How many unique ancestors do you have?

How many ancestors do you have? The answer
seems easy at first, like a game of 2048. Your 2 parents each have 2 parents, making
4 grandparents, followed by 8 great-grandparents, 16 great-great-grandparents, 32 great-great-great-grandparents… Just keep doubling, right? Not for long. While your 11th generation could *just* have 2,048
unique ancestors, doubling will eventually fail. Here’s why. 28-year generations mean
that we should expect our average 31st generation ancestors to have been born around the year
1117, when the population of Earth was less than 600 million. But the 31st generation
requires more than 2 billion ancestral lines. How? Scholars who lived in the 12th century were fascinated by impossible doublings like these. In one legend, an Indian king
is so impressed by the game of chess that he offers its inventor a reward of his choosing.
The inventor’s request—1 grain of wheat on the first square of the chessboard, 2 on
the second, 4 on the third, the doubling continuing until the 64th square—amounts to more than
10 to the 19th grains. The king could never have paid this amount, over 9 times the value
of the global economy in 2014. That same number is your number of ancestral lines in the 64th generation, less than 2,000 years ago. If they were all unique individuals, they couldn’t
fit on the surface of Earth, plus 12 Jupiters. So, we need to refine the question. How many
unique ancestors do you have? For this we need a real world example, and for counting
ancestors the most complete family trees available are those of European royalty. The ancestors
of the current queen of Denmark double for a few generations, but in the 5th generation,
she has 28 instead of the expected 32. Because her parents were double 3rd cousins, these
ancestors (and all of their ancestors) repeat. Continued cousin marriage makes her ancestry
look more like a family spinning top than a family tree. Instead of over 1,000,000 20th
generation ancestors, she has only about 3,000, mostly of the tiny ruling class of Renaissance
Europe—8 millionths of the world population. It’s well known that European royals often married close cousins; they’ve mostly stopped. But your ancestors did too. Small populations
mean that nearly every possible mate is closer than a 4th cousin. Your ancestors grow much
like the queen’s if they lived in small villages or isolated islands, or if they maintained
traditions that encouraged close cousin marriage, such as tribal, religious, or caste endogamy.
Ethnic diversity and other forms of exogamy in your background will tend to make your
number of 20th generation ancestors higher than the queen’s 3,000. People with geographically
diverse ancestry have more small in-marrying groups in their ancestry. At the other extreme
are groups like the Sentinelese, who some believe are the most isolated community in
the world. If they have truly remained uncontacted, and have rejected migrants to their Andaman
Sea island for centuries, their number of 20th generation ancestors could be just a
few hundred. Sustained first cousin marriage could make someone’s number even less. We know that your number of ancestors at any given point is less than the world population.
Some people never become ancestors. Others have lines that go extinct. However, our number
of ancestors does approach the world population, and in historical time. If we assume that
the queen’s ancestors continue to accumulate at about the same rate, they converge toward
the total population of Earth about 150 generations ago in the third millennium BC. This fits well with one model of the timing of the identical ancestors point—the point in time before
which every living human today shares the same set of ancestors. Anyone who was alive
at or before this identical ancestors point either became the ancestor of everyone living
today or the ancestor of no one living today. In a series of papers, Chang, Rohde, and Olson
have run an increasingly complex series of simulations of human population history that
attempt to calculate the date of this point. Their most recent paper identifies a mean
identical ancestry date of 2158 BC, assuming relatively high migration. This is especially
relevant for Eurasia and Africa, core regions of humanity that have been connected by trade and migration routes for thousands of years. A recent identical ancestors point for these
regions of the world is starting to become supported by genetic data. In 2013, Ralph
and Coop studied long genomic segments in Europeans’ DNA that reflect these shared ancestors.
Their distribution shows that ancestry can spread very quickly, with “individuals from
opposite ends of Europe… expected to share millions of common genealogical ancestors
over the last 1,000 years.” The same applies to densely populated parts of Asia, which have held the majority of humanity for all of recorded history. If we integrate the area
under this curve, we get the number of years that our ancestors lived over the past 150
generations. Dividing by a life expectancy of 40 gives the figure of about 400,000,000
ancestors since the 22nd century BC. By the same logic, there are another 1,500,000,000
people from the 150th generation to the 250th generation—Chang, Rohde, and Olson’s more
conservative IAP—then 4,000,000,000 more people back to 10,000 BC. Using this model,
a first estimate, the queen has about 6,000,000,000 ancestors since the domestication of wheat, a bit less than the current population of Earth, plus billions more in the Paleolithic
past. A a more complete answer than this requires a definition for the beginning of humanity, or a decision to count pre-human ancestors, changing our numbers radically. So for this answer, let’s stick to the relatively recent human past. You have 6,000,000,000 ancestors
in the past 12,000 years, plus or minus a few billion. 5,000 years ago, you could find
your ancestors in every village on the planet. Today, their descendants are everyone you know. For more videos on big history, language, and a universe of connections, please subscribe
here to Roots and Routes.


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