I have probably heard the answer to this before but just don't remember things well enough, and don't know how quite to search for it. I understand (if I've understood correctly) that there are several aspects to the OT ceremonial laws: for instance, to give a sense of total distinction from other nations, of being set apart to God in a way that touched every area of life; as well as to point forward to this set apartness being in Christ (among other things). Would one of the reasons for the OT dietary laws with the 'unclean' creatures be a reminder that Adam had involved all creation in a curse; and that the creatures themselves were in need of a sort of 'redemption'? So also the firstlings of all their animals had to be redeemed in some way -- not just as a general pointer forward to Christ (certainly that), but also specifically to emphasise that even the animals needed a second Adam? And so now, though the whole creation is still groaning in travail until the sons of God are brought to light, the creatures are already clean and 'purified' (the ESV has Christ making a 'declaration' of this at Mark 7:19 which the KJV does not?); and 'touch not, taste not, handle not' does not apply -- because Christ has already (as the fulfillment of Psalm 8) redeemed the creation?