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Preface
There
are mysteries in all families. Those that arrest me, that set me back
on my heels, are the mysteries of heredity -- the past whispered in bone
and blood; the dozens of ancestors rolled up in one skin, to be read in
“curve and voice and eye,” as Thomas Hardy wrote, “the seeds of being
that heed no call to die” but turn up again and again on the doorstep
like a ne’er-do-well uncle. It seems astonishing that a sweep of eleven
generations hardly modifies the night blindness of one family or the trembling
jaw of another, that fifty or a hundred years may fail to alter a familial
pattern of whorled eyebrow or “wolf’s” teeth, the musical genius of the
Bach family, or the dimpled chin of my husband’s tribe.
In
the last decade or so, a startling new message has come out about the
long hold of heredity. Members of the human family carry traits that have
held on down the line not just for generations but for eons, traits that
mock all boundaries of time and kind. Scientists probing the deep workings
of organisms from yeast to humans have turned up news that despite our
outward differences of life and limb, we are run by similar genes and
proteins, similar cell parts and mechanisms, which have weathered evolution
over ages, passing nearly intact through hundreds of millions of years
of rising and falling forms. These shared molecules and routines affect
nearly all the turnings of life, from birth and growth to perception and
behavior.
This
book is a pilgrimage to the heart of heredity. It is a natural history
not in the literal sense of a systematic inquiry, but rather in the etymological
sense, a telling of stories about life, lineage, chance, and fate; about
family, kin, and kind. It explores both the projecting traits of the human
family -- the one we’re born into and the one we create -- and also the
bigger, deeper inheritance that ties us to the rest of life in profound,
even shocking ways.
.
. .
I
like to hang around the doorway of biological surprise. For years I have
collected news of curious findings, of young spiders that eat their mothers,
of a giant fungus infecting miles of Michigan forest spawned by a single
spore in the last ice age, of fish with fingers, caterpillars with lungs,
genes with secrets. I don’t profess to worship everything, but I do harbor
strange sympathies fired by such discoveries, a kind of naturalist’s faith.
This is the news that sweeps me away, the gnomic workings of the living
order, nature’s inventive jack-in-the-box surprises that shift our view
of life like the sudden twist of a kaleidoscope.
Here
is an item from my files. When scientists deciphered the intimate details
of mating in yeast, that single-celled fungus that raises our bread and
brews our beer, they got a shock. The molecule that draws two yeast cells
into sex closely resembles one made by our own brain cells to regulate
reproduction.
The
likeness seemed a fluke at first. But then other examples popped out of
the box: genes that shape the bodies of fruit flies so like our own body-shaping
Hox genes that one can put a human Hox gene into a developing fruit fly
embryo, and it will carry out the job of the fly’s gene without a hitch;
genes that shape the marvelous globe of the human eye strangely similar
to those that carve the compound eye of a fruit fly; the tiny genetic
mechanisms that drive our biological rhythms, keeping us in tune with
the big swings of night and day, matching those in algae. So, too, do
we share with other organisms the ancient genes that dictate cell death,
the phenomenon that underlies metamorphosis, turning tadpoles into frogs
and caterpillars into butterflies and also shapes our bodies, whittling
away the webbing between fingers before birth, eliminating inappropriate
sexual organs. Common to all of us, as well, is a suite of small, sturdy
messenger molecules, offering clues to such mysteries as why the cells
of the human brain respond to the chemical messages of the poppy plant
and to the potent sexual attractants of a Himalayan deer.
What
are chemicals found in the human body doing in plants, fungi, bacteria?
How can genes that shape a fruit fly be near twins of my own?
Disparate
organisms, it seems, are more radically alike than we ever imagined. Our
deepest selves -- our very cells and molecules - - are alive with reminders
of old, enduring connections with other creatures, resemblances that run
right down to the root of the tree of life. These items of shared inheritance
have formed a library of wonders in my mind’s eye. That there is a certain
sameness among life’s various forms follows from the notion that we all
arose, ultimately, from a common ancestor. We are shaped by fate, by what
came before. But life has chanced to venture in wildly different directions.
In learning to suck energy from sunlight and in swallowing shocking amounts
of oxygen, in heaving up from the beneficent chemical crucible of the
sea and in exploring leafy interiors and desiccated desert, life has split
into discrete identities, strewn about fresh designs, unimaginably varied
feet, teeth, tongues, antennae, wings, leaves, brains.
