Where did death come from?
Physical death comes from God.
1 Corinthians 15:42-44, 53-54
So also is the resurrection of the dead. It is sown a perishable body, it is raised an imperishable body; it is sown in dishonor, it is raised in glory; it is sown in weakness, it is raised in power; it is sown a natural body, it is raised a spiritual body. If there is a natural body, there is also a spiritual body....... 53For this perishable must put on the imperishable, and this mortal must put on immortality. But when this perishable will have put on the imperishable, and this mortal will have put on immortality.....
Scripturally speaking, humans were made mortal. Death came from God. It is a normal, effective function of divine design. When a plant - any plant - matures and produces flowers the pollination of that plant causes the petals of the flower to
die and from there the fruit develops. When the fruit matures it contains a seed and the fruit dies when it is eaten or, if not eaten, then the fruit drops to the ground, rots and
dies, and the new seed is planted. That seed
dies in order to become a new plant. If this condition did not exist then cows and deer, and crows, etc. would live forever. All the animals would be
immortal, not just humans. Furthermore, if an animal happened to fall off a cliff it would necessarily survive the splat at the end of the fall. We'd have
flat immortal goats and squirrels

. Supposing there was no physical death prior to Genesis 3:6-7 would necessarily mean an entirely different set of physics for the entire planet and all its flora and fauna.
If the op's inquiry is intended to ask, "
Where did physical human death come from?" then the answer is "
God. Physical death exists as a function of God's design."
If the op intends to ask, "
Where did 'spiritual' death come from?" then a categorical error has occurred because evolutionary science is not concerned with explaining anything spiritual. It would be like asking a geologist to scientifically explain why poems exist. Rocks are the domain of geology. Poetic literature is not the domain of geology.
Lastly, the words, "
spiritual death," are nowhere to be found in the Bible. The
concept of "
spiritual" death is a man-made invention, one that was devised to discriminate the distinction between physical death and the nature of death Adam and Eve, and all other humans, experience when disobeying God. The concept is laden with problems and is inconsistent with whole scripture in various ways. For example, Adam, according to 1 Corinthians 15, is called a "
living being," and no reference is made to his act of disobedience. Jesus is called a "
life-giving spirit," and no reference is made to his sinlessness. This is the basis for the juxtaposition between the "
natural" and the "
spiritual." In fact, the context provided by the larger text is corruptibility, not corruptedness. Adam was not made already corrupted. He was made corruptible. Adam was not made dead. He was made mortal. Genesis explicitly states each plant and animal produced its own kind and, for plants, that necessarily entails the plant and its various components dying.
The death that occurred as a consequence of Adam's disobedience is best called "
transgressional death," of "
sinful death," because scripture states the living, breathing, blood-pumping human is dead in transgression, or dead in sin. The person who has disobeyed God is not physically dead, but s/he is, nonetheless, dead in sin. Being dead in sin is a function of God's design for creation (
if a person disobeys God,
then they die transgressionally). Whether ever disobeying God, or not, the mortal creature is going to die. The only thing that can prevent physical death might be the tree of life/Jesus...... and
if the tree of life entailed a resurrection, then physical death was always going to be experienced. Either way, none of that is the domain of scientific evolutionary theory.