Who we are

I've been a math teacher for most of my career, and I am fascinated with numbers. I love how they tell a story and line up in the most unusual ways. The Fibonacci sequence is one of my favorite math topics. But here is another cool concept: if we could travel back in time, tracing the branches of our family trees, the number of our ancestors doubles with each generation.

- BY JOANNE STEENBERG

In order for us to exist, we had to have two parents, four grandparents, eight great-grandparents, 16 second great-grandparents, 32 third great-grandparents, 64 fourth great-grandparents, 128 fifth great-grandparents, 256 sixth great-grandparents, 512 seventh great-grandparents, 1,024 eighth great-grandparents, 2,048 ninth great-grandparents. And that's just 12 generations (approximately 400 years).

This exponential growth just blows my mind. In just 30 generations-roughly 750 years-you would theoretically have over a billion ancestors. But this number far exceeds the world's population at that time. The explanation lies in something genealogists call "pedigree collapse," where ancestral lines intertwine, meaning that we basically share many of our ancestors.