ronment, excretes waste material of lower chemical energy, and surfs
the energy difference between food and shit to go on living. Life is a
breakfast cereal, a board game, a very long sentence, a bitch and then
you die. I’ll let you in on a dirty little secret: We don’t really know what
life is. We may as well try and catch the wind as pin life down with a
tidy definition.
Even if we found a decent definition that worked for all life on Earth,
we wouldn’t know if it applied anywhere else. We have no outside per-
spective. The fact that we have only one form of life to study was not
obvious when biology was a new science. After all, trillions of diverse
creatures are on Earth to compare and contrast. Now we know that
they—and we—are all branches of one sprawling evolutionary shrub
with a single root. Our limited and parochial knowledge of the nature
of life makes any confident statement about life elsewhere an affront to
the scientific method. Nevertheless, we can’t help it, because we so des-
perately want to know about life in the universe. We will study Earth
life with the finest-toothed combs we can find, drawing great and uni-
versal significance from what may be random or unique events.
Of course, we can always use our definition itself to limit what it is
we are looking for, declaring that any extraterrestrial phenomenon that
does not conform is, by definition, not alive. A better approach is to
accept the ambiguity. Though we cannot precisely define life, we can
describe many of its properties and make reasonable guesses about
which ones are universal. It may be that life, like true love, is impossible
to define, but you know it when you see it. And perhaps finding
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extraterrestrial life will be more like falling in love than confirming a
specific hypothesis. When it happens, we’ll know.
H O W I T S T A R T E D
Now that I’ve established that we don’t know what life is, I’ll continue
to describe where we think it came from. After the rains, the first
oceans were laced with amino acids and other goodies. A seething brew
of organic goo began to whoop it up. Carbon chemicals combined in
new ways, evolving without memory or intention, but with plenty of
time. It was a self-organizing organic orgy, each molecule getting it on
with all comers, unafraid of the consequences of complete promiscuity.
For some it proved fatal, but a lucky few wound up in long-term
arrangements of greater stability. Once some of these learned to start
copying themselves, there was no turning back.
The first self-replicating molecules didn’t have to be very good at it.
Any random assemblage that could make even imperfect copies of itself
found its chemical type increasing in number. Structures with self-
replication proclivities became more abundant, interacting and combin-
ing to form new molecules, with novel properties and behavior. Some
of these new models were even better at self-replicating. You see how
this could quickly get out of hand. In the right kind of environment,
with a ready supply of organics and without catastrophic interruption,
what was there to stop it? Maybe this ocean just had to come alive.
We believe that it did. The story goes as follows: Chemical evolu-
tion led inexorably to self-replicating molecules, which in turn evolved
into the first primitive cells. Through Darwinian selection, these cells
evolved into modern organisms.
This statement seems so reasonable and consistent with what we
know about the natural world that we scientists accept it as true. The
problem is, it’s difficult to prove. No one has succeeded in creating life
from nonlife in the laboratory. It would be hard to repeat the experi-
ment the way nature did it originally, because that probably took mil-
lions of years. Even the most patient scientists or the most aimless grad
students don’t have that kind of time on their hands. In our labs we try
various tricks to speed things up: concentrating the most promising
chemicals and adjusting temperature, acidity, or other conditions to
encourage evolutionary activity. We’ve come up with many promising
and suggestive results, chemical brews that point down the path toward
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self-replication. But nothing we’ve created has crawled out of a flask
and introduced itself, or even met the obvious minimal requirements
for a living organism.*
Though we don’t want to admit it, our belief that chemical evolution
can lead to life is still an article of faith. Let’s call it informed faith, to
give justice to the great strides that we’ve made in finding the potential
pathways of life’s origins. The creationism-versus-evolution debate has
unfortunately pushed science into a defensive corner from which we
exude overconfidence, pretending to have certainty in places where
we really have only reasonable inference. Instead of saying, in effect,
“We have proof whereas you only have faith,” we could, more hon-
estly, say, “At least our faith is testable in principle, and wherever tested
has been borne out by observation.”
