Politics are saturating the U.S. news media once again as candidates vie for the presidential elections, still more than a year away. Every four years, news services “discover” that various candidates are related to one another. This week’s news is that Donald Trump and Hillary Clinton are 19th cousins. Their common ancestors include John of Gaunt, the duke of Lancaster, and third wife Katherine Swynford at the end of the 14th century — a century before Columbus sailed the ocean blue.
I have one reaction: “Ho hum, so what else is new?”
Of course, most every person in North America whose ancestors have been here for several generations is related to most everyone else in North America whose ancestors have been here for several generations. In fact, you and I are probably related, and we are also probably related to Barack Obama, Donald Trump, Justin Bieber, Celine Dion, and the local mailman.
Let’s have some fun with mathematics. Let’s assume that there is a new generation born every twenty-five years. Your two parents were born about 25 years before you were born, your four grandparents were born about 50 years before you were born, your eight great-grandparents were born about 75 years before you were born, your sixteen great-great-grandparents were born about 100 years before you were born, and so forth back into antiquity. In other words, the number of your ancestors doubles every 25 years as you go back in time.
If you take this back just 1,000 years, you’ll find that you have well over 500 billion ancestors in a single generation.
Obviously, that’s impossible as there are fewer than seven billion people on this planet. While mathematically correct, the practical reality is that you don’t have 500 billion distinct ancestors, but rather a much, much smaller number of ancestors who reappear over and over and over again in your family tree. The reality is that we are all the products of much interbreeding, and with that we have cousins… many cousins. Thousands of cousins… Yes, even millions of cousins. Every one of us has millions of cousins.
Proving all these relationships may be a challenge, but the relationships certainly do exist. You and I are undoubtedly related, and we are also both related to most every politician, Hollywood star, and professional athlete you have ever heard of. We are also related to less notable people, including the truck driver who delivered your furniture and the plumber who fixed the leaky faucet.
I mentioned earlier that most every person in North America whose ancestors have been here for several generations is related to most everyone else in North America whose ancestors have been here for several generations. However, the relationships go much further back in time and cover all areas outside of North America as well. We are related to everyone on every continent although we may have to go back many more generations to prove the connections.
By definition, cousins are two people who share a common ancestor. Joe Chang proved (see www.stat.yale.edu/~jtc5/papers/Ancestors.pdf) that the number of generations back to the most recent common ancestor in a randomly mating population is very closely approximated by the base two logarithm of the size of the population. So, for a population of a million people, it takes 20 generations to reach the most recent common ancestor — or about 500 years — because 2 to the 20th power is about a million. For a randomly mating population of a billion people, it takes 30 generations — 750 years — because 2 to the 30th power is a billion. In simpler terms, most of us can find a cousin relationship with most anyone else by tracing our family trees back an average of 750 years.
Of course, we are not all related only to “the good guys.” We don’t get to pick and choose our relatives. I suspect that you and I and Barack Obama and Donald Trump are all related to Adolph Hitler, Genghis Khan, Alexander the Great, Attila the Hun, Charles Manson, and Osama bin Laden. However, we undoubtedly will need to go back more than 750 years to find some of those connections.
We are all related. All these news stories are simply proving the point.
As Joseph T. Chang, Douglas L.T. Rohde, and Steve Olson observe in their 2004 paper on the subject of everyone being related to everyone else (Nature, Volume 431, Issue 7008, pp. 562-566), we’re all interrelated, and getting more so all the time:
“No matter the languages we speak or the color of our skin, we share ancestors who planted rice on the banks of the Yangtze, who first domesticated horses on the steppes of the Ukraine, who hunted giant sloths in the forests of North and South America, and who labored to build the Great Pyramid of Khufu. [And] within two thousand years, it is likely that everyone on earth will be descended from most of us.”
The next time you read or watch a news story saying that two political candidates are related, turn the page or change the station. That’s not news.