Final answer:
Homo erectus was the first hominin with a significantly larger brain size compared to earlier hominins and modern apes, showcasing a notable increase in encephalization within human evolution.
Step-by-step explanation:
The hominin species that was the first to have a brain size significantly larger than earlier hominins and modern apes is Homo erectus. This species evolved from a common ancestor shared with Australopithecus about two million years ago, and was the first hominid to walk upright. It is also notable for developing more advanced stone tool-making capabilities than its predecessors. Homo erectus had a brain size of roughly three-fourths that of modern humans, indicating significant encephalization, the increase of brain size over time in human evolution. Compared to Australopithecus, with its smaller brain size more similar to that of great apes, and Homo habilis, which had a brain size of around 600 to 750 cubic centimeters, Homo erectus marked a considerable leap in brain development within hominins.