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

86 THE TREE OF LIFE How Should We Visualize the Tree of Life? 87
parent organism survives to coexist for a while with its offspring. But such
microdetails of the fan-out will not in general concern us at this time. There
is no serious controversy about the fact that all the diversity of life that has
ever existed on this planet is derived from this single fan-out; the controversies
arise about how to discover and describe in general terms the various
forces, principles, constraints, etc., that permit us to give a scientific
explanation of the patterns in all this diversity.
The Earth is about 4.5 billion years old, and the first life forms appeared
quite "soon"; the simplest single-celled organisms—the prokaryotes—appeared
at least 35 billion years ago, and for probably another 2 billion years,
that was all the life there was. bacteria, blue-green algae, and their equally
simple kin. Then, about 1.4 billion years ago, a major revolution happened:
some of these simplest life forms literally joined forces, when some bacterialike
prokaryotes invaded the membranes of other prokaryotes, creating the
eukaryotes—cells with nuclei and other specialized internal bodies (Margulis
1981). These internal bodies, called organelles or plastids, are the key
design innovation opening up the regions of Design Space inhabited today.
The chloroplasts in plants are responsible for photosynthesis, and mitochondria,
which are to be found in every cell of every plant, animal,
fungus—every organism with nucleated cells—are the fundamental oxygenprocessing
energy-factories that permit us all to fend off the Second Law of
Thermodynamics by exploiting the materials and energy around us. The
prefix "eu" in Greek means "good," and from our point of view the eukaryotes
were certainly an improvement, since, thanks to their internal
complexity, they could specialize, and this eventually made possible the
creation of multicelled organisms, such as ourselves.
That second revolution—the emergence of the first multicelled organisms—had
to wait 700 million years or so. Once multicelled organisms were
on the scene, the pace picked up. The subsequent fan-out of plants and
animals—from ferns and flowers to insects, reptiles, birds, and mammals—
has populated the world today with millions of different species. In the
process, millions of other species have come and gone. Surely many more
species have gone extinct than now exist—perhaps a hundred extinct spe-cies
for every existent species.
What is the overall shape of this huge Tree of Life spreading its branches
through 35 billion years? What would it look like if we could see it all at
once from a God's-eye view, with all of time spread out before us in a spatial
dimension? The usual practice in scientific graphing is to plot time on the
horizontal axis, with earlier to the left and later to the right, but evolutionary
diagrams have always been the exception, usually plotting time on the
vertical dimension. Even more curiously, we have accustomed ourselves to
two opposite conventions for labeling the vertical dimension, and with these
conventions have come their associated metaphors. We can put ear-
lier on top and later on the bottom, in which case our diagram shows
ancestors and their descendants. Darwin used this convention when he spoke
of speciation as modification with descent, and of course in the title of his
work on human evolution, The Descent of Man, and Selection in Relation to
Sex (1871). Alternatively, we can draw a tree in normal orientation, so it
looks like a tree, on which the later "descendants" compose die limbs and
branches that rise, over time, from the trunk and the initial roots. Darwin also
exploited this convention—for instance, in the only diagram in Origin—but
also, along with everyone else, in uses of expressions that align higher with
later. Both metaphor groups coexist with little turbulence in the language
and diagrams of biology today. (This tolerance for topsy-turvy imagery is not
restricted to biology. "Family trees" are more often than not drawn with the
ancestors at the top, and generative linguists, among others, draw their
derivational trees upside down, with the "root" at the top of the page.)
Since I have already proposed labeling the vertical dimension in Design
Space as a measure of amount of Design, so that higher = more designed, we
must be careful to note that in the Tree of Life (drawn right-side-up, as I
propose to do ) higher = later (and nothing else ). It does not necessarily
mean more designed. What is the relation between time and Design, or what
could it be? Could things that are more designed come first and