What major events are already history by the time Genesis 12 opens?
By Genesis 12, the Flood is history, the Tower of Babel is history, and the early chapters of Adam have passed. The human race had multiplied to perhaps millions, not billions, and the timeline stands around 2000 B.C., where the Jewish nation begins with Abraham's call.
Les explains that Genesis 1:1 sets out two spheres of God's influence, heaven and earth, before moving to Genesis chapter 12. There, the Flood has been history, the Tower of Babel is history, and the whole human race has continued. He notes this is only 200-400 years after the Flood, with perhaps a few million people, not billions yet, around 2000 B.C. At that point comes the beginning of the Jewish nation, or the call of Abraham. Genesis 12:1 records the Lord telling Abram to leave his country, kindred, and father's house for a land God will show him; land means what it says.
Now the LORD had said unto Abram, Get thee out of thy country, and from thy kindred, and from thy father’s house, unto a land that I will shew thee: