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The Earliest Humans Climbed Trees and Walked

There has long been a belief among evolutionary anthropologists that our earliest ancestors could be either good climbers (like apes) or efficient walkers (like humans), but they couldn’t be both.

Elaine Kozma

But new research has thrown this either/or debate into question. In a groundbreaking analysis of fossils and biomechanics, Graduate Center doctoral candidate Elaine Kozma (Anthropology), along with Professor Herman Pontzer (GC/Hunter, Anthropology) and their team of researchers, discovered that the earliest hominins (think Lucy) had the bone and muscle structure to both walk upright and to climb trees.

The researchers found their answer in a 10-year journey led by Kozma that brought together several types of analyses including fossil measurements, anatomy research, and biomechanics, and finally by focusing on the pelvic bone.  

“We knew from our chimpanzee work that if you had to pick one really important factor that affected how upright or crouched your gait is, it is the pelvis,” says Pontzer. Specifically, it’s the ischium, familiarly known as the sit bone, which is where the hamstring muscles attach to the pelvis.   

What’s important about the ischium is its length and its angle. In chimpanzees, it’s long and angles down toward the feet. In humans, it’s short and angles more toward the back.

For this study, published in the Proceedings of the National Academy of Sciences; the researchers combined biomechanical analyses of hip extension during walking and climbing of living apes, humans, and monkeys with analyses of pelvic bone structures of the earliest hominins, including a 4.4-million-year-old Ardipithecus, the earliest hominin with a preserved pelvis, and Lucy, a younger Australopithecus (about 3.2 million years old). One of the mathematical challenges was figuring out how to orient the pelvis in space, because the angle of the pelvis is critical to understanding the way the hominins moved.

The researchers found that although hominins had a long ischium, which gave them the power to climb, it was angled slightly back, which allowed them to walk upright and take longer strides. “The Ardipithecus was able to climb powerfully and walk efficiently, and we hadn’t thought that was possible before,” says Pontzer. “Because it had a long ischium, people thought it just walked like an ape,” he says. “This shows they were very efficient walkers, more similar to humans than to apes, but they didn’t give up their powerful climbing abilities.”

There has always been evidence that early hominins had arms and hands and an ape-like foot that allowed them to climb well, yet they appeared to walk in a more human upright manner. This study provides an explanation of how they were able to do both.

Submitted on: MAY 27, 2018

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