Final answer:
The hominin that matches the description of being shorter, heavier, and more muscular with larger brains than modern humans is Homo erectus. They had distinctive features like a sagittal keel, large jaws, and a sloped forehead, which differed from Homo sapiens. Homo erectus was also known for advanced tool-making and efficient bipedalism.
Step-by-step explanation:
The hominin species that was shorter, heavier, and more muscular than modern humans, and had a larger brain within a differently shaped skull, is Homo erectus. Homo erectus walked fully upright with a bipedal gait, with humanlike feet featuring arches and ankles. The skulls of Homo erectus did not resemble modern humans entirely—they featured a sagittal keel, large jaws and teeth, a prominent brow ridge, and a sloped forehead, indicative of their robust anatomy which differed from Homo sapiens. While Homo habilis, often called "handy man", is known for using stone tools and having a larger brain capacity than prior species like the australopithecines, it is Homo erectus that stands out as being muscular and robust with a brain size close to that of modern humans. Other distinguishing features of Homo erectus include a low forehead and broader pelvis, which aided in their efficient bipedal locomotion. Relative to earlier hominins like australopithecines, Homo erectus had a more advanced tool-making capability and a larger brain approximately three-fourths the size of modern humans. This species marks an important evolution in the genus Homo for its more efficient bipedalism and increased brain capacity.