In
this world of dreamlike change, the lexicon of genes, like human languages,
is thought to evolve along unreturning tracks. We know that nature is
constantly making random changes in almost all genes, and that two species
that diverged from a common ancestor hundreds of millions of years ago
are likely to have accumulated a lot of little alterations. As eons pass,
so do variants of genes, vanishing on the same wind that took the tyrannosaurs.
It seems strange and wonderful that among organisms so spirited with individuality
and detail -- pepper frog, salp, dragonfish, basset hound -- there should
be so much solid common ground.
Over
the last few years I have wandered the body, looking for these legacies
and slim continuances, seeking to ferret them out of their holes and sun
them a little, to brush their surface in places, give them a stab or pinch
them to the bone if I could. I have tracked the labyrinthine world of
laboratories, too, asking about the molecular bricks that underlie the
splendid medley of living forms: what makes them work so beautifully that
they have demanded little change in hundreds of millions of years? What
happens if they go awry? If organisms of such diverse stripes are made
of similar genes, how is newness born in the world?
By
exploring this deep-down world, I hope to create new shelves in my mind
for the recent profusion of genetic discoveries, the news of the sequencing
of genomes from the tubercle bacillus to Homo sapiens, the findings of
genes linked with cancer, Alzheimer’s disease, migraine, and baldness,
passed down from father to son, grandmother to granddaughter; genes affecting
intelligence, sexual preference, spatial ability, anxiety, sense of well-being
-- some of them discovered in small, so-called model organisms such as
worms, fruit flies, mice.
What
is a gene, anyway? Are there genes “for” particular traits? Are the letters
DNA and RNA an Open Sesame to all the familial secrets of life? Can we
starve all of nature’s mysteries into molecular oneness, explain the fruit
solely by its root?
And
what does one make of the notion that our genes mirror those in yeast?
Two decades ago scientists discovered that humans and chimpanzees appear
to have in common about 98 percent of their DNA. Chimps are one thing,
yeast is quite another. The news that, when it comes to molecules, we
are so perilously close to our tailed, finned, and spoorish brethren goes
against the stories I grew up on, biblical tales of human supremacy and
uniqueness, stories of how I was “fearfully and wonderfully made,” as
it is written in the Psalms, to get up before the sun and buy a river,
to buzz above all creatures, “over the fish of the sea, over the fowl
of the air, over the cattle, over every creeping thing that creepeth upon
the earth.” Fear not therefore: ye are of more value than many sparrows.
(Luke 12:7)
We
have for so long picked ourselves out from the horde of other creatures,
reckoned ourselves the peak and point of nature’s whole history. What
to do now, with this news of our deep-down similarity, our profound kinship,
with “lowly” organisms?
The
physicist Michio
Kaku once wrote that finding the key to weather and seasons required
a leap into another dimension, up into outer space. Understanding humanity’s
place in the matrix of life requires just such a leap, but downward, into
the diminutive world of genes and cells.
Raised
as I was on gerbils and birds, on the love of the whole organism, not
its microscopic parts, I find it a stretch to descend into the darkness
of a molecular world. I know my bats, weasels, and wood frogs far better
than I do the crabbed atoms of a hemoglobin molecule. I am far more comfortable
exploring the elements of the violet family than those of the periodic
table. The human mind may have mastered the black hole and the quark,
but most of us have difficulty grasping the very big and the very small.
We tend to think easily only of things on our own scale, midway between
the atom and the sun. The first microscopists, confronted with the bizarre
creatures swimming beneath their lenses, sought desperately to see bodies
like their own, searched for sign of head or tail, denied as long as they
could the many orifices and multiple stomachs, the brainless chunks of
transparent flesh. So, too, we may seek in vain the familiar in the minute
parallel planet of genes and proteins.
To
make things worse, the language of this world veers into the cold domain
of chemistry, where the common nouns are “nucleic acids” and “amino acids”;
the common verbs, “regulate,” “synthesize,” and “catalyze.” One scientist
grappling with the absence of a precise definition for the term “gene”
offered this to snarl the brain: “It is the nucleotide sequence that stores
the information which specifies the order of the monomers in a final functional
polypeptide or RNA molecule, or set of closely related isoforms.”
