The following is re-posted from another list I get. Much of my thoughts about cM segment length are discussed here.
And I think that all the tiny 1+ cM segment matches we have represent these remote ancestors from maybe 500-1000 years ago.
Now, if we could only point to one particular segment and say "that one is DNA from Willem van de Hagen" and "that one is DNA from Jacome de Bruges," etc.
Doug da Rocha Holmes
Sacramento, California
Pico & Terceira Genealogist
916-550-1618
Sacramento, California
Pico & Terceira Genealogist
916-550-1618
· THE LOOM:
·
· 3 days ago
Charlemagne’s DNA and Our Universal Royalty
by Carl Zimmer
Nobody in my past was hugely famous, at least that I know of. I vaguely recall that an ancestor of mine who shipped over on the Mayflowerdistinguished
himself by falling out of the ship and having to get fished out of the
water. He might be notable, I guess, but hardly famous. It is much more
fun to think that I am a bloodline descendant of Charlemagne. And in
1999, Joseph Chang gave me permission to think that way.
Chang
was not a genealogist who had decided to make me his personal project.
Instead, he is a statistician at Yale who likes to think of genealogy as
a mathematical problem. When you draw your genealogy, you make two
lines from yourself back to each of your parents. Then you have to draw
two lines for each of them, back to your four grandparents. And then
eight great-grandparents, sixteen great-great-grandparents, and so on.
But not so on for very long. If you go back to the time of Charlemagne,
forty generations or so, you should get to a generation of a trillion
ancestors. That’s about two thousand times more people than existed on
Earth when Charlemagne was alive.
The
only way out of this paradox is to assume that our ancestors are not
independent of one another. That is, if you trace their ancestry back,
you loop back to a common ancestor. We’re not talking about first-cousin
stuff here–more like twentieth-cousin. This means that instead of
drawing a tree that fans out exponentially, we need to draw a web-like
tapestry.
In a paper he published in 1999 [pdf],
Chang analyzed this tapestry mathematically. If you look at the
ancestry of a living population of people, he concluded, you’ll
eventually find a common ancestor of all of them. That’s not to say that
a single mythical woman somehow produced every European by magically
laying a clutch of eggs. All this means is that as you move back through
time, sooner or later some of the lines in the genealogy will cross,
meeting at a single person.
As
you go back further in time, more of those lines cross as you encounter
more common ancestors of the living population. And then something
really interesting happens. There comes a point at which, Chang wrote,
“all individuals who have any descendants among the present-day individuals are actually ancestors of all present-day individuals.”
In 2002, the journalist Steven Olson wrote an article in the Atlantic about
Chang’s work. To put some empirical meat on the abstract bones of
Chang’s research, Olson considered a group of real people–living
Europeans.
The
most recent common ancestor of every European today (except for recent
immigrants to the Continent) was someone who lived in Europe in the
surprisingly recent past—only about 600 years ago. In other words, all
Europeans alive today have among their ancestors the same man or woman
who lived around 1400. Before that date, according to Chang’s model, the
number of ancestors common to all Europeans today increased, until,
about a thousand years ago, a peculiar situation prevailed: 20 percent
of the adult Europeans alive in 1000 would turn out to be the ancestors
of no one living today (that is, they had no children or all their
descendants eventually died childless); each of the remaining 80 percent
would turn out to be a direct ancestor of every European living today.
Suddenly,
my pedigree looked classier: I am a descendant of Charlemagne. Of
course, so is every other European. By the way, I’m also a descendant of
Nefertiti. And so are you, and everyone else on Earth today. Chang
figured that out by expanding his model from living Europeans to living
humans, and getting an estimate of 3400 years instead of a thousand for
the all-ancestor generation.
Things
have changed a lot in the fourteen years since Chang published his
first paper on ancestry. Scientists have amassed huge databases of
genetic information about people all over the world. These may not be
the same thing as a complete genealogy of the human race, but
geneticists can still use them to tackle some of the same questions that
intrigued Chang.
Recently,
two geneticists, Peter Ralph of the University of Southern California
and Graham Coop of the University of California at Davis, decided to
look at the ancestry of Europe. They took advantage of a compilation of
information about 2257 people from across the continent. Scientists had
examined half a million sites in each person’s DNA, creating a
distinctive list of genetic markers for each of them.
You
can use this kind of genetic information to make some genealogical
inferences, but you have to know what you’re dealing with. Your DNA is
not a carbon copy of your parents’. Each time they made eggs or sperm,
they shuffled the two copies of each of their chromosomes and put one in
the cell. Just as a new deck gets more scrambled the more times you
shuffle it, chromosomes get more shuffled from one generation to the
next.
This
means that if you compare two people’s DNA, you will find some chunks
that are identical in sequence. The more closely related people are, the
bigger the chunks you’ll find. This diagram shows how two first cousins
share a piece of DNA that’s identical by descent (IBD for short).
Ralph
and Coop identified 1.9 million of these long shared segments of DNA
shared by at least two people in their study. They then used the length
of each segment to estimate how long ago it arose from a common ancestor
of the living Europeans.
Their results, published today in PLOS Biology,
both confirm Chang’s mathematical approach and enrich it. Even within
the past thousand years, Ralph and Coop found, people on opposite sides
of the continent share a lot of segments in common–so many, in fact,
that it’s statistically impossible for them to have gotten them all from
a single ancestor. Instead, someone in Turkey and someone in England
have to share a lot of ancestors. In fact, as Chang suspected, the only
way to explain the DNA is to conclude that everyone who lived a thousand
years ago who has any descendants today is an ancestor of every
European. Charlemagne for everyone!
If
you compare two people in Turkey, you’ll find bigger shared segments of
DNA, which isn’t surprising. Since they live in the same country,
chances are they have more recent ancestors, and more of them. But there
is a rich, intriguing pattern to the number of shared segments among
Europeans. People across Eastern Europe, for example, have a larger set
of shared segments than people from within single countries in Western
Europe. That difference may be the signature of a big expansion of the
Slavs.
Ralph
and Coop’s study may provide a new tool for reconstructing the history
of humans on every continent, not just Europe. It will also probably
keep people puzzling over the complexities of genealogy. If Europeans
today share the same ancestors a thousand years ago, for example, why
don’t they all look the same?
[Update: Adjusted the estimated generations since Charlemagne to thirty. Corrected Ralph's affiliation.]
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