I recently read that the Australian Aborigines numbered close to a million during the time of Botany Bay and comprised of ~250 tribes/nations. The Native Americans of North America also boasted impressive numbers of between 5 and 20 million people. Given these numbers, I wonder why their societies did not evolve up from hunter-gatherers to establish civilisation as we know it.
Could it be because of the size of their respective continents? While the Indigenous Australians were isolated, the Native (North) Americans were not. Civilisation sprung up not too far away in Central and South America. [I'm assuming - perhaps naïvely - that the Mayans, Toltecs, Aztecs, and Incas were decidedly distinct from the Native American tribes with limited contact between the two. Even if they are related, it still begs the question as to why only some nations decided to establish cities etc.]
Thank you for your time!