Final answer:
The earliest change in primate morphology that differentiated human ancestors from other primates was the development of bipedalism in the genus Australopithecus around 3.9 million years ago.
Step-by-step explanation:
Earliest Changes in Primate Morphology
The first significant morphological change that differentiated our ancestors from other primates occurred in the genus Australopithecus, which evolved about 3.9 million years ago. These early human ancestors were already walking on two feet (bipedalism), an adaptation that paved the way for further evolutionary changes and set them apart from other primate ancestors. The transition to bipedalism precedes other notable changes such as the increase in brain size, development of tool-making technology, and the emergence of the genus Homo, which is marked by fully bipedal creatures with larger brains but still relatively small compared to modern humans.
Understanding our evolutionary past involves looking at our closest relatives within the primate order. Our species falls into the hominid family, which includes not only Homo sapiens but also other great apes such as chimpanzees, gorillas, and orangutans. However, it is the transition to walking on two legs by the Australopithecus that represents the earliest change in our direct lineage defining the hominin branch of the evolutionary tree.