A new study published in the journal Science Advances sheds light on how early human ancestors moved and lived. Researchers, led by Kristian J. Carlson, a professor at the Keck School of Medicine of the University of Southern California, examined fossil data from seven human ancestors that lived between 1.5 and 3.7 million years ago. By analyzing the strength of arm and thigh bones, which adapt to the physical demands of an individual's lifestyle, the team aimed to understand how these early humans used their bodies and navigated their environments. The study found that Australopithecus, an early human ancestor that lived about 2 to 4 million years ago, had strong arms, suggesting they spent significant time climbing trees. However, their leg bones showed a more human-like strength pattern, indicating they also walked upright on the ground. In contrast, early Homo individuals, who lived roughly 1.8 to 2.3 million years ago, had stronger leg bones and weaker arm bones, a pattern more similar to modern humans. These findings suggest a major shift occurred as Australopithecus evolved into Homo, involving changes in movement and behavior. The researchers describe the relative strength of limbs as a "threshold trait," meaning it represents a significant change in how these early humans lived and interacted with their environment. To estimate bone strength, the researchers used computed tomography (CT) scans, which use X-rays to create detailed images of bones. These scans allowed them to measure the thickness of bone shafts and calculate how resistant the bones would have been to bending and twisting. By comparing the strength of upper arm, thigh, and shin bones within each individual, they could infer how much time these ancestors spent on the ground versus in trees. Australopithecus had strong arms relative to their thighs, similar to modern apes. However, the strength of their thigh and shin bones showed a human-like pattern, indicating they were using their lower limbs for upright walking while still relying on their arms for climbing. Early Homo, on the other hand, had stronger thighs compared to their arms, a pattern more like that of modern humans. The study also touches on the debate over how much time Australopithecus spent on the ground versus in trees. While some scientists have suggested they were mostly ground-dwelling, this research indicates they still spent considerable time in trees. The findings also suggest a connection between walking on the ground and the increase in brain size seen in early humans. Researchers propose that the shift toward walking longer distances on the ground may have influenced both the change in limb strength and the growth of larger brains, possibly through increased physical activity or changes in foraging behavior.