Little Foot fossil shows that the early human ancestor clung close to trees

A highly anticipated, high-tech analysis of the upper body of the famous fossil ‘Little Foot’ opens a window into a pivotal period when human ancestors diverged from monkeys, new USC research finds.

Little Foot’s shoulder assembly proved key to interpreting an early branch of the human evolutionary tree. Scientists at USC’s Keck School of Medicine focused on what’s called the chest girdle, which includes collarbones, shoulder blades, and joints.

While other parts of Little Foot, particularly the legs, exhibit human-like traits for walking upright, the shoulder components are distinctly ape-like, making the arms surprisingly well-suited for hanging from branches or moving trees up and down instead of using a projectile. or dangling on the torso like people.

The Little Foot fossil provides the best evidence yet of how human ancestors used their arms more than 3 million years ago, said Kristian J. Carlson, lead author of the study and associate professor of clinical integrative anatomical sciences at the Keck School of Medicine.

“Little Foot is the Rosetta stone for early human ancestors,” he said. “If we compare the shoulder assembly with living humans and monkeys, it shows that Little Foot’s shoulder was probably a good model of the shoulder of the common ancestor of humans and other African apes such as chimpanzees and gorillas.”

The ape-like features are likely to cause significant intrigue, as scientific teams around the world have examined different parts of the skeleton to find clues to its human origins. The USC-led study, which also involved researchers from the University of Wisconsin, the University of Liverpool and the University of Witwatersrand in South Africa, among others, was published today in the Journal of Human Evolution.

The magazine dedicated a special issue to Little Foot analyzes of a global research group, which looked at other parts of the creature’s skeleton. The process is somewhat like the story of blind men and the elephant, each examining a part in coordination with others to explain the entirety of something that is not fully understood.

The Little Foot fossil is a rare specimen as it is an almost complete skeleton of an Australopithecus individual much older than most other human ancestors. The creature, probably an old female, was about four feet long and had long legs suitable for bipedal movements when it lived some 3.67 million years ago. Called “Little Foot” because the first bones found were a pair of small foot bones, the remains were discovered in a cave in South Africa in the 1990s. Researchers have spent years excavating it from its rock envelope and subjecting it to high-tech analysis.

While not as widely known as the Lucy Skeleton, another Australopithecus individual excavated in East Africa in the 1970s, Carlson said Little Foot is older and more complete.

The USC-led research team focused on the shoulder assemblies because Little Foot is the oldest and most intact example of this anatomy ever found. Those bones provide telltale clues about how an animal moves. In human evolution, he said, these parts had to change shape before our ancestors could live without trees, walk across the open savannah, and use their arms for functions other than bearing the individual’s weight.

The scientists compared the creature’s shoulder parts to apes, hominins and humans. Little Foot was a creature adapted to live in trees, as the chest girdle suggests a creature that climbed trees, hung under branches, and used its hands above its head to support its weight.

For example, the scapula or scapula has a large, high rim to attach heavy muscles to, similar to gorillas and chimpanzees. The shoulder joint, where the humerus joins, sits at an oblique angle, useful for stabilizing the body and reducing tensile loads on shoulder straps when a monkey hangs under branches. The shoulder also has a solid, ape-like reinforcing structure called the ventral rod. And the collarbone has a distinctive S-shaped curve common in monkeys.

Those conclusions mean that the shoulder structural similarities between humans and African monkeys are much more recent and last much longer than has been suggested, Carlson said.

“We see irrefutable evidence in Little Foot that our ancestors’ arm was still used 3.67 million years ago to support substantial weight during tree movements to climb trees or hang under branches,” he said. “In fact, based on comparisons to living humans and monkeys, we propose that Little Foot’s shoulder morphology and function is a good model for that of the common ancestor of humans and chimpanzees 7 million to 8 million years ago.”

The scientists were able to create remarkably clear images of the fossils. That’s because the bones, painstakingly excavated for years, are in good condition and uniquely complete. The scientists examined them using micro-CT scans, which can detect tiny features on an object’s surface, look deep into a bone, measure an object’s density, and generate a 3D model without affecting the fossil. to damage.