Complex sentence: The Ndebele tribe originally in the early 18th Century lived in grass huts, and it wasn't until they began using mud-walled houses in the mid- 18th Century did these symbols begin to be created on their houses and walls.
Split into simpler sentences: The Ndebele tribe originally in the early 18th century lived in grass huts. They began using mud-walled houses in the mid- 18th century when these symbols begin to be created on their houses and walls.