It is certainly not immediately obvious that the beauty and complex-
ity of life on Earth all came about through billions of years of random
variation and selection. Our prescientific forebears can be forgiven for
their intuitive inference that such a wonderful design requires a super-
human designer. Science has given us reason to doubt this need, but sci-
ence has also revealed the design to be far more intricate, complex, and
finely tuned than anyone imagined hundreds of years ago. Modern
thinkers, too, are reasonable to doubt that natural selection could come
up with all this. If you have never, ever, doubted it, then you’ve never
really thought about it, only accepted the ideology and authority of
your teachers. Within each living cell, from paramecia to paramedics, is
a chemical factory far more complex and elegant in design than the
most sophisticated chemical plant ever built by humans.
H O W I T W O R K S
Without a chemistry book and a chemist, how does nature know how
to construct these intricate factories? What keeps them running?
Proteins. What makes the proteins? DNA.
Life on Earth is largely a game played between two types of macro-
molecules (giant molecules), proteins and nucleic acids (DNA and RNA
are nucleic acids). Each is a long, thin, tangled chain of thousands of
*If we succeed in creating life from scratch in the lab, we may still not know how it actually happened historically. But our belief that it did happen will gain several notches in credibility.
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nearly identical subunits. Because they are made up of an enormous
number of these smaller units, they are analogous to sentences made up
of many letters. In the run-on sentences of protein, the letters are amino
acids. Proteins are the basic structural materials of living organisms.
They are what you are made of—except for some parts such as fa
t,
bones, and teeth, but even these are constructed under the close super-
vision of proteins.
Even more important, proteins control the chemical machinery of
life. Every chemical reaction in every cell of your body—all your life’s
work—is mediated by proteins acting as catalysts. The colloquial use of
this term is closely analogous to its meaning in chemistry. Someone
who is known as a catalyst makes connections, brings people together.
A chemical catalyst grabs this molecule over here and that one over
there and says, “Why don’t you two get together? Let’s make some-
thing happen.”
Proteins are organic catalysts with an incredible ability to recognize
other molecules, pull them together, and moderate their interactions.
They regulate all the chemical reactions that, collectively, we call life.
How do they do this? It all has to do with the unique 3-D shape of each
protein.
A typical protein is made of thousands of amino acids. An amino
acid looks like this:*
They are all exactly the same, except for the side group, here labeled
R, which is different in each one. This amino acid, the simplest, called
glycine, has a single hydrogen atom for its side group:
*H, N, and C are atoms of hydrogen, nitrogen, and carbon, respectively. Notice that each carbon atom insists on forming exactly four bonds with other atoms. Thus the C that is
“double-bonded” to an oxygen.
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Here’s one, called tryptophan, that’s among the most complex:
When thousands of amino acids are strung together in a particular
order, you’ve got a protein. It is the specific sequence of amino acids
that gives each protein its unique abilities. Here’s how: When you string
a large number of amino acids together to make a protein, the different
side groups (the R’s), all dangling off the main string, interact with one
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another. Some of them are strongly attracted. Other pairs find each
other repulsive and can’t get enough distance between them. These
forces of attraction and repulsion cause the protein chain to fold up
into a complex twisted shape.
Imagine a long rope with a large number of cats tied to it at regular
intervals. Some of the cats would try to get as far from each other as pos-
sible, whereas others would seek each other out to fight, groom, or play.
Now, picture this happening in a fluid tank where they all have kitty
Aqua-Lungs, or in the weightlessness of an orbital cat house where all
the cats can move about in three dimensions. The final shape of the rope
would be quite twisted because of the complexities of feline social life.
Proteins become twisted and folded because of the social interactions
among all the side groups of their amino acids. Each protein folds in its
own way, and the final shape is precisely determined by the specific
amino acid sequence. It is the 3-D, folded shapes of these molecules that
give them their amazing ability to “recognize” and bind to other mole-
cules. Each protein has evolved so that its amino acid sequence causes it
to fold up into just the right shape to precisely fit specific molecules and
encourage them to react in ways needed to keep our cells running.
How do the amino acids know what order they should assemble
themselves in to make a protein fold in just the right way to work its
magic? That’s where the other group of macromolecules, nucleic acids,
come in. DNA is a nucleic acid.* It stores and passes down the infor-
mation on how to string together amino acids in the right way to make
the proteins needed by living cells.
