Family Secrets
Clandestine gene-mixing was often hushed up and is lost from the historical record
"sssh!" by Stephen Dagnall is licensed under CC BY-NC-SA 2.0.
[Trigger warning - this column discusses sexual violence in the context of human genetics]
Last week’s column presented some basic maths showing the power of exponential growth in genetics. Everyone has two parents, four grandparents and eight great-grandparents. This number doubles every generation (defined as 25 to 30 years). The numbers get strange very quickly. To cut a long story short, the calculations show that everyone is related to everyone else and much more recently than you might imagine.
This result feels very wrong to our intuitions. What about people in remote and rural areas? Or tribes in the Amazon? Or in two areas that are separated by vast oceans?
One way of trying to understand this is to run the same maths we talked about last week backwards. Take a minute to imagine someone who lived 1,000 years somewhere on Earth. That is around 40 generations ago. Imagine that this person had two descendants who had children who survived.
For the sake of easy maths, imagine that on average each descendant has two children who lived to have their own descendants. Over the course of 40 generations, that becomes an impossible number, vastly higher than the 107 billion members of our species who have lived over the whole of our history. Trying to keep track of 40 generations will baffle our brains, which find it hard to peer beyond our lifetimes.
Another way of creeping up on the riddle is to think about how population growth has been speeding up over time. The United Nations (UN) has estimated that the world population only hit 1 billion for the first time around 1804. It hit 2 billion in 1927 (123 years) and 3 billion in 1960 (33 years). The next milestones were 4 billion in 1974 (14 years); 5 billion in 1987 (13 years); 6 billion in 1999 (12 years); and 7 billion by 2011 (12 years again). We just passed 8 billion in November, but the rate of growth appears to be slowing.
Let’s go back to 1804, when the population based the billion point for the first time. It is just 218 years ago, or just over eight generations. The human population was so much smaller before this date that it was much easier for genes that flowed from one area to another to have an outsized impact a few generations later. Meanwhile, the worldwide population 1,000 years ago is estimated to have been somewhere between 254 million and 310 million. That is smaller than the current population of the US and roughly equivalent to modern-day Indonesia (274 million).
In last week’s blog, we mentioned that everyone alive today is descended from every member of the global population who had descendants around 3,400 years ago. The world population around 1,000 years BCE was somewhere between 50 million and 115 million - somewhere between contemporary Colombia and Ethiopia. With such small numbers, a little mixing 136 generations ago could go a very long way.
Another way or tackling the problem is to confront some dark truths. In every generation, some percentage of women were raped; and some percentage of the violations led to live births. Some percentage of the rapists would be soldiers from armies invading a foreign land; while others would be slave owners or members of tribes that kidnapped their enemies’ women.
Meanwhile, some percentage of children would be born as a result of clandestine love affairs between people who in theory weren’t allowed to marry each other, maybe because they belonged to different cultures. And some percentage of women would be drawn to prostitution to survive. Some percentage of prostitutes would give birth to their clients’ children. Some percentage of these clients would be sailors and travellers from far-off lands.
We will never know the exact percentages, of course. Most of these stories would be hidden from the pages of history. Many of them became family secrets that were hushed up or subject to rumours that were never written down anywhere. Within a generation or two, the secret would be mostly forgotten.
Seeing the world like this provides a skeleton key for aspiring novelists interested in sweeping family dramas. Imagine a farmer’s wife who is raped by a soldier from an invading army. Her husband is killed in the war. The widow struggles to bring up her youngest child with her other children, but never fully accepts her.
The child runs away when she is a teenager and ends up as a prostitute in a nearby port town. She conceives three children on the job with three different clients. It is difficult to know the identity of the fathers for sure, but her eldest son looks remarkably like a sailor from a distant land on another continent, who spent some time in the port. She raises the kids as best as she can. Her eldest son joins the army and ends up as a mercenary in another country.
After retiring from the army, the mercenary opens an inn in his new country and settles down with a local woman. They have five children. One of them marries a local farmer, who gets in early on a new economic trend and becomes rich. They have a couple of kids. Their daughter is extraodinarily beautiful and she captures the eye of a local grandee, who makes her his concubine. Their three children are on the cusp of high society. Their eldest son marries well and becomes a good soldier with a taste for strategy. He is ennobled after helping win a civil war. One of his daughters is also famed for her beauty. A marriage is negotiated with a foreign prince. They have two children, but the prince’s family is overthrown in a palace coup and he is killed. The widow and her children have to flee overseas to a country that is hostile to her late husband’s enemies.
The speed of the imaginary scenario above is a little exaggerated. It spans ten generations (around 250 years) and up to five different countries (the farmer’s widow, the rapist from an invading army, the sailor who visited a prostitute in a port town, the mercenary who moved to a new country and the family that fled to exile). However, it is worth repeating that it only takes a little mixing each generation to give us results that appear strange to us. This is particularly trye when we go back many generations and the worldwide population was much lower.
We can easily imagine prostitutes sending unwanted children to live with relatives in rural areas; members of Amazonian tribes who have been raped by intruders many centuries ago, but bring up the kids as full members of the tribe; and sailors spreading their genes far and wide. In a small population, the new genes would spread like wildfire in just a few generations, only to become very common as human populations began to boom.
Most of the stories might be lost to us, but we can still see the results today if we look closely. Did you know that Cleopatra traced her descent back to one of Alexander the Great’s generals? Or that Roman emperor Lucius Septimius Severus was born in what is now Libya?
Did you know that Vikings lived for centuries in the city that is now called Istanbul? Or that one of the most famous Samurai of Japan was African? Or that the grandchildren of Genghis Khan ruled lands as far apart as what is now China, Iraq, Iran, Syria and Mongolia? Or that more than 1 million Europeans were enslaved in what is now is Algeria?
Did you know that US President Calvin Coolidge claimed to be of Native American heritage? Or that Afro-American black nationalist Malcolm X had red hair? Or that one in 20 migrants around the world today trace their roots to India? The comments are open. See you next week!
Further Reading
How to Argue With a Racist by Adam Rutherford
Most Europeans Share Recent Ancestors by Ewen Callaway
Rome: A History in Seven Sackings by Matthew Kneale
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