Final answer:
Body proportions evolved from the tree-climbing Australopithecus afarensis to the efficiently bipedal Homo erectus, with changes in limb length, pelvis shape, and brain size reflecting a transition to ground-based bipedalism.
Step-by-step explanation:
Regarding the question of body proportions, allowing for the height difference between Australopithecus afarensis and Homo erectus, their body proportions evolved significantly over time. Australopithecus afarensis, which lived from 3.9 to 2.9 million years ago, had a mix of ape-like characteristics such as long arms and curved fingers and toes suited for climbing trees and an adapted pelvis signaling the transition to bipedal locomotion. On the other hand, by about 2.8 million years ago, early species of Homo, such as Homo erectus, had developed body proportions that supported nearly as efficient bipedal locomotion as modern humans, with their longer legs, arched feet, and a broader pelvis.
H. erectus also displayed significant brain growth, with brain sizes between 1,000 and 1,200 cubic centimeters. Despite these advancements, H. erectus still had anatomical differences from modern humans, including more prominent jaws and teeth and a sloping forehead. Through these transitional phases, the body proportions of hominins developed from those advantageous for both tree-dwelling and bipedal activities to ones favoring entirely ground-based bipedalism.