"And Adam knew Eve his wife; and she conceived, and bare Cain, and said, I have gotten a man from the LORD." (Genesis 4:1 KJV)
It is thought that Eve had expected Gods promise of 3:15 was to be her son Abel. Imagine her disappointment when Cain killed Abel and became a curse among mankind! "No more deliverer" she probably thought. If one accepts this theory, then Shem became her new hope, and symbolized her efforts to "help things out a bit"! The same thing happens later with Sarah, when she saw that she was barren and gave her handmaid to Abraham to "help God keep His promise"! Don’t we often, because of unbelief, try to "help God out"? Our foolishness in doing so hurts Him more than it helps Him, and it brings reproach unto us because it is blatant unbelief! So was the case with Seth. (5:3)
The fifth chapter of Genesis is the chapter of "procreation"! God made man "in His image, after His likeness" and gave man the ability to do the same! So, Adam "created" Cain and Abel, and Seth (and probably many others), and they too began to "procreate," after their kind, and in their own images. Thus begins the godly line, through Seth, through which Jesus is eventually born. The "fruitfulness" of man is a gift from God. He gives "seed" and only He, we need make no mistake about that. His plan for redemption, in the Old Testament, is the underlying reason for the events that took place, and that we often misunderstand. One sees here the earth being populated over a period of over one thousand years! A calculator and a little extra time will prove that it was nearly two thousand years until the flood took place.