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

98 THE TREE OF LIFE Retrospective Coronations 99
DNA of the different people alive today and deduce from that how recently
Mitochondrial Eve lived, and even where she lived. According to their
original calculations, Mitochondrial Eve lived in Africa, very, very recently—
less than three hundred thousand years ago, and maybe less than half that.
These methods of analysis are controversial, however, and the African Eve
hypothesis may be fatally flawed. Deducing where and when is a far trickier
task than deducing that there was a Mitochondrial Eve, something that
nobody denies. Consider a few of the things we already know about Mitochondrial
Eve, setting aside the recent controversies. We know that she had
at least two daughters who had surviving children. (If she had just one
daughter, her daughter would wear the crown of Mitochondrial Eve.) To
distinguish her title from her proper name, let's call her Amy. Amy bears the
title of Mitochondrial Eve; that is, she just happens to have been the maternal
founder of today's line of people. 6 It is important to remind ourselves that in
all other regards, there was probably nothing remarkable or special about
Mitochondrial Eve; she was certainly not the First Woman, or the founder of
the species Homo sapiens. Many earlier women were unquestionably of our
species, but happen not to have any direct female lines of descendants
leading to people living today. It is also true that Mitochondrial Eve was
probably no stronger, faster, more beautiful, or more fecund than the other
women of her day.
To bring out just how unspecial Mitochondrial Eve—that is, Amy—probably
was, suppose that tomorrow, thousands of generations later, a virulent
new virus were to spread around the Earth, wiping out 99 percent of the
human race in a few years. The survivors, fortunate to have some innate
resistance to the virus, would probably all be quite closely related. Their
closest common direct female ancestor—call her Betty—would be some
woman who lived hundreds or thousands of generations later than Amy, and
the crown of Mitochondrial Eve would pass to her, retroactively. She may
have been the source of the mutation that centuries later came into its own as
a species-saver, but it didn't do her any good, since the virus against which it
is to triumph didn't exist then. The point is that Mitochondrial Eve can only
be retrospectively crowned. This historically pivotal role is determined not
just by the accidents of Amy's own time, but by the accidents of later times
as well. Talk about massive contingency! If Amy's uncle hadn't saved her
from drowning when she was three, none of us (with our particular
mitochondrial DNA, thanks ultimately to Amy) would ever have
existed! If Amy's granddaughters had all starved to death in infancy—as so
many infants did in those days—the same oblivion would be ours.
The curious invisibility of the crown of Mitochondrial Eve in her own
lifetime is easier to understand and accept than the near-invisibility of what
every species must have: a beginning. If species aren't eternal, then all of
time can be divided, somehow, into the times before the existence of species
x, and all subsequent times. But what must have happened at the interface? It
may help if we think of a similar puzzle that has baffled many people. Have
you ever wondered, when hearing a new joke, where it came from? If you are
like almost everybody else I have ever known or heard of, you never make up
jokes; you pass on, perhaps with "improvements," something you heard from
someone who heard it from someone, who... Now, we know the process
cannot go on forever. A joke about President Clinton, for instance, cannot be
more than a year or so old. So who makes up the jokes? Joke-authors (as
contrasted with joke-purveyors) are invisible. 7 Nobody ever seems to catch
them in the act of authorship. There is even folklore—an "urban legend"—to
the effect that these jokes are all created in prison, by prisoners, those
dangerous and unnatural folks, so unlike the rest of us, and with nothing
better to do with their time than to fashion jokes in their secret underground
joke-workshops. Nonsense. It is hard to believe— but it must be true—that
the jokes we hear and pass on have evolved from earlier stories, picking up
revisions and updates as they are passed along. A joke typically has no one
author; its authorship is distributed over dozens or hundreds or thousands of
tellers, solidifying for a while in some particularly topical and currently
amusing version, before going dormant, like the ancestors from which it
grew. Speciation is equally hard to witness, and for the same reason.
When has speciation occurred? In many cases (perhaps most, perhaps
almost all—biologists disagree about how important the exceptions are), the
speciation depends on a geographical split in which a small group— maybe a
single mating pair—wander off and start a lineage that becomes
reproductively isolated. This is allopatric speciation, in contrast to sympatric
speciation, which does not involve any geographic barriers. Suppose
we watch the departure and resettlement of the founding group. Time passes,
and several generations come and go. Has speciation occurred? Not yet,
certainly. We won't know until many generations later whether or not these
individuals should be crowned as species-initiators.
There is not and could not be anything internal or intrinsic to the individuals—or
even to the individuals-as-they-fit-into-their-environment—from
6. Philosophers have often discussed strange examples of individuals known to us only via
definite descriptions, but they have usually coniined their attention to such boring—if
real—individuals as the shortest spy. (There has to be one, doesn't there?) I suggest that
Mitochondrial Eve is a much more delicious example, all the more so for being of some
genuine theoretical interest in evolutionary biology.
7. There are, of course, the writers who make their living writing funny lines for television
comedians, and the comedians themselves, who create much of their own material,
but, with negligible exceptions, these people are not the creators of the joke stories
("Did you hear the one about the guy who...?") that get passed around.