Our common biblical ancestor is not Adam, it’s Noah; everyone but him died in the flood. Likewise, if you accept evolutionary theory, there would have been multiple near-extinctions where the genetic tree was pruned to only a few branches. Both Noah or his evolutionary equivalent would have had other men around.
No, its Adam.
Noah was his descendant, therefore Adam is our earliest common ancestor.
The Bible takes account of only the known world at the time that God transmitted its essentials to humanity. Those individuals had little to no idea of what was going on barely beyond their horizon, let alone the rest of the world.
I suspect that the flood stories came from the deluge that may have opened the Black Sea to the Mediterranean ~ 8,000 years ago. If that hypothesis is correct it must have caused total havoc for the people there, and would have been passed down as oral history until it was recorded.
Noah indeed may have built an arc. But there is no geological evidence of continents submerged at the same time.