Could be the remnants of ancient homesteads.
A circle encloses the maximum area within a given length of wall,A compound inside the outer wall would be divided into animal pens,storage,human shelter,garden?,privy,and whatever else might be needed.Bigger groups of people would have the time (and need) to wall (make secure) bigger areas.
You don’t need space aliens to explain every bit of ancient history.
Wow! Three other posters beat me to the same conclusion;I think we have a consensus.