Story, first published: Thrilling Wonder Stories, Dez. 1938
A scientist invents a mechanical time brain (time is non-existent but a time-sense exists in the human brain), and he and two others go on a hunting expedition to the interglacial period seventy thousand years back. The scene then shifts to the fifty-sixth century, when time-travel is an everyday occurrence. Aliens are using time travel to steal and store their loot in the past, in the same era where our twentieth-century time travelers have located. After the usual battles with wild animals and an encounter with a friendly old Neanderthal, the time travelers from the past and those from the future put a stop to the Centaurians (their appearance in the past makes them responsible for the myth of the devil), but the story ends with a puzzle - why were they stealing? Perhaps Simak intended a sequel, but never got around to writing it.
Ewald, Robert J.: When the Fires Burn High and the Wind is from the North, p.31