Cain and Abel were the first people born into the world, descendants of Adam and Eve. But Cain killed Abel. – And the Bible tells how Cain afterward found a wife for himself from a foreign land. How is this explained? The question is often posed in a deliberately misleading way, so that the Word of God would appear faulty. Mockingly, it is asked: “Where did Cain get a wife, when there were no other people?” But the Bible does not say that there were no other people at that time! For Adam and Eve had many sons and daughters, including Seth. Genesis 4:25. “And the days of Adam after he had begotten Seth were eight hundred years: and he begat sons and daughters.” Genesis 5:4. The descendants of the first human pair spread out into a “foreign land,” including Cain, who had murdered his brother. In due course, Cain took a wife from his sisters, or even a descendant of his sisters. Genesis 4:17. There is therefore no contradiction in the matter. However, the Bible does not present events in chronological order, but the events as presented are completed by the following pages. An example is the creation account (Genesis 1), which is completed in chapter 2.
It should be noted that Adam’s lifespan was 930 years, and during his lifetime humanity multiplied into hundreds of thousands, who spread out in different directions. There they met one another, took wives, and bore children.
Also Seth, the third son of Adam and Eve (Genesis 5:3), took a wife from Adam’s daughters, or from the daughters of his daughters.
Thus humanity began to multiply through the one human pair created by God. This is reasonable and logical. It was only during Moses’ time that unions among close relatives were forbidden. Leviticus 18.
PS. The natural water, air, and food in Adam’s time were 100% pure. Because of this, the lifespan of people was nearly a thousand years. For example, the oldest person in the world, Methuselah, lived to be 969 years old. Genesis 5:27.
The straightforward answer is we have no idea, there is total silence in the biblical text. Any attempt to answer the question is going to depend on numerous interpretive approaches, and speculation.
This is a claim and a conclusion that lacks anything but your own guesswork.
The claim that “the natural water, air, and food in Adam’s time were 100% pure” could be a correct statement; but it lacks any definition of what is meant by “pure” and it lacks any substantive argument–it’s a claim without definition or rationale. But even permitting the statement to be correct, your use of the claim to reach a conclusion: that this is why the lifespans of the antediluvian patriarchs was so long appears out of no where, without attempt to demonstrate how such a conclusion could be reached.
There is no biblical answer given to the long lifespans of the antediluvian patriarchs, only a biblical answer as to why the lives of human beings would be shortened. We could populate this discussion with plenty of speculations as to why human beings are said to have lived so long, but the text doesn’t tell us and all we’d have are our own speculations. And, again, these would depend greatly on our own interpretive methods and approaches.
Just like in the case of Cain’s wife, we are presented with information without any answers to a great deal of number of accompanying questions we may want to pose.
I could note that it is not uncommon in stories humans have told in the ancient past to speak of even more ancient generations of people who lived incredibly long lives. This appears to reflect an idea among certain ancient cultures to view even more ancient people to have lived during a golden age, or to have been under a special blessing, or approximation to the divine–granting extraordinarily long lifespans. So this is a common motif–what does this mean for our interpretation of the biblical material? Again we’d be dealing with interpretive approaches and speculations–rather than engaging in strict exegesis.