The main Avodah of the early generations was through yearning and longing for holiness and Hashem. Avraham merited completeness in this and from then and on some form of the Torah started being revealed. This was continued and furthered by the other Avos, Shivatim and their decedents until the generation of the desert who with Moshe managed to bring down the actual Torah. (LH YD Hil Dam 1)