In 2026, CT scans showed 3.7-million-year-old “Little Foot” had ape-like arm strength but human-like leg loading, suggesting a mix of climbing and upright walking

Deep in the Sterkfontein Caves of South Africa was a nearly complete 3.7-million-year-old skeleton of one of our ancestors known as Little Foot. Now, a new study in Science Advances led by researchers at USC's Keck School of Medicine suggests this ancient relative had arms like an ape's but legs that already moved like ours. By using CT scans to look inside the fossilized bones of Little Foot, the team saw something that is reshaping part of the human story. Our ancestors, it turns out, were walking upright while still spending time in trees.

A skeleton with a story to tell

Formally known as StW 573, Little Foot's discoverers determined from her teeth and pelvis that she was an older adult female who lived roughly 3.67 million years ago and stood about four feet tall. She was found piece by piece in the Sterkfontein Caves from the 1990s onwards, and the paleontologists spent years carefully excavating her nearly complete skeleton embedded in the rock formations. As almost all parts of her body have been well preserved from the head down to the feet, Little Foot is considered a highly informative Australopithecus specimen. Unlike most early hominid fossils, which are known from a jaw fragment or a few bones, Little Foot is a nearly complete skeleton, allowing scientists to compare her arms and legs side by side, something rarely possible with older finds.

How scientists read bones like diaries

Bone is not fixed once it forms; rather, it is strengthened or weakened based on use throughout one's lifetime, just as muscles are affected by exercise. Arms or legs that bear weight or absorb stress grow relatively stronger over time. Bone strength acts as a fossil record for the activities that the body performed most often. To study those patterns, the research team used CT scans, which take layered X-ray images to reveal a bone's internal structure, not just its outer shape. The study says the team analyzed the relative bone strength in arms, thighs, and shins in seven individuals whose fossils dated back between 1.5 and 3.7 million years.

Ape arms, human legs

The results were unexpected. Little Foot had proportionally stronger arm bones than leg bones, a pattern observed in tree-dwelling apes today. But when researchers examined her thigh and shin bones, they found a pattern much closer to modern humans. "Australopithecus combined an ape-like upper limb strength with a human-like pattern in the legs, suggesting they had a unique movement strategy that has no modern comparison," said lead author Kristian Carlson, Keck School of Medicine of USC. That suggests Little Foot's arms were still used for climbing, while her legs had adapted to supporting her weight upright on the ground.

What early Homo did differently

However, the picture is quite different for early members of our own genus, Homo, which lived about 1.8 million to 2.3 million years ago. Fossils from Dmanisi in Georgia dating back to about 1.8 million years ago show that the leg bone was more robust than the arm bone, a situation closely resembling modern humans. According to researchers from the University of the Witwatersrand involved in the study, the situation reflects a fundamental behavioral change and not just an anatomical change. While Australopithecus roamed in a habitat that included trees as well as ground level, early Homo appears to have relied more heavily on walking.

A threshold in our evolution

This transition, Carlson says, is what the team terms a "threshold trait," a way of classifying the difference between Australopithecus and Homo as a meaningful behavioral split, even though the underlying bone changes themselves built up gradually over generations. "There is no living species that provides a good behavioral comparison for them," he said of Australopithecus, explaining just how unusual its mixed movement style really was.

Walking and the growing brain

Interestingly, this broader transition toward Homo-like, ground-based movement, which researchers date to roughly 1.8 to 2.3 million years ago, well after Little Foot's own time, lines up with another major change in human evolution: the rapid growth of brain size. The researchers are careful not to claim that one directly caused the other. "We are not saying that one change caused the other," Carlson said, as reported in the study. "But the timing is intriguing. We hope the possible relationship between changes in limb use, ranging behavior, and brain size will stimulate further discussion."

Little Foot suggests how evolution does not always progress in neat and linear paths. For quite some time in our evolutionary past, our ancestors have neither been completely tree-dwelling nor completely ground-dwelling. Instead, they have been adapting gradually through minor changes in their bones. So, next time you go for a walk, it might be worth remembering that this simple, everyday act carries millions of years of trial, compromise, and eventual commitment behind it.