Final answer:
Australopithecus around 2.5 million years ago displayed bipedalism, a slender build, and a brain size smaller than modern humans. They had longer arms for tree movement, and a pelvic shape that allowed bipedal walking. Their skulls had larger teeth and a prominent jaw compared to modern humans.
Step-by-step explanation:
Around 2.5 million years ago, the hominin species known as Australopithecus exhibited characteristics that were both similar to and distinct from modern humans. Members like Australopithecus africanus had a slender build and a brain size of approximately 500 cubic centimeters, which is less than one-third the size of the modern human brain. Their bipedalism allowed them to walk on two legs, although their long, curved fingers and arms indicate they also moved in the trees. Pelvic bone shape was more similar to that of humans than apes, suggesting a walking pattern akin to humans. The skull was smaller than that of modern humans with a sloped forehead, larger teeth, and a prominent jaw. Another species, Australopithecus afarensis, known from fossils like 'Lucy,' had some features that were distinctly different from modern humans, such as longer arms and more elongated finger and toe bones, yet also shared a similar hip structure supportive of bipedal locomotion. Australopithecus also displayed significant sexual dimorphism with males being considerably larger than females.