The famous double helix is made of two strands of DNA, each built
up from a long string of subunits called nucleotides. Just as amino
acids are the individual repeating “letters” in a protein molecule, the
nucleotides are the letters in DNA. There are only four nucleotides in
DNA: adenine, thymine, guanine,† and cytosine. These four structures
function as the four letters in the genetic code, and we abbreviate them
with the letters A, T, G, and C. The information content of the genetic
code is entirely contained in the ordering of the A, T, G, and C
nucleotides along strands of DNA.
*So-called because Friedrich Miescher, the German chemist who first isolated them in 1869 (while experimenting with pus!), didn’t know what they were but suspected they came from the nuclei of cells.
†So-called because it was first isolated in bird shit or guano. I wonder how many more dis-gusting bodily fluids I can work into the footnotes of this chapter. . . .
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Now for the vital connection between nucleic acids and proteins.
Contained in the sequence of nucleotides composing a strand of DNA
are coded instructions for stringing together a list of amino acids in the
right sequence for making specific proteins. That’s all it is. Nothing
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more and nothing less. The entire genetic code—the information on
how to make all of you out of a single fertilized cell—is a set of instruc-
tions for making a large batch of proteins that will fold into the shapes
needed to get the job done.
Each “word” in the genetic code is three letters long. That is, every
three nucleotides in a strand of DNA codes for one amino acid in a pro-
tein. For example, the sequence GGT stands for the amino acid gly-
cine, and CAA stands for glutamine. So a sequence of DNA reading
CAACAAGGT contains the instructions “Add two glutamines and
then a glycine.” A DNA code for making an average protein contains
thousands of these little words. Inside each cell is chemical machinery
that can read the DNA chain, putting together amino acids in the spec-
ified order. The resulting proteins then promptly fold into their 3-D
shapes and make the “desired” chemistry happen in your cells. If “love
is just a four-letter word,” life is just a long series of three-letter words.
T H E T W I S T
That’s quite a stunt for dumb old nature to pull off—encoding the 3-D
shapes of our all-purpose molecules (proteins) within the linear
sequence of a different molecule (DNA). Why didn’t we think of that?
But, wait, it gets better. DNA molecules can perform another amazing,
essential trick: they can make identical copies of themselves. That gives
us heredity, without which we would still be nothing more than a
skanky brew of chemicals sloshing around in the ponds of a dead rock.
In DNA’s double helix, each coded strand lives in a twisted pair with
another. This allows each molecule to contain a template for its own
reconstruction. The two strands of the double helix are identical except
for the letters of the code, which form a complementary message.
Bonds form between the nucleotides on each strand, joining the two
together like the rungs of a twisted ladder. Each nucleotide reaches
across and bonds to one on the sister strand. Because of their shapes,
; they are choosy about whom they will bond with. G and C bond only
with each other. Likewise, A and T are a faithful, exclusive pair. So,
the sequence of the nucleotides on one strand is exactly specified by the
sequence on the other. Each contains a complete description of the
other’s structure.
When they’re in the mood to replicate, the DNA molecules, with
some protein midwives to help them unzip, sequentially break the
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bonds forming the rungs of the ladder, leaving two naked strands dan-
gling free. The nucleotides strung along the two resulting individual
DNA strings, suddenly finding themselves single again, are quick to
bond to any attractive nucleotides floating by. In this way, each of the
two separated strands immediately builds itself a new partner. But G
will only bond to C, and A will only bond to T. The result? When each
nucleotide along these chains hooks up with its desired counterpart, the
two new double chains are in every way identical to the parent double
helix. Each of these will then build the same proteins as its parent did.
Really, each one is its parent. The parent molecule never died, but sim-
ply replicated. In this sense there is only one molecule of DNA on
Earth. The ones dividing right now in your toes and in the grass
beneath them are all pieces of the original founder. And so it divides,
never forgetting, forever and ever, amen.
Image unavailable for
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To recap: What keeps the complex chemical factories in the cells of
all Earth life running? Proteins. What makes the proteins? DNA. How
does DNA copy itself? Proteins. A very clever design, indeed, yet it
seems to have arrived here through evolution by natural selection.
When I learned the details of this surprisingly complex machinery for
the first time (and here I’ve only scratched the surface), I felt that intel-
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lectual honesty required me to rethink my opinion of evolution.
Doesn’t it strain credulity to think that the intricate, streamlined, fan-
tastically clever, and totally uniform building code found in all life, even
its simplest known forms, could ever come into existence through such
an aimless process? I’ve heard this many times, and I’ve thought it
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