To
state the absolutely obvious, every person living today is directly
descended from the very first cells that lived billions of years ago.
However, to make the time scale easier to grasp, let’s just state
that every human alive today is necessarily directly descended from
someone who was alive a thousand years ago, when, according to one
estimate, the human population of the world was about 265 million.
Of
those 265 million, we can assume not everyone managed to produce
offspring, who, in turn, produced offspring, over forty-one
generations (assuming four generations per century) between the years
1000 and the year 2000. Families were wiped about by wars, plagues,
other natural disasters, and some people would have been celibate or
infertile. However, a certain, evidently substantial fraction of
those 265 million did manage to breed successfully, and today they
have an estimated seven billion descendants.
From
the perspective of a person concerned with genealogy, the important
question is: which of the 265 million humans, who were alive a
thousand years ago, are my ancestors? If you make a simple but
fallacious calculation, you might be led to the conclusion that you
are descended from them all. A person born in the the year 2000 has
two parents who were born in 1975, four grandparents who were born in
1950, eight great-grandparents born in 1925, and sixteen
great-great-grandparents who were born in 1900. Taking it back to
the year 1000 would mean 241
ancestors,
which is 219,902,325,552,
or
more than thirty
times
the present human population of the world. We could not possibly be
not descended from two-hundred,
twenty
billion different people. Hence, many of our ancestors must, in
fact, sit in more than once place on our family trees.
For
example, suppose two of your great-grandparents, not necessarily a
man and wife, were first cousins (let’s say your mother’s
grandmother and your father’s grandmother). That would mean that
two of your great-great-grandparents were siblings, and those
siblings would, by definition, have the same parents. So instead of
thirty-two different descendants in that generation, you would only
have thirty. That’s the process that whittles down the number of
our actual ancestors from the impossible billions that the arithmetic
offers. In the villages where most of our ancestors probably lived,
in times when people tended to live and breed close to the places
where they were born, the pool of available spouses could not have
been very large, so many of our ancestors were related to each other.
Instead of fanning out more or less infinitely, our lines of direct
descent overlap and tangle. Not only are we directly descended from
someone who lived a thousand years ago, we are descended from that
person (and from many of our other actual ancestors in that
generation) along multiple paths.
Some people I know are very proud that they can trace their ancestry back several centuries, but, obviously, that usually means they can name one person out of the many from whom they are descended in each previous generation. I would be interested in my own genealogy only if I could discover more than a name and place of residence. If I knew what one of my ancestors did with her life in the twelfth century, that might be interesting.