But
despite the dull terminology used to describe it, the cosmos of molecules
and cells has surprising beauties and minute dramas every bit as beguiling
as those of a bushmaster or a Bengal tiger. In DNA, proteins, even in
the molecules of water encapsulated in our cells, are shapely details,
beautiful clues that hold the key to everything from the acuity of the
eye to the memory of the immune system. In their daily workings are tales
of seduction, compromise, duplicity, deception, stubbornness, art, magic,
death.
I
first learned of the Hox body-shaping genes when I was a few months pregnant
with my second child. The idea that the molecular mechanisms shaping my
baby’s growth were the same as those fashioning the fruit fly I found
oddly comforting. Think of all the bending and breaking in the boughs
of life. The notion that species as remotely related as humans and flies
are shaped by the same genes -- genes that have slipped in and out of
the Cambrian, the Devonian, the Permian, the Pleistocene, requiring little
revision in all that time -- suggests that they must perform their task
beautifully and will not easily be wrenched off course.
Fish,
fruit flies, wondrous babies: we may be a feast of distinct entities,
but we share the odd economies of nature from birth to death. I’m thrilled
to find that we’re connected with other organisms, not by something as
vague or slippery as animal nature, but by a strong ribbon of measurable
molecules, molecules so alike that they can be swapped between species
separated by half a billion years of evolution.
I
think our minds are built for the pleasure of discovering likenesses or
links between vastly different things. It is why we delight in learning
that the words “fate” and “symphony” share an ancient root meaning “to
speak”; that the opening of Beethoven’s Fifth neatly repeats the call
of the white-breasted wood wren; that pointing a single finger to draw
attention to something of interest is bound tightly to the learning of
language. (The earlier a baby extends a demonstrative digit, the more
words he or she will know by the age of two.) It is why we love syzygies
and rhymes and why we are undone by Romeo’s words when he finds Juliet
in the tomb and thinks her dead: “Death hath suckt the honey of thy breath.”
It is why we believe Emerson when he tells us that “the world is a Dancer;
it is a Rosary; it is a Torrent; it is a Boat; a Mist; a Spider’s snare.”
The
language of science holds a hunch here. Though some scientific terms are
Latinate and pompous, or simply weedy (deoxyribonucleic acid, for instance,
a great millipede of a term that puts the mind off with its literalness),
there are other terms -- pithy, germinal, long-lived, and prophetic --
that link the unlike and suggest the blooming mysteries of both language
and life. The word “gene” goes back to an Indo-European root word that
meant beginning and birth. This gave rise to the Old English gecynd, meaning
family, kin, or kind. The Greek and Latin variants blossomed into a bunch
of gen words with a multitude of jobs: genus, genius, gender, gentle,
generous, generation, genealogy, genesis. One Latin stem became gnatus,
unfurling into innate, native, natural.
That
so short and spare a word as “gene” would persist through the revolutions
of language and pop up in all these new, masterful forms impresses me.
So do these shared ancestral genes, which are something like word roots.
Knowing them is a way of prizing what is essential in our common heritage.
That we are still abob with these ancient bits of biological wisdom, that
they have endured over eons in creatures as genealogically distant as
worms and widowed aunts, is to me as much a cause for celebration as a
Bach cantata or bird song.
These
fragments of shared biology arose by chance and became fate. I have come
to think of them as points of entry or small portholes through which to
view the natural history of heredity. Or, perhaps, like the scriptural
mustard seed cast into the family garden, from which one might draw radii
to every corner of nature.
This
book is a tracing of those radii. It is a journey in four parts, starting
with the roots of all flesh, the inherited molecules that keep our bodies
and those of other mortals alive and thriving; then moving to the generation
of our being as single, sentient organisms, from our beginnings in sperm
and egg -- those seeds of inheritance -- to the birth of bodies with vision
and the capability of sex. Thence to our relations with other living things,
how we recognize, compete, and conspire with them in the deepest, most
intimate ways to make something more useful, more skillful, more beautiful
than what we might have made alone. And finally to our common passage
through time, from the immediate tick of the present, by which our bodies
stay in tune with the swing of sun and moon, to our long passage out of
the past, from earliest beginnings.
Copyright
© 2001 by Jennifer G. Ackerman
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