Albert, you make here an interesting point.
As argued in my previous post, I think we are unambiguously taught by Jesus Christ that from the beginning humans are tempted by Satan to deny worship to God. Adam and Eve succumbed to such a temptation according to Genesis 3.
One can then conclude that the consequence of this disobedience was “to revert to one’s evolved animal instincts”, in line with what you claim.
This has interesting implications:
The original state of humans (as beings endowed with conscience, free will, and sense for accountability) was a state of “Original Blessing” (your wording), that is, a state of Grace and fullness of Virtue, where humans were called to love a Creator who is “All Good”.
The state of “evolved animal instincts” (with killing, illness, death, extinction, etc.) existed before the state of “Original Blessing” and also during this state outside humanity, and was foreseen by God to the aim of facilitating Redemption in case humans sinned.
If humans had not sinned, they would have remained exempted of evils like death and illness.
Thus, the creation was subjected to frustration, and is groaning right up to the present time, not by its own choice, but by God’s will: He wanted to facilitate that humans freely accept to become children of God again after a possible fall. However in the end of history the creation itself will be liberated from its bondage to decay and brought into the freedom and glory of the children of God. [See Romans 8:19-22].
To a certain extent I share the claim by LT_15:
Nonetheless I would like to precise that death was not introduced into the animal kingdom by Adam’s sin backwards in time: It was introduced by God since the beginning of creation as means to move humans to freely repent in case they sinned.
Death before the fall does not make God a liar but is rather a sign of His mercy: He creates a world where sinners can live in and are stimulated to freely atone.
Finally, generations may have passed before the arrival of sin. Accordingly “Adam’s sin” means the sin of the first human sinner and not necessarily the sin of the first human.