In the summer of 2009, anthropologist Jeremy DeSilva was studying fossilized foot bones during a research trip in South Africa when a man burst into his lab and asked, “Do you want to see something cool?” Though he had never met his visitor, DeSilva recognized him as Lee Berger, a fellow anthropologist with a reputation as an Indiana Jones–style fossil hunter. Berger led DeSilva down the hall to his lab and stopped before a table shrouded in a black sheet. Then he ripped the sheet away “like a magician, and my jaw dropped.”

The sheet had been hiding a humanlike pelvis and spine, chimp-like torso and feet, primitive arm bones, and surprisingly modern hands—all of which belonged to a single 1.977-million-year-old female skeleton that Berger was still in the process of excavating from a nearby cave site. Malapa Hominin 2 (MH2) resembled no other early human ever found. She had “a completely new combination of anatomies that frankly, prior to seeing this thing with my own eyes, I would not have thought possible in an early human.” Would she have walked like us, or climbed trees like a chimpanzee? Though DeSilva is an expert in early human locomotion, he was stumped.

A Detective Story

DeSilva, an assistant professor of biological anthropology at Boston University, first became fascinated with early humans while working as an educator at the Museum of Science in Boston, where he was assigned to update the human evolution exhibit. “I didn’t know anything about this stuff, so I started reading, and I became absolutely obsessed,” he says. “I would look at the pictures of ancient creatures and wonder about them as individuals. They lived and breathed and ate and slept and had babies. And they were our ancestors!”

Chimpanzees and humans shared a common ancestor until four to seven million years ago, when the earliest humanlike creature, Ardipithecus, came into existence. This creature evolved into the Australopithecus, which lived two to four million years ago and is represented by a famous skeleton, Lucy. Australopithecines evolved into the genus Homo (two million years ago), which developed into modern humans. Within each of these genera, multiple species existed with different anatomical combinations, and DeSilva wanted to know how these creatures with such different anatomies would have moved.

He went straight from the Museum of Science to graduate school, where he studied the locomotion of early apes and humans. “We can’t go back in time to see these things or run them on a treadmill, and yet we really want to bring them back to life as best we can to understand the changes that occurred in the past to make the anatomy we have today. It’s a wonderful detective story.” He began to study modern humans and chimpanzees for clues about how our earliest ancestors would have moved. For his PhD dissertation, he visited Kibale Forest National Park in southwest Uganda to film a population of wild chimpanzees and decipher how they can so quickly and efficiently travel from the ground to the treetops.

During DeSilva’s first day of filming, chimpanzees began to climb in front of him, and “what they were doing with their feet was extraordinary.” The grasping ability of their big toe has been well documented, but DeSilva noticed something striking about their ankles: chimpanzees could pull the tops of their feet against their shinbones to keep their bodies close to the tree trunk. “I would try to replicate the way their feet were moving,” he says. “Right there, I realized these guys are doing something with their feet that I can’t.”

The differences between human and chimpanzee feet leave their mark on bones; because human feet have a narrower range of motion, the forces are distributed equally across the ankle joint, molding the ankle bone into a square. Chimpanzees exert more pressure on the front of their ankle joint, so the front of the bone is wider than the back. Likewise, while the base of a human heel bone is broad to accommodate the impact of our stride, a chimpanzee’s heel bone comes to an almost beak-like point. The shapes of the chimpanzee’s ankle joint and heel bone are signatures of its ability to climb.

“I could then look at the fossils and say if they look more chimp-like or humanlike,” DeSilva says. “I studied foot bones from all over Africa, and they all more or less looked the same, which suggested to me that early humans were all walking more or less the same way. This would mean that humanlike bipedalism evolved early on and hasn’t changed much in three million years.”

DeSilva’s theory aligned with the common belief that early humans stopped climbing and living in trees once they began walking upright. But the bones under the sheet exploded this theory.

A New Type of Human

Berger had used Google Earth to mark approximately 200 clumps of trees in the barren South African landscape known as the Cradle of Humankind. The tree clumps signaled the existence of underground streams that had forged deep, vertical limestone caves, into which centuries of rain had washed the remains of early humans. The MH2 skeleton, along with at least six other partial skeletons, had been preserved in one of these caves until the turn of the century, when mining explosives catapulted the fossilized bones to level ground.

In a 2010 Science article, Berger named MH2 and her companions Australopithecus sediba an entirely new species within the genus Australopithecus. He compiled a team of experts to study the sediba’s unusual anatomy and invited DeSilva to join the foot team.

“When the casts of these fossils arrived, I was shocked,” DeSilva says. In the last year, Berger had excavated more fossils from MH2’s skeleton, including a heel bone with the chimpanzee’s distinctive beak-like point, signifying a foot built for climbing. “I almost thought it was a joke, that they had put a chimpanzee heel into the box just to see if I would notice.”

DeSilva and his colleagues described the sediba’s foot and ankle structures in a September 2011 Science article, and speculated that the creature had moved differently from other early humans; it had likely walked on two legs and possessed the ability to climb trees more efficiently. The team members did not speculate on how exactly the creature might have moved—because they had no idea. “I walked around the laboratory in South Africa trying to figure out how these joint systems worked together, and I couldn’t figure it out,” DeSilva says. “I was truly lost.”

Biomechanics and Bones

Kenneth Holt (SAR’83), associate professor of physical therapy and athletic training at BU College of Health & Rehabilitation Sciences: Sargent College, was intrigued when he heard that DeSilva would be presenting a lecture about the evolution of upright walking at Sargent College. It had been nearly 20 years since DeSilva had taken his class on biomechanics (the study of the mechanical laws governing a living body) as a graduate student, but Holt had been telling his students about him ever since.

DeSilva had conducted his final project on the 3.2-million-year-old skeleton Lucy to learn how the forces operating at her hip joint allowed her to walk on two legs. “I always use Jeremy’s project as an example of how you can use biomechanics to understand how bones work,” he says. “I tell my students to think of a question that they’ve never been able to answer, and see if biomechanics can help answer it.”

Early in his career as a physical therapist, Holt had developed a theory about how the biomechanics of the foot affects the body as a whole. He determined that the way in which the foot hits the ground influences the forces that generate around the foot and carry through the body. In a disorder like hyperpronation, for instance, the foot hits the ground on the outside edge, and the ground pushes back with a force that drives the foot to roll onto its inside edge, resulting in a chain reaction throughout the body: The body pitches forward, becoming unbalanced. To compensate, a hyperpronator leans backward to realign his upper body over his feet, resulting in exaggerated curvature in his spine that causes his head to tip back, eyes pointing at the sky. He counteracts this awkward posture by forcing his head forward, which causes stress on the neck muscles, sometimes resulting in a hunched back.

A hunched back is an example of bones adapting to the stresses imposed upon them; to Holt, abnormal bone growth is a hallmark of problems that start in the feet. In short, he realized that the body adapts to the biomechanics of the foot.

“Professor Holt was somebody I always looked up to and wanted to impress because I just found him so brilliant,” DeSilva says. When he spotted his former professor in the audience for his lecture at Sargent, “I was pleased to be able to show off the fossils I was working on. Little did I know that he would fill in the huge missing piece.”

The Missing Piece

DeSilva began the lecture by introducing the sediba and describing the strange anatomy of the creature’s feet. He passed around the heel bone, then threw his hands in the air and said, “I have no idea what this means.”

The bone was making its way through the audience, and when Holt had the chance to study it up close, he thought it looked familiar. He raised his hand and asked, “Is this skeleton’s AIIS unusually big?”

The anterior inferior iliac spine, or AIIS, is the bump on the outside of the pelvis where the rectus femoris muscle attaches. In a modern human hyperpronator, the AIIS bone becomes enlarged due to the stress inflicted on the muscle. The skeleton did have an enlarged AIIS, which not only indicated that its feet were hyperpronated—but that it was an upright walker.

Chimpanzees do not possess this bone structure, “so if you find a pelvis with that bulge, it’s a good indication that you’ve got something walking on two legs,” DeSilva says. Holt added to the evidence for this theory by correctly predicting abnormal bone growth in key areas throughout the rest of the skeleton. “Even though Ken works on people who live today, and I think about the ones who lived millions of years ago and don’t exist anymore, the same principles apply,” DeSilva says. “So we can use humans and chimpanzees today as our models to come up with reasonable hypotheses for how the Australopithecus sediba were moving, based on the shapes of their bones.”

DeSilva and Holt paired up to study the sediba’s locomotion, determining that while the modern human’s body is not built to walk on hyperpronated feet, MH2’s body was. Its skeleton displayed an evolutionary progression uniquely adapted to both walking and climbing—which challenges the assumption that early humans stopped living in trees once they began to walk on two legs.

The research suggests that there were multiple ways to be an upright walker, a hypothesis that anthropologists—including DeSilva—had rejected until now. “We can no longer generalize Australopithecus locomotion,” DeSilva says. “The different species moved in different ways and utilized trees in different amounts. I have no doubt that this thing was living, to some degree, in trees.”

A Provocative Hypothesis

Holt and DeSilva detailed their research in “The Lower Limb and Mechanics of Walking in Australopithecus sediba,” one of six articles on the new species’ anatomy published by Science in April 2013. The story has been picked up by journals and media outlets around the world, including National Geographic, BBC News, USA Today, and the Boston Globe, and has provoked a flurry of scholarly responses.

One colleague called the theory “a provocative hypothesis,” while another suggested in a Nature commentary that the MH2’s hyperpronated gait could be pathological and not representative of how all of the individuals walked. “Those critiques are reasonable,” DeSilva says. “We based our hypothesis mostly on that single female skeleton, but the bones from the other individuals are consistent. Just as all humans today walk a little differently, all sedibas two million years ago probably walked a little differently, too. So was MH2’s form of locomotion the norm? I think so—but we won’t know until we have enough fossils to test it. The wonderful thing about the Malapa site is that we are going to have those fossils.”

And BU students will have the opportunity to help unearth them. DeSilva is writing a grant to start a field school for BU undergraduates and graduate students to participate in the excavation. While there are enough bones at this one cave to keep anthropologists busy for decades, Berger’s Google Earth map indicates at least 200 sites yet to be explored. The Cradle of Humankind is “such a vast area, and you start to imagine what else is out there,” DeSilva says. “Then you look at that area compared to the continent of Africa, and…oh jeez! Just think about how much more future generations are going to find.”