In his purport to the Srimad Bhagavatam's Third Canto, chapter 31,text 44, Srila Prabhupada partly explains the mysterious nature of karma:
"It is clear that a particular body is given to the living entity for a particular type of activity. This process is going on perpetually, from a time which is impossible to trace out. Vaisnava poets say, therefore, anadi arama-phale, which means that these actions and reactions of one's activity cannot be traced, for they may even continue from the last millennium of Brahma's birth to the next millennium. We have seen the example in the life of Narada Muni. In one millennium he was the son of a maidservant, and in the next millennium he became a great sage."