There was definately two “first” humans. Science has shown us that there is a “mitochondrial Eve.” That is, a women that every human being alive today is descended from. She lived about (I believe off of the top of my head) around 100,000 years ago. We know about her through molecular genetic evidence and we know of her time frame of existence due to the molecular clock calculated from internal evidences of the mitochondria.
Likewise, there is also a “Y-Chromosome Adam” that was the first male of our species. So yes, there were two “first” parents. Were these two formed from dirt approx. 6,000 years ago? For heavens sakes, NO. But, the exact date of their existence is not as important as the fact that they did exist.
Did the biblical authors believe they were really two peoples from around 4,000 b.c.? YES. But again, I would suggest that-that is irrelevant as they did not have the genetic evidence we have today. The length of time does nothing to change the fact that (whenever we came about) we fell into a state of sin and that God came to this world in the person of Jesus and brought about salvation for all who desire it.
Just my 2 cents though.
David