This is a giant jigsaw puzzle, in which some of the pieces are hidden. You may know an ancestor and their parents and then reach a dead end. It is helpful to focus on the parents, their siblings, the children of their siblings and track the migration. By researching these other “pieces” you may discover additional documents which assist in putting together a better picture of the exact piece you are missing. The best example I have for this is in researching a tree where there is little information on the family due to a divorce which occurred, and subsequent remarriages. In focusing on the parent’s and their siblings, building out their lines a discovery was made. That discovery was a male child who took the last name of the second step father after reaching the age of 18. Up until that discovery the “assumption” had been that he had died without children. This discovery opened the door and allowed the sharing of information with the DNA connections who had a completely different last name than the family member I was researching. Subsequent discussions, identified that this male child changed his name again after his second marriage; he married again and had at least one child with a different surname than that of his grandparents.
For those interested in genetic genealogy, DNA genealogy, and those interested in discovering family origins resulting from adoption or unknown parentage. You can become a DNA Detective.