True, these are fore-runners of the chariot, but all are aware of the rolling action, sun, moon, animals on grass or water. A sled or a travois is also a kind of 'ratham'. When time came, they attached wheels to the 'ratham'.
'Ratha': chariot, pleasure, delight, limb, body, wagon, two-wheeled war-chariot, cart, love, car, warrior, goer, member, champion, rattan plant (Calamus Rotang - Bot.), hero, affection, part, Sandan tree (Desmodium oojeinense- Bot.), joy, any vehicle or equipage or carriage.
Chariot: ratha. garta, dhvajin, patākin, pattrin, mūlavibhuja, yāma, vāhin, shatānga, syandana, cakravat, yugya, vāhana, vāhyaka, pattra
Now 'pattrin', 'pattra' means a leaf, board, and 'yugya' means yoked. 'Mūlavibhuja' means something like a travois (vibhuja - two arms), two poles meeting at one point. So, 'ratha' (chariot) was once basically a board yoked to a animal.
Word meanings from
http://spokensanskrit.de/