Final answer:
Hominins likely crossed the Wallace Line in Indonesia to migrate from Asia to Australia, using rafts or watercraft over open water during the ice ages when land bridges were exposed due to lower sea levels.
Step-by-step explanation:
The geographical barrier in Indonesia that hominins would have had to cross to migrate from Asia to Australia is the Wallace Line. During the ice ages, particularly around eighteen thousand years ago when the last glaciation period was at its peak, sea levels were much lower than they are today. This exposed land bridges between the islands of Southeast Asia and possibly facilitated overland migrations.
However, Wallace and Weber hypothesized that a land bridge never connected the Asian side with the Australian side. Therefore, it is believed that early humans reached Australia not by walking on exposed land or ice sheets but rather by using some form of watercraft. They likely took rafts over open water to accomplish their migration, as marked by archaeological evidence of human settlement in areas like Papua New Guinea and Australia, dated to be at least forty-five thousand years old.
Ultimately, the ability of Homo sapiens to construct and navigate seagoing canoes and rafts evidence advanced problem-solving skills and ingenuity long before the development of civilization as we define it today.