The little Floresians lived on the island until at least 13,000 years ago, and possibly to historic times. But they were not a pygmy form of modern humans. They were a downsized version of Homo erectus, the eastern cousin of the Neanderthals of Europe, who disappeared 33,000 years ago. Their discovery means that archaic humans, who left Africa 1.5 million years earlier than modern people, survived far longer into recent times than was previously supposed.
Q : I think the suject of "was previously supposed", which is omitted, is "a time archaic humans survived". It is not a specific part of a sentence. It could be found through the context. What do you think?