Humans Are All More Closely Related Than We Commonly Think – Scientific American

Posted By on October 7, 2020

The late esteemed English actor Christopher Lee traced his ancestry directly to Charlemagne. In 2010 Lee released a symphonic metal album paying homage to the first Holy Roman emperorbut his enthusiasm may have been a tad excessive. After all, says geneticist Adam Rutherford, literally everyone with European ancestry is directly descended from Charlemagne.

The family tree of humanity is much more interconnected than we tend to think. Were culturally bound and psychologically conditioned to not think about ancestry in very broad terms, Rutherford says. Genealogists can only focus on one branch of a family tree at a time, making it easy to forget how many forebears each of us has.

Imagine counting all your ancestors as you trace your family tree back in time. In the nth generation before the present, your family tree has 2n slots: two for parents, four for grandparents, eight for great-grandparents, and so on. The number of slots grows exponentially. By the 33rd generationabout 800 to 1,000 years agoyou have more than eight billion of them. That is more than the number of people alive today, and it is certainly a much larger figure than the world population a millennium ago.

This seeming paradox has a simple resolution: Branches of your family tree dont consistently diverge, Rutherford says. Instead they begin to loop back into each other. As a result, many of your ancestors occupy multiple slots in your family tree. For example, your great-great-great-great-great-grandmother might have also been your great-great-great-great-aunt, he explains.

The consequence of humanity being incredibly inbred is that we are all related much more closely than our intuition suggests, Rutherford says. Take, for instance, the last person from whom everyone on the planet today is descended. In 2004 mathematical modeling and computer simulations by a group of statisticians led by Douglas Rohde, then at the Massachusetts Institute of Technology, indicated that our most recent common ancestor probably lived no earlier than 1400 B.C. and possibly as recently as A.D. 55. In the time of Egypts Queen Nefertiti, someone from whom we are all descended was likely alive somewhere in the world.

Go back a bit further, and you reach a date when our family trees share not just one ancestor in common but every ancestor in common. At this date, called the genetic isopoint, the family trees of any two people on the earth now, no matter how distantly related they seem, trace back to the same set of individuals. If you were alive at the genetic isopoint, then you are the ancestor of either everyone alive today or no one alive today, Rutherford says. Humans left Africa and began dispersing throughout the world at least 120,000 years ago, but the genetic isopoint occurred much more recentlysomewhere between 5300 and 2200 B.C., according to Rohdes calculations.

At first glance, these dates may seem much too recent to account for long-isolated Indigenous communities in South America and elsewhere. But genetic information spreads rapidly through generational time, Rutherford explains. Beginning in 1492, you begin to see the European genes flowing in every direction until our estimates are that there are no people in South America today who dont have European ancestry.

In fact, even more recent than the global genetic isopoint is the one for people with recent European ancestry. Researchers using genomic data place the latter date around A.D. 1000. So Christopher Lees royal lineage is unexceptional: because Charlemagne lived before the isopoint and has living descendants, everyone with European ancestry is directly descended from him. In a similar vein, nearly everyone with Jewish ancestry, whether Ashkenazic or Sephardic, has ancestors who were expelled from Spain beginning in 1492. Its a very nice example of a small world but looking to the past, says Susanna Manrubia, a theoretical evolutionary biologist at the Spanish National Center for Biotechnology.

Not everyone of European ancestry carries genes passed down by Charlemagne, however. Nor does every Jew carry genes from their Sephardic ancestors expelled from Spain. People are more closely related genealogically than genetically for a simple mathematical reason: a given gene is passed down to a child by only one parent, not both. In a simple statistical model, Manrubia and her colleagues showed that the average number of generations separating two random present-day individuals from a common genealogical ancestor depends on the logarithm of the relevant populations size. For large populations, this number is much smaller than the population size itself because the number of possible genealogical connections between individuals doubles with each preceding generation. By contrast, the average number of generations separating two random present-day individuals from a common genetic ancestor is linearly proportional to the population size because each gene can be traced through only one line of a persons family tree. Although Manrubias model unrealistically assumed the population size did not change with time, the results still apply in the real world, she says.

Because of the random reshuffling of genes in each successive generation, some of your ancestors contribute disproportionately to your genome, while others contribute nothing at all. According to calculations by geneticist Graham Coop of the University of California, Davis, you carry genes from fewer than half of your forebears from 11 generations back. Still, all the genes present in todays human population can be traced to the people alive at the genetic isopoint. If you are interested in what your ancestors have contributed to the present time, you have to look at the population of all the people that coexist with you, Manrubia says. All of them carry the genes of your ancestors because we share the [same] ancestors.

And because the genetic isopoint occurred so recently, Rutherford says, in relation to race, it absolutely, categorically demolishes the idea of lineage purity. No person has forebears from just one ethnic background or region of the world. And your genealogical connections to the entire globe mean that not too long ago your ancestors were involved in every event in world history.

So the next time you hear someone claim to be descended from royalty, take heart: you are, too. You are very special, and you are very generic, in a sense, Manrubia says.

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Humans Are All More Closely Related Than We Commonly Think - Scientific American

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