Focusing on the northern origin of Ryukyu.
 Gregory Smits  "Maritime Ryukyu, 1050-1650", 
University of Hawaiʿi Press, c2019 

"Rethinking Early Ryukyuan History" - Japan Focus
 Gregory Smits
 "My recent book, Maritime Ryukyu, 1050-1650, .....
  Maritime Ryukyu situates the Ryukyu islands within nautical networks extending northward through the Tokara islands, the Satsunan Islands 薩南諸島, coastal Kyushu, the islands of Iki 壱岐 and Tsushima 対馬, and the southern part of the Korean Peninsula.
  Ryukyuan culture and people did not spring from the soil of Okinawa. They came from northern locations, and Maritime Ryukyu highlights the deep Japonic roots of the Ryukyu islands. It also advances other new arguments about early Ryukyuan history.