i heard from my mashgiach that the tur says betzibur. when you finish it betzibur you will get arichus yamim since its betzibur and hashem doesnt kill a tzibur… or something upon those lines, and now when they came down to mitzraim “Shivim nefesh” they became a tzibbur …
another peshat he gave why davka in shemos was to tell you even if you didnt do bereishis you should start from shemos and carry on (and be mashlim bereishis another time)