One Million Years B.C. (1966) on Netflix

Tumak, a member of the Rock Tribe, is expelled from their cave after running afoul of their leader Akhoba, who also happens to be his father. After several days of wandering, he stumbles upon several female members of the Shell Tribe, a group that lives on the coast. Loana, the daughter of the chief, sees that he is in terrible shape from his ordeal and nurses him back to health. This causes her betrothed, Payto, to become jealous and eventually the two of them get into a major fight and Tumak is expelled as a result. However, Loana decides to join him and follows him back to the caves of his people. While there Loana teaches the Rock people civility and this causes Tumak to become the new leader (Akhoba was severely injured while Tumak was away). This doesn't sit well with Tumak's brother Sakana who begins to plot to have Tumak overthrown.