When I was pregnant with my first child, I was shocked by the idea that it required no thought at all for me to sculpt a whole other person. That it was somehow built into my species - all species - to generate copies of itself, not identical, of course, but remarkably alike in the spectrum of life.
I wanted perfection. I imagined the baby's growth as a slow-motion, silent explosion inside me, neat and orderly, like a chain reaction. I willed one perfect cell to form, and another, a million perfect cells, body shape uncurling or building up, beginning with bones - white weightless wires, naked at first, then growing the thinnest silk of flesh. Cell after cell, the bones would gain heft, the muscles thicken to fill the spaces of the network.
But at night I worried about what I held. The genetic burst that delivers the knack for numbers or musical talent, the stray taste for chemistry, the hand for drafting or bones for dance, can as easily blight limbs or punch a hole in the heart. That my sister Beckie had been born microcephalic - head too small, body twisted, intelligence trapped in infancy - blunted the expectant buzz of maternity. Beckie had slipped from dark into light, a primal piece of life, shaped and unshaped. Over the years I watched her helplessness drive my mother to resignation, at times to despair. My mother's grief belonged to the phylum of fatigue, the problem of how to get through the days, seasons, years, with a child who would never feed herself, never talk or walk without help, never know the childish pleasure of naming dinosaurs or birds.
It was little comfort knowing that the root of my sister's deformity lay not in ancestry but in some other secret, perhaps a first-trimester virus, a wayward scrap of DNA that found its way into my mother's blood and unhinged her baby's whole development. I was told by relatives how unlikely it would be for a family to suffer twice in this way, as if family misfortune could be drawn off in one member, the others somehow proportionally relieved. Though I knew my chances of having a child like Beckie were no greater than those of any other mother, I did not hold much stock in odds.
My imagination played in a thousand dark ways. The horrors would gather in me as a migraine does, a flicker of