No doubt, something actually happened at the origins of mankind.
No doubt, that something included rebellion against God.
No doubt, that rebellion had lasting consequences for all subsequent generations of humans.
As I understand it, Catholic doctrine (most of which is shared by Lutherans such as I, by the way) says that the first humans had preternatural gifts beyond their natural biological endowments, but that those gifts were lost due to their rebellion against God.
Lacking those preternatural gifts, natural biological processes such as sickness and death affect humans as they do the other animals.
Eternal life of the sort Jesus Christ (the “first fruits”) has would have been (without sin) an immediate culmination of each earthly human life, with the transition to that life happening smoothly, uninterrupted by death. Jesus took upon Himself the consequences of our sin, made that pioneering transition via agonizing suffering and death, and thereby created a path for us to follow Him into eternal life.
I got the above-stated understanding largely from
inters.org/death