees to avoid predators, which they probably had to do more often than adults. It also may have helped them grasp onto their mothers since they were probably carried around a lot, like baby chimpanzees.
It also turns out that Selam’s heel is different from those of adult A. afarensis. Kimberly Hickok at LiveScience reports that it is much more delicate than the adult heel, which is similar to ours. “So that suggests [A. afarensis] grew their heels very differently than we do,” DeSilva tells Hickok. “Even though we have the same anatomy they had, we got it differently.”
While juveniles spent more time in trees, it’s likely the adults took to the branches as well. In 2012, after 30 years of intense debate about whether Lucy and Selam were strictly bipedal or also arboreal, a study of their shoulder blades showed they could swing through the jungle with the best of them. It’s likely they spent the day foraging on foot and climbed into the trees to sleep. “If you were living in Africa 3 million years ago without fire, without structures, and without any means of defense, you’d better be able get up in a tree when the sun goes down,” DeSilva says in a press release.
However, even if the species did sleep and hide in the trees, that doesn’t mean having two feet planted on the ground wasn’t important. In fact, Carol Ward of the University of Missouri who is currently examining Selam’s spine and ribs tells Hickock that the child’s grasping power doesn’t really compare with apes. “Even if a baby could have fit more things between its first and second toe, it would not have had the grasping capability like an ape," she says, pointing out that Selam’s foot is more adapted to walking than climbing. “[It shows] how important life on the ground was for these animals, and that effective climbing was much less important.”
Whatever their lifestyle, they were pretty successful. So far researchers have found fossils from 300 A. afarensis individuals—though very few children—and the species lasted more than 900,000 years, three times as long as our own has walked the earth.