We have no correspondence or stories about them. As a geneticist I am aware that each child receives half of his or her genes from the father or from the mother. If I had traced it back to the late 1600s, I would have only one percent or less of my ancestor's genes. If I went through my mother's line through a line of mothers as ancestors, I would be tracing the history of my mitochondria because males and females alike owe all their mitochondria to their mothers. The mitochondria are parts of cells that do our breathing and give us energy from the oxygen our blood brings to the cells.
Cousin marriages reduce the number of ancestors.
They were more frequent about 150 years or so ago because most people married someone within 25 miles of where they were born. You needed a horse to travel then. It is no surprise that if we could trace our ancestry back to the Middle Ages those of European ancestry would be related to most people who were alive then in the country of our origin. Very few people can do this outside of royalty because not all marriages were recorded and not all records survive. We are, of course, all related to one another as distant cousins whether we believe our ancestors go back to Adam and Eve or to some members of our species that left Africa about 200,000 years ago to form the other branches of humanity throughout the world.