Complex sentence: Little is known about how Hammer became interested in photography or who taught her the art but her first works date to the 1860s and it is assumed she continued to practice until she moved back to Zealand in the 1880s.
Split into simpler sentences: Little is known about how Hammer became interested in photography or who taught her the art but her first works date to the 1860s. It is assumed she continued to practice until she moved back to Zealand in the 1880s.