Final answer:
Hominins were skilled at extracting marrow due to their bipedalism, which freed their hands to use tools, and a dietary shift towards increased meat consumption that included breaking bones for marrow.
Step-by-step explanation:
Hominins became adept at obtaining bone marrow from the long bones of large animals primarily due to the development and use of tools. Bipedalism, characteristic of hominins, was a significant anatomical adaptation that allowed the freeing of hands for carrying and using tools.
This evolution meant that hominins could use stones and other objects to break open bones to access the nutrient-rich marrow. Additionally, their dietary habits shifted towards an increased meat consumption, as seen in Homo erectus, which may indicate a greater reliance on hunting and marrow extraction as a food source. Moreover, fossils from various Homo species have shown evidence of cutmarks made by tools on bones, suggesting the skilled processing of carcasses for meat and marrow.