And some call those people "primitives". Sound like they were all hip enough to consider the future. Of course don't get the idea that I think there is any evidence supporting the scribblings of Mr. Dick.
I said nothing about 'primitives'. However, the composite nature of the Pentateuch has been suspected since the early 18th C (see Henning Witter). It had to wait till 1780 before Johann Gottfried Eichhorn was brave enough to make his findings public.
I believe I was wrong about saying the details about the 'clean animals' were added. They belonged to an earlier account which related that sacrifices had begun with Cain and Abel and continued with Noah onwards (the J or Jahvist narrative). The later account, interpolated in the story, is known as the Priestly narrative (P) and in this writer's perspective there would be no sacrifices till the consecration of Aaron, so only two animals of each kind were needed (see the last chapter of Exodus).
So far from these being entirely my own speculations, they relate to a tradition of scholarship going back a long way. If you're interested, you'll find an excellent guide in "Who wrote the Bible?" by Richard Elliott Friedman (and American -unfortunately not Canadian-, forsooth!). If you're feeling very intellectually curious, you could try the work of Julius Wellhausen. I won't be holding my breath on that one.
P.S. These scholars argue that there were five hands at work in the Pentateuch - four original sources and one redactor.