Foot fossil pegs hominid kids as upright walkers 3.3 million years ago

Walking was afoot long ago among toddler-aged members of a hominid species best known for Lucy’s partial skeleton.

A largely complete, 3.3-million-year-old child’s foot from Australopithecus afarensis shows that the appendage would have aligned the ankle and knee under the body’s center of mass, a crucial design feature for upright walking, scientists report July 4 in Science Advances.

“The overall anatomy of this child’s foot is strikingly humanlike,” says study director Jeremy DeSilva, a paleoanthropologist at Dartmouth College in Hanover.

But the foot retains some hints of apelike traits. Compared with children today, for example, the A. afarensis child — only about 3 years old at the time of death — had toes more capable of holding onto objects or anyone who was carrying her, the team found. Those toes included a somewhat apelike, grasping big toe. “Young children having some ability to grasp mom could have made a big energetic difference for Australopithecus afarensis adults as they traveled,” DeSilva says.

Scientific debate about whether A. afarensis , which may have been ancestral to humans, primarily walked upright or hung out in trees has raged for nearly 40 years. Accumulating lower-body fossils, including foot bones ( SN: 3/12/11, p. 8 ), as well as ancient footprints ( SN: 1/21/17, p. 8 ), point to an adept two-legged stride among A. afarensis adults. But little was known about whether A. afarensis tykes walked early in life or slowly developed a stride-worthy stance.

The A. afarensis foot fossil, about the size of an adult human thumb, comes from a child’s partial skeleton previously excavated at Dikika, Ethiopia (SN: 9/23/06, p. 195). Relatively small, still-developing canine teeth in the upper jaw had already indicated that the child was female. Researchers finished removing rock that had encased much of the Dikika child’s left foot in 2013.

DeSilva and colleagues compared bones from the Dikika foot fossil with corresponding bones of humans, chimps, bonobos, gorillas and orangutans. Samples included adults and juveniles.

The researchers discovered that the child’s midfoot bones supported a low arch. A key, humanlike bone on the outer side of the child’s foot contributed to arch support. Other evidence indicates that a moderately arched foot, typical of people but not other apes, reduces physical stress and lower leg injuries while walking upright. Previously discovered foot fossils are too incomplete to say whether A. afarensis adults had feet with low arches or any arches at all, the team adds.

But the Dikika child had a surprisingly small, apelike heel bone, DeSilva says. Other fossil finds indicate that A. afarensis adults had sturdy heel bones like those of human adults and children. A. afarensis heel bones became more robust in response to regular walking throughout childhood, DeSilva suspects.

Walking-related features of the Dikika child’s foot highlight the importance of a ground-based lifestyle for Lucy’s species, says paleoanthropologist Carol Ward of the University of Missouri in Columbia. “These bones are not entirely human, but all the hallmarks of a humanlike foot seem evident in the arrangement of joints and proportions within the foot.”

Even if a young A. afarensis could have fit more items between its big toe and second toe than a human child can, the ancient youngster would not have had a grasping ability even close to that of a modern-day ape, Ward says.