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

88 THE TREE OF LIFE How Should We Visualize the Tree of Life? 89
gradually lose Design? Is there a possible world in which bacteria are the?
descendants of mammals and not vice versa? These questions about possibility
will be easier to answer if we first look a bit more closely at what has
actually happened on our planet. So let us be clear that for the time being, the
vertical dimension in the diagrams below stands for time, and time alone,
with early at the bottom and late at the top. Following standard practice, the
left-right dimension is taken as a sort of single-plane summary of diversity.
Each individual organism has to have its time line, distinct from all others,
so, even if two organisms are exact atom-for-atom duplicates of each other,
they will have to appear side by side at best. How we line them all up,
however, can be according to some measure or family of measures of
difference in individual body shape—morphology, to use the technical term.
So, to return to our question, what would the overall shape of the entire
Tree of Life look like, if we could take it all in at a glance? Wouldn't it look
rather like a palm tree, as in figure 4.1?
This is the first of many trees, or dendrograms, we will consider, and of
course the limited resolution of the ink on the page blurs quadrillions of
separate lines together. I have left the "root" of the tree deliberately fuzzy and
indistinct for the time being. We are still exploring the middle, saving the
ultimate beginnings for a later chapter. If we were to zoom in on the trunk of
this tree and look at any cross-section of it—an "instant" in
time—we would see billions upon billions of individual unicellular organisms,
a fraction of which would have trails leading to progeny slightly higher
up the trunk. (In those early days, reproduction was by budding or fission;
somewhat later, a kind of unicellular sex evolved, but pollen-wafting and
egg-laying and the other phenomena of our kind of sexual reproduction have
to wait for the multicellular revolution in the fronds.) There would be some
diversity, and some revision of design over time, so perhaps the whole trunk
should be shown leaning left or right, or spreading more than I have shown.
Is it just our ignorance that prevents us from differentiating this "trunk" of
unicellular varieties into salient streams? Perhaps it should be shown with
various dead-end branches large enough to be visible, as in figure 4.2,
marking various hundred-million-year experiments in alternative unicellular
design that eventually all ended in extinction.
EARTH FORMED --------------------------------------------------------------------
FIGURE 4.3
There must have been billions of failed design experiments, but perhaps
none ever became very distant departures from a single unicellular norm. In
any event, if we were to zoom way in on the trunk, we would see a luxuriant
growth of short-lived alternatives, as in figure 4.3, all but invisible against
the norm of conservative replication. How can we be sure of this? Because,
as we shall see, the odds are heavily against any mutation's being more
viable than the theme on which it is a variation.