Arm and thigh bone strength in seven fossils separates Australopithecus from early Homo

Arms that are strong relative to legs point to time spent moving in trees; legs that are strong relative to arms point to walking upright on the ground ^[3]. Both halves of each comparison came from the same skeleton, so that individual's body size sits in the numerator and the denominator alike, and no body-mass estimate enters the ratio ^[17]. Fossil biomechanics usually has to guess at mass.

"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 Kristian J. Carlson, a professor of clinical medical education at the Keck School of Medicine and the study's lead author ^[6][7].

Seven individuals cover 2.2 million years of the record, an average of one per roughly 310,000 years ^[15]. The phys.org account does not report how many of the seven belong to each group ^[18].

Then the timing. Australopithecus as a genus is dated to roughly two to four million years ago ^[9], and the early Homo individuals in this sample sit at the young end of that span, an overlap of about 300,000 years ^[16]. That figure compares a genus range against specimen ages, so it is coarse ^[16]. Carlson described relative limb strength as a "threshold trait" that marks "an important and fundamental shift in behavior between Australopithecus and Homo" ^[10]. What the paper adds to the timing argument is evidence from how individuals loaded their limbs within a lifetime, rather than from anatomy accumulated over generations, and it points to a major shift in movement by roughly two million years ago ^[12].

The brain-size step is the softest one. Brain size began rising around the same period, and language, tool use and changes in how ancestors found food have all been proposed as causes ^[13]. Carlson and colleagues posit that walking greater distances on the ground contributed both to the limb strength change and to the brain enlargement, and the account presents that as speculation ^[14].