A typical assertion of many forms of Theistic Evolution and "Progressive Creationism" is that Man before the Fall was already mortal. But is that substantiated by the Scriptures?
It must be noted that biologically, there is no reason why Man could not be immune from the problems of old age. Barring external threats to life (i.e. decapitation, major blood loss through severing arteries, severe burning by fire etc.), it is not clear why Man cannot constantly regenerate his cells and live forever, if the genetic makeup especially concerning telomerase is different. Bacteria after all could last for an extremely long time if not killed. An accelerated healing factor like that seen in Wolverine (X-men) would function the same way of course, and while the efficiency of Wolverine's healing factor is probably a stretch too far, I do not see why something analogous to it is not somehow plausible given the right genes. Creatures like the lizard for example regenerate its tail.
Scripturally, if one were to accept Genesis 1-11 as real history (and there is no reason from the text why Genesis 1-11 is not history while 12-50 is history), then one must reckon with the saying in Genesis 2:17 that Adam would surely die if he ate of the Tree of the Knowledge of Good and Evil. Unless one wants to say that God was lying, then death must surely be the consequence of sin, which Romans 5:12 states. So what kind of death is God speaking of in Genesis 2:17? Surely some would say "spiritual death," and that is true. But is it merely spiritual death? It is to be noted that in the OT, there is no real dichotomy between the physical and the spiritual. Spiritual death is correlated with physical death or the descent into Sheol (e.g. Ps. 16:10, 18:5 etc). So if God's command threatens death, it cannot be just limited to spiritual death, as if the threat is purely spiritual without a physical or material component.
It is thus better to read Genesis 2:17 as threatening both physical and spiritual death, as congruent with the way the OT frames life and death. That Adam and Eve were not immediately killed shows the mercy of God and a partial covering for sin, typified in the shedding of blood in the provision of clothes of animal skins (Gen. 3:21, which in Hebrew is merely "skin" pointing towards substitutionary death of Christ). Yet just because the sentence is not immediately carried out does not mean that it is false. Adam and Even, and their descendants after them, still physically die. Presumably, the faithful among them continued to offer up sacrifices (like Cain and Abel and the line of Seth did), yet they still die.
Thus, if physical death is indeed part of God's punishment, it makes no sense for there to be human death before the Fall. After all, what kind of threat is it when death was already part of reality before the Fall? Especially if Theistic Evolution was held to, the threat of spiritual death wouldn't make much sense either, since hominids of any sort (Homo neanderthalensis, Homo erectus etc) have been dying for ages, and which stage of human evolution did the hominids began to have a soul which would go to hell (or do hominids by default go to heaven until they began to have a "sin nature" and which stage of evolution did that come into being)?
Yes, Adam and Eve in the garden are not glorified creatures. Yet there is no reason why they cannot have conditional immortality (before the Fall) of the sort that they could potentially not die if they weren't killed by external forces, since biologically such is possible. God's threat of death therefore would make better sense if humans did indeed have conditional immortality before the Fall, a threat that would make no sense in Theistic Evolution, and to a lesser degree, in Progressive Creationism.