Final answer:
Homo erectus skull shape is characterized by sagittal keels and thick skull bones, with its distinctive shape more closely correlated with the increased brain size, a product of encephalization.
Step-by-step explanation:
Homo erectus crania display a highly distinctive shape, partly because of sagittal keels and thick skull bones, but probably more correlated with increased brain size. The distinctive features of Homo erectus, such as projecting nasal spines, pronounced brow ridges, and a protruding occipital bun, are thought to be adaptations to its environment and lifestyle. The progressive increase in brain size over evolutionary time, a process known as encephalization, corresponds with an increase in behavioral, cognitive, and cultural complexity, making it a significant factor in the distinctiveness of Homo erectus crania.