Final answer:
The main obstacle related to hominins' increasing brain size was the limited ability for the birth canal to expand due to pelvic changes needed for bipedal locomotion. This obstetric dilemma occurred alongside evolving brain sizes that began with Homo habilis and continued with Homo erectus, contributing to behavioral and cognitive developments.
Step-by-step explanation:
The key obstacle that hominins' increase in brain size had to overcome was b. larger skulls demanding larger birth canals, even though the requirements of upright bipedalism impose limits on the expansion of the human pelvic opening. As hominins evolved to walk on two legs, the structure of the pelvis changed to support this locomotion. This change limited the degree to which the birth canal could expand, presenting a challenge as brain and skull size increased in hominins like Homo erectus and eventually Homo sapiens. Furthermore, evidence from fossils shows that there was a progressive increase in brain size over time among the genus Homo, commencing with Homo habilis and more prominently observed in H. erectus. This increase in brain size is believed to be linked to cognitive, behavioral, and cultural complexities, enabling hominins to adapt to new environments and challenges. In addition to bipedalism, tool use, reduced prognathism, and smaller jaws may also classify a species under the genus Homo, as put forth by noted paleoanthropologists.