Final answer:
The increase in cranial capacity of Homo habilis, making it 1/3 larger than that of the australopithecines, is attributed to the evolutionary development of complex cognitive functions and adaptation to environmental changes, marking a shift towards greater encephalization in the hominin lineage.
Step-by-step explanation:
The enlarged cranium of Homo habilis compared to that of the australopithecines is believed to be directly attributed to the evolutionary pressures of developing more complex cognitive functions and adapting to environmental changes. The increase in cranial capacity is associated with encephalization, which reflects the growth of brain size and complexity over time. This growth in brain size enabled early humans like Homo habilis to enhance their behavioral, cognitive, and cultural complexity, leading to advancements such as the use of tools, symbolic language, and the development of culture and technology. Although the brain size of Homo habilis was substantially less than that of modern Homo sapiens, the increase in size from the australopithecines marked a significant shift in the hominin lineage towards greater encephalization.