Final answer:
The oldest Homo fossils show that early humans had smaller postcanine teeth and larger brains compared to their predecessors. Evolutionary trends indicate a reduction in jaw and teeth size and an increase in brain volume, all linked to dietary changes and advancements in tool use.
Step-by-step explanation:
The oldest fossil evidence for Homo dates back to approximately 2.8 million years ago, and these early humans had relatively smaller postcanine teeth and larger brains compared to their ancestors. Over time, the trend in hominin evolution has shown a decrease in the size of the face, jaws, and front teeth, which is linked to dietary changes and the development of technology such as tool use. The increased consumption of meat and the use of fire for cooking led to softer food, reducing the need for large chewing teeth. This, along with the decreased prognathism evident in species such as H. erectus and H.habilis, allowed for an increase in brain size. Smaller teeth and jaw sizes facilitated an increase in cranial capacity, with H. erectus showing a vast range from about 550 cubic centimeters (cc) to 1250 cc.
Noteworthy is the transition from bipedal australopiths like Australopithecus afarensis, with larger teeth and more prominent jaws, to later Homo species that exhibit smaller dentition and increased brain volume. While the brain size of Homo naledi remained small, it still had small front teeth, indicating that larger brain size was not universal among all Homo species. Nonetheless, the evolutionary trajectory of our genus indicates an overall trend towards more efficient energy use by developing a smaller digestive system to support a larger brain, consistent with the "expensive tissue hypothesis."