Pedigree collapse

Or Ancestor Paradox

In genealogy, pedigree collapse describes how reproduction between two individuals who share an ancestor causes the number of distinct ancestors in the family tree of their offspring to be smaller than it could otherwise be.

Without pedigree collapse, a person's ancestor tree is a binary tree, formed by the person, the parents (2), the grandparents (4), great-grandparents (8), and so on. However, the number of individuals in such a tree grows exponentially and will eventually become impossibly high. For example, a single individual alive today would, over 30 generations going back to the High Middle Ages, have 230 or roughly 1 billion ancestors, more than the total world population at the time.
This paradox is explained by shared ancestors. Instead of consisting of all different individuals, a tree may have multiple places occupied by a single individual. This typically happens when the parents of an ancestor are related to each other (sometimes without your own knowledge). For example, the offspring of two first cousins has at most only six great-grandparents instead of the usual eight. This reduction in the number of ancestors is referred to as pedigree collapse. It collapses the ancestor tree into a directed acyclic graph.
This happens when people marry people within their own caste, village, region and families and have children with them. That occasional marriages between relatives reduce the required number of ancestors to more acceptable levels.

Small, isolated populations such as those of remote islands represent extreme examples of pedigree collapse, but the common historical tendency to marry those within walking distance, due to the relative immobility of the population before modern transport, meant that most marriage partners were at least distantly related. 

There was a real bottle neck to the population a few thousand years back, the population was at a near extinction number. We are not very diverse as a species. We are quite inbred.

Go back more than about ten generations - less if your family all came from the same area - and the same people start cropping up in more than one position in your chart, so you find out that your mother’s mother’s father’s mother father’s father was also your father’s father’s mother’s mother’s father’s mother’s father. The further back you go, the more the lines collapse back together.

( This should actually be considered in reverse way)

The published conclusion was that all current human mtDNA originated from a single population from Africa, at the time dated to between 140,000 and 200,000 years ago.

The understanding of the evolution of the mtDNA pedigree helped population geneticists to trace the ancestors of modern humans to their roots in Africa and their subsequent distribution in the world. It was determined that the Mitochondrial Eve lived in sub-Saharan Africa 200,000 years ago (1).

In human genetics, the Mitochondrial Eve (more technically known as the Mitochondrial-Most Recent Common Ancestor, shortened to mt-Eve or mt-MRCA) is the matrilineal most recent common ancestor (MRCA) of all living humans. In other words, she is defined as the most recent woman from whom all living humans descend in an unbroken line purely through their mothers and through the mothers of those mothers, back until all lines converge on one woman (2).

Footnotes: 

1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4347471/#:~:text=The%2....

2. https://en.wikipedia.org/wiki/Mitochondrial_Eve