Submitted by Mythical_Atlacatl t3_11huevk in askscience
atomfullerene t1_jawxqjk wrote
There's no evidence of this, but it's not just H. erectus that follows this pattern. There's a very clear transition of fauna between SE Asia and Australia that occurs in this region. The most famous division here is the Wallace line, which divides areas that were connected to the Asian mainland during the ice age from islands that remained separate, but early hominids actually dispersed a bit further, about to the Weber line. This is similar to elephants and some other large mammals, and implies they managed to cross short stretches of open water to reach these islands. But they apparently couldn't cross the larger gap between Timor and Australia.
It's worth noting that this dispersal happened during glacial periods when sea levels were lower and the geography of the area was quite different. You could walk all the way out to around Bali because sea levels were lower. And H. erectus and kin were all over south Asia and had been for more than a million years.
Really, it's not unusual to see that sort of spread. There's a fair amount of overlap between South Asian and African large mammals. When the Sahara is a grassland/savanna (which happens sometimes), large mammals with adaptable habits can disperse pretty easily between Africa and Asia, which is what you are seeing with hominins. But crossing the open ocean is much more difficult, and it seems that only modern humans managed it.
Here's a paper about hominins in SE Asia, which also has a map of sea levels and one of known hominin fossil sites
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