If time travel were possible and you were to travel back to the year 1400, it is true that, over the course of several centuries, the family tree of every individual alive at that time would have expanded to encompass a vast number of descendants. As you move backward in time, the number of individuals who are your direct ancestors doubles with each generation.
However, it's important to note that this does not mean that every person in the past would be a direct ancestor of yours. While the number of slots in your family tree increases exponentially as you go back in time, the actual number of people living at any given point in history is limited. This discrepancy leads to what is known as the "identical ancestors point" or the "pedigree collapse."
Pedigree collapse occurs when individuals in a family tree share common ancestors. As you trace your ancestry back through multiple generations, you start to encounter repeated individuals occupying multiple positions in your family tree. These repeated individuals are your ancestors through different branches of your family tree.
Over many generations, the shared ancestry becomes more widespread, and many people from the same population would end up being related to each other in various ways. However, it is worth noting that this relationship becomes increasingly distant as you go back in time. The concept of "relatedness" becomes diluted, and the actual genetic and familial connections between individuals become less significant.
Furthermore, human populations are not isolated, and migration, intermarriage, and genetic exchange occur between different regions and groups. These factors further complicate the notion of everyone being related in a strictly genealogical sense.
Therefore, while the potential for shared ancestry exists among individuals from the past, the actual genealogical relationship between you and every person from the year 1400 would become increasingly tenuous and diffused over time.