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

A long-awaited, high-tech analysis of the upper body of the famous “Little Foot” fossil opens a window to an essential period in which human ancestors moved away from monkeys, new USC research shows.

The Little Foot shoulder assembly proved essential for the interpretation of an early branch of the human evolutionary tree. Scientists at the Keck School of Medicine at USC have focused on the so-called pectoral girdle, which includes the collarbones, shoulder blades and joints.

Although other parts of the Small Leg, especially its legs, have human-like features for upright walking, the shoulder components are clearly similar, supporting the arms surprisingly suitable for hanging branches or shining up and down trees, more rather than throwing a projectile or hanging on the torso like humans.

The Little Foot fossil provides the best evidence 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 integrative clinical anatomical science at Keck School of Medicine.

“The little foot is the Rosetta Stone for early human ancestors,” he said. “When we compare the whole shoulder 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 monkeys, such as chimpanzees and gorillas.”

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

The journal dedicated a special issue to Little Foot analyzes from a global research group that looked at other parts of the creature’s skeleton. The process is somewhat similar to the story of the blind men and the elephant, each examining one part in coordination with the other to explain the whole thing that is not fully understood.

The Little Foot Fossil is a rare specimen because it is an almost complete skeleton of an Australopithecus individual that is much older than most other human ancestors. The creature, probably an old woman, was about 4 feet tall, with long legs suitable for bipedal movement when she lived 3.67 million years ago. Called the “Little Foot,” because the first recovered bones consisted of a few small bones in the foot, the remains were discovered in a cave in South Africa in the 1990s. Researchers have spent years excavating it from its rock cover and subjecting it to high-tech analysis.

Although not as well known as the Lucy skeleton, another Australopithecus individual dug up in East Africa in the 1970s, Carlson said Little Foot is older and fuller.

The USC-led research team focused on shoulder assemblies, as the Small Foot provides the oldest and most intact example of this anatomy ever found. These bones provide revealing clues about how an animal moves. In human evolution, he said, these parts had to change shape before our ancestors could live tree-free life, walk in the open savannah, and use their arms for functions other than supporting the weight of the individual.

Scientists have compared the creature’s shoulder parts with monkeys, hominins and humans. The little leg was a creature adapted to life in trees, because the pectoral girdle suggests a creature that climbs trees, hangs under branches, and uses its hands above its head to support its weight.

For example, the scapula or scapula has a large, high ridge to attach heavy muscles resembling gorillas and chimpanzees. The shoulder joint, where the humerus connects, is at an oblique angle, useful for stabilizing the body and reducing traction loads on the shoulder ligaments when a monkey hangs under branches. The shoulder also has a sturdy, reinforcing structure, the ventral bar. And the clavicle has a distinctive S-shaped curve commonly found in monkeys.

These findings mean that the structural similarities between African humans and apes are much more recent and have persisted much longer than proposed, Carlson said.

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

Scientists have managed to obtain remarkably clear images of the fossils. This is because the bones, carefully excavated for many years, are in good condition and completely complete. Scientists examined them using micro-CT scans, which can detect tiny features on the surface of an object, look deep inside a bone, measure the density of an object and generate a 3D model without affecting the fossil.