Darwin's Dangerous Idea - Evolution and the Meaning of Life

96 THE TREE OF LIFE Retrospective Coronations 97
3. RETROSPECTIVE CORONATIONS: MITOCHONDRIAL EVE AND
INVISIBLE BEGINNINGS
When we tried to see whether Lulu's descendants split into more than one
species, we had to look ahead to see if any large branches appeared, and then
back up if we deemed that somewhere along the line a speciation event must
have happened. We never addressed the presumably important question of
exactly when speciation should be said to occur. Speciation can now be seen
to be a phenomenon in nature that has a curious property: you can't tell that it
is occurring at the time it occurs! You can only tell much later that it has
occurred, retrospectively crowning an event when you discover that its
sequels have a certain property. This is not a point about our epistemic
limitations—as if we would be able to tell when speciation occurs if only we
had better microscopes, or even if we could get in a time machine and go
back in time to observe the appropriate moments. This is a point about the
objective property of being a speciation event. It is not a property that an
event has simply by virtue of its spatio-temporally local properties.
Other concepts exhibit similar curiosities. I once read about a comically
bad historical novel in which a French doctor came home to supper one
evening in 1802 and said to his wife-. "Guess what / did today! I assisted at
the birth of Victor Hugo!" What is wrong with that story? Or consider the
property of being a widow. A woman in New York City may suddenly
acquire that property by virtue of the effects that a bullet has just had on some
man's brain in Dodge City, over a thousand miles away. (In the days of the
Wild West, there was a revolver nicknamed the Widowmaker. Whether a
particular revolver lived up to its nickname on a particular occasion might be
a fact that could not be settled by any spatio-temporally local examination of
its effects.) This case gets its curious capacity to leap through space and time
from the conventional nature of the relation of marriage, in which a past
historical event, a wedding, is deemed to create a permanent relation—a
formal relation—of interest in spite of subsequent wanderings and concrete
misfortunes (the accidental loss of a ring, or the destruction of the marriage
certificate, for instance.)
The systematicity of genetic reproduction is not conventional but natural,
but that very systematicity permits us to think formally about causal chains
extending over millions of years, causal chains that would otherwise be
virtually impossible to designate or refer to or track. This permits us to
become interested in, and reason rigorously about, even more distant and
locally invisible relationships than the formal relationship of marriage. Speciation
is, like marriage, a concept anchored within a tight, formally definable
system of thought, but, unlike marriage, it has no conventional
saliencies—weddings, rings, certificates—by which it can be observed. We
can see this feature of speciation in a better light by looking first at another
instance of retrospective crowning, the conferring of the title of Mitochondrial
Eve.
Mitochondrial Eve is the woman who is the most recent direct ancestor, in
the female line, of every human being alive today. People have a hard time
thinking about this individual woman, so let's just review the reasoning.
Consider the set A, of all human beings alive today. Each was born of one
and only one mother, so consider next the set, B, of all the mothers of those
alive today. B is of necessity smaller than A, since no one has more than one
mother, and some mothers have more than one child. Continue with the set
C, of mothers of all those mothers in set B. It is smaller still. Continue on
with sets D and E and so forth. The sets must contract as we go back each
generation. Notice that as we move back through the years, we exclude many
women who were contemporaries of those in our set. Among these excluded
women are those who either lived and died childless or whose female
progeny did. Eventually, this set must funnel down to one— the woman who
is the closest direct female ancestor of everybody alive on earth today. She is
Mitochondrial Eve, so named (by Cann et al. 1987) because since the
mitochondria in our cells are passed through the maternal line alone, all the
mitochondria in all the cells in all the people alive today are direct
descendants of the mitochondria in her cells!
The same logical argument establishes that there is—must be—an Adam
as well: the closest direct male ancestor of everybody alive today. We could
call him F-Chromosome Adam, since all our F-chromosomes pass down
through the paternal line just the way our mitochondria pass through the
maternal line. 5 Was F-Chromosome Adam the husband or lover of Mitochondrial
Eve? Almost certainly not. There is only a tiny probability that
these two individuals were alive at the same time. (Paternity being a much
less time-and-energy-consuming business than maternity, what is logically
possible is that F-Chromosome Adam lived very recently, and was very, very
busy in the bedroom—leaving Errol Flynn in his, um, dust. He could, in
principle, be the great-grandfather of us all. This is about as unlikely as the
case in which F-Chromosome Adam and Mitochondrial Eve were a couple.)
Mitochondrial Eve has been in the news recently because the scientists
who christened her think they can analyze the patterns in the mitochondrial
5. Note one important difference between the legacies of Mitochondrial Eve and Y-
Chromosome Adam: we all, male and female, have mitochondria in our cells, but they
all come from our mothers; if you are male, you have a V-chromosome and got it from
your father, but most—virtually all, but not quite all—females have no Y-
chromosome at all.