The Greek formation describes feet on opposite sides. Through Latin, Middle English antipodes named people on the other side of the globe. The geographical sense now concerns opposite places.
Slash-separated forms are alternatives, not successive chronological stages.
Sources
- Merriam-Webster: antipode / antipodes
The headword, Word History and Did you know? explain the plural, Greek components and earlier reference to people.
- Online Etymology Dictionary: antarctic
Also built on *anti-*: antarctic, from Greek antarktikos "opposite the north" (anti- "opposite" + arktikos)