Complex sentence: It is not known what language these early Scandinavians spoke, but towards the end of the 3rd millennium BC, they were overrun by new tribes who many scholars think spoke Proto-Indo-European, the Battle-Axe culture.
Split into simpler sentences: It is not known what language these early Scandinavians spoke. Towards the end of the 3rd millennium BC, they were overrun by new tribes who many scholars think spoke Proto-Indo-European, the Battle-Axe culture.