Complex sentence: Lucy was only tall, weighed and looked somewhat like a Common Chimpanzee, but although the creature had a small brain, the pelvis and leg bones were almost identical in function to those of modern humans, showing with certainty that these hominids had walked erect.
Split into simpler sentences: Lucy was only tall, weighed and looked somewhat like a Common Chimpanzee. Although the creature had a small brain, the pelvis and leg bones were almost identical in function to those of modern humans, showing with certainty that these hominids had walked erect.