Early hominins' four smaller toes had adapted to bipedal walking by around 4.4 million years ago, but the big toe remained better-suited to grasping and climbing for a few million more years, until some time early in the evolution of our genus, Homo. That’s the conclusion of a new study by Stony Brook University anthropologist Peter Fernandez and his colleagues, who studied the size and shape of the metatarsals (the bones of the mid-foot) in modern humans, fossil hominins, and an assortment of monkeys and apes.

Spring in our step

Primate feet evolved mostly to grasp while climbing, which is why chimpanzees have more flexible feet than humans and why their big toes are opposable, like our thumbs. Humans—and our hominin predecessors—are the weird exceptions in the primate family tree, with the basic architecture of our feet adapted to bear weight while walking.

Some of those adaptations, like the one Fernandez and his colleagues studied, look pretty subtle to anyone who is not a specialist in primate foot anatomy. Their study focused on how much the head of the metatarsal protrudes toward the top of the foot, sticking out above the shaft of the bone like a dome, at the joint with the phalanx (one of the bones that makes up the toes) at the base of the toe.

During walking, humans hyperextend the joints of our forefoot, especially the one between the metatarsals and the phalanges, every time we push a foot off from the ground to take a step. That extension pulls the soft tissue on the bottom of the foot tighter, which helps turn the foot into “a relatively stiff, propulsive lever,” as Fernandez and his colleagues put it. And that high, rounded top at the head of the metatarsal gives the joint more range of motion, which makes that hyperextension more effective. In other words, that little protrusion on the ends of our metatarsals puts a little more spring in our step—and that gave our ancestors an evolutionary advantage.

A history in toes

Hominins had started evolving that trait by around 4.4 million years ago, according to the oldest fossils with well-preserved foot bones. That's Ardipithecus ramidus, which lived between 5.8 million and 4.4 million years ago. Its second, third, and fourth toes all had domed metatarsal heads that looked a lot more like those of modern humans than modern apes. Fernandez and his colleagues didn’t examine the fifth, smallest toe in much detail because it plays much less of a role in walking than the others.

The second toes of Australopithecus afarensis, Homo floresiensis, Homo naledi, and an earlier representative from the genus Homo, all looked more human than ape-like. Interestingly, the third and fourth toes for the A. afarensis (a hominin species that lived between 3.9 million and 2.9 million years ago, best known from a specimen called Lucy) fossils in the study had less-domed metatarsal heads—more like modern apes than humans—although the third metatarsal had suffered some damage since burial that may have skewed the measurements. The early Homo genus’ metatarsal heads were domed like ours, and Homo naledi (whose exact relationship to modern humans is still unclear) also fell within the modern human range of measurements.

But in Fernandez’s study, the third metatarsal head of H. floresiensis (the short-statured hominin also known as the hobbit, which lived on Flores from about 100,000 to 50,000 years ago) fell just outside the modern human range of measurements but still closer to us than to modern apes. In the fourth toe, the metatarsal head was domed just like a modern human’s, but other aspects of the shape of the bone looked more ape-like. The lesson? Evolution is a complicated process, and our own evolutionary history is less linear and more piecemeal than you might assume at first glance.

A weird mix of traits

The partial foot of an A. afarensis toddler found near Dikika, Ethiopia, suggests much the same conclusion. We already know, from other fossil evidence and from the well-preserved set of footprints near Laetoli, Kenya, that A. afarensis walked upright—similar to, though perhaps not exactly like, modern humans. But in a study published earlier this year, Dartmouth College anthropologist Jeremy DeSilva and his colleagues reported that the Dikika child’s foot looked like a modern human’s in some ways but more like an ape’s in other ways.

The length and shape of the Dikika child’s cuboid bone—the one responsible for making our mid-foot rigid and providing support for the arch of the foot—suggests that A. afarensis had a stiffer mid-foot than apes. That would have been much better for supporting the body's weight while walking but also much less flexible and mobile for grasping things while climbing. And A. afarensis had a more arched foot than apes but would have been relatively flat-footed compared to modern humans.

A. afarensis also had a much more mobile big toe. It still stuck out from the foot at an angle compared to the other toes but not a fully opposable one like modern apes, based on the angle of the cuneiform (the bone that the base of the metatarsal would attach to). And the shape of the big toe stayed decidedly more ape-like until much later in our lineage, too. In Fernandez and his colleagues’ study, A. afarensis and Australopithecus africanus (which lived 3.3 million to 2.1 million years ago) both had metatarsals that look much more like modern gorillas, in particular, than like modern humans.

So while most of the rest of the foot was starting to adapt to walking, even as early as 4.4 million years ago, the big toe was still more like an ape’s: built for climbing. That suggests that for several million years, even bipedal hominins were still spending a lot of time in the trees.

Walk this way

“It shows that the big toe stayed in an ape-like form longer in human evolution, either because these critters weren't walking quite like we humans or because they were still spending some of their time up in the trees,” Fernandez told Ars. Climbing may have offered a way to escape predators or get to food sources.

In modern humans, the second toe is more important for the mechanics of walking than the other three small toes, although we also rely heavily on our big toe. But if earlier hominins had big toes built for climbing, species like A. afarensis used their smaller toes, perhaps especially the second one, to provide most of the push-off force when taking a step, letting that second toe do most of the work that the big toe does now. And that means their gait may have been slightly different.

The footprints at Laetoli support that idea. “There appears to be less weight transfer and push off the big toe in those footprints, and we find the same thing in the bones. They match beautifully,” DeSilva told Ars Technica.

It seems that there wasn’t evolutionary pressure to bring the big toe into line with the smaller four, where it could be used to push off the ground for walking, until the early members of the genus Homo came along—or else earlier hominins still faced pressure to retain some ability to grasp and climb with their feet.

PNAS, 2018. DOI: 10.1073/pnas.1800818115 (About DOIs).