How many ancestors do you really have?

A girl in Penang is working on a school project: draw your family tree. She draws her parents, then her four grandparents, then her eight great-grandparents. Halfway down the page the paper runs out. The tree has grown faster than her paper can.

The story

Everyone has two parents, four grandparents, eight great-grandparents, and so on. Each step back doubles the number. That is the rule of "2 to the power N". Ten generations back is 2 to the power 10, which is 1,024 people in that one row. Twenty generations back is more than a million. Mathieson and Scally (2020) call this the genealogical ancestors: people in your family tree.

There is a catch, and it is a pleasant one. These numbers are a ceiling. In real life the same person can appear in the tree twice, because cousins sometimes marry, and a village has only so many people. The number of different ancestors is smaller than the ceiling, but still huge.

A second catch is about DNA. A parent passes on only half of their DNA to each child, and the pieces are shuffled each time. After many generations, some ancestors leave no DNA in you at all. Mathieson and Scally explain that your genetic ancestors, the ones who gave you real pieces of DNA, are only a small share of your family-tree ancestors. Two brothers or sisters have the same family tree but not the same inherited DNA.

The two together give a surprising picture. In Europe, Ralph and Coop (2013) found that two people from opposite ends of the continent share millions of ancestors in their family trees going back about 1,000 years, but only a few of them gave either person any DNA. Models by Rohde and colleagues (2004) suggest that the most recent common ancestor of everyone alive today lived only a few thousand years ago. So we are all cousins, and much closer than people think.

Where you can still see it today

In a family tree drawn on paper that grows too wide for the page.
In a village or a taman where almost everyone seems related, since the tree folds back on itself.
In your region estimate, which reflects pieces of DNA from only some of your ancestors.

What DNA can tell, and what it cannot

A region estimate is built from the pieces of DNA you actually inherited. It speaks only for the ancestors who left something in you. It cannot name the many ancestors who left nothing.

Your mother line and father line follow just two paths among the many. They cannot say who the rest of your ancestors were.

DNA does not decide race, religion or bumiputera status. It also does not rank people: every one of us has a very large, very mixed family tree.