One interesting and puzzling question is when and how humans developed the ability to make ocean crossings. Although much of the Indonesian archipelago turns into a peninsula during the glacial peaks (Sundaland), it’s never possible to walk to Sahul (Australia/New Guinea). I don’t think it was ever quite possible to walk to the main Philippine islands, either. Palawan had a land connection with Borneo at times of extreme glaciation, but the last time that happened was something like four hundred thousand years ago, well before modern humans.
Homo erectus had a limited ability to cross these water barriers, as far as we can tell. They seem to have managed to get to Flores (judging from old tools and of course hobbits) , which meant that they had to make two short sea crossings, Bali to Lombok and then Lombok to Flores. Apparently some kind of tool-maker reached Timor as well, and some other islands of Wallacea. But none managed to get all the way to Sahul – if they had, they would have spread widely and left plenty of very old stone tools, which have not been found.
Modern humans were better at crossing water barriers: they made it to Sahul and the Philippines. The two colonizing populations were evidently related – you can see this in some of the remaining Philippine Negritos, like the Mamanwa, that also have that characteristic Denisovan admixture. These people went on to colonize the Solomon Islands about 40,000 years ago, but didn’t go further: after the Solomons you have to cross 200 miles of open ocean to go further, and that didn’t happen until the Polynesians arrived, relatively recently.
This is hard to square with the fact modern humans apparently couldn’t manage to settle Cyprus and some other Mediterranean islands ( judging by the late survival of their odd fauna, such as 3-foot elephants) until just a few thousand years ago, but that’s what it looks like.