Cain was afraid of the people "out there."  Excavation of human remains through the years has been extensive.  And those remains are far, far, older than 6000 years ago when Cain was alive.  As of late, through our ever increasing knowledge of the human genome, scientists have found that all of us have some Neanderthal genes.  This is not surprising since the person that Cain married was not a person from the garden of Eden.   The Neanderthal was much older than 6000 years ago, and the people in Nod probably carried some of those genes.  God doesn't tell us about those people.

You remember the story.  Cain killed Able, and when God confronted him, Cain denied it saying, "I don't know where he is.  Am I my brother's keeper?"  Of course he knew where Able was.  And he should have been smart enough to know that God knew.  God placed a curse on Cain--cursed from the earth that held his brother's blood.  "You will be a fugitive and a vagabond," God told him.

Cain told God, "My punishment is greater than I can bear...you have driven me out...from the face of the earth...and....everyone that finds me will slay me."  So God set a mark on Cain lest any finding him should kill him.  Who are these "everyone that finds me?"  Who are these "any finding him?"  Whoever they were, they lived in Nod, on the east of Eden.  (That's where Steinbeck got the name for his classic book, East of Eden.)

The generations after Cain are in the fourth chapter of Genesis.  All of Cain's offspring were from the land of Nod.  Later, Adam had a son named Seth.  We aren't told where his wife came from, but he had many descendants.  The Bible follows the story of Seth's descendants, not Cain's.

So.  Here we are.  Although God created Eden for his perfect man and woman, they messed up, and from then on, things went downhill.  But God had a plan to put his Spirit into the flawed people who were born in the following years.  The people born after Adam didn't have God's spirit within.  They had an empty place that longed to be filled.  I once told you that scientifically, man had no instincts--but said that I personally believe he had one.  All humans search for something to fill their emptiness.  They search for God, or design a god, idol, made with their hands.

I believe humans were here before Adam, but that Adam was different.  Unique.  Spirit filled.