Up in the North West of Australia there are mysterious rock paintings. The Gwion Gwion (or Bradshaw) figures are unique, painted by a society that has vanished.
Lost civilisations have famously sparked incredible adventure stories.
The obvious answer is that the people were kidnapped by aliens.
The aliens were slavers. They captured the people of the area and took them away to a distant planet to serve a particular function (I haven't decided what yet). Humans proved competent but not great slaves so the aliens never went back for more. After all, there was a sufficient group for the humans to breed and maintain the slave population.
Millenia pass. The alien society evolves. Now, slavery is considered immoral (even slavery of non-alien beings). But what do they do with the slaves? It's not like humans are their equals.
So the aliens look up the records. They re-discover Earth. They return the human ex-slaves to Earth.
But these humans have spent millenia away from Earth. What happens when the aliens drop them into the Australian wilderness? Do the humans survive? Do they take over the Earth and enslave the earthling humans?
Improbability Wednesdays always leave me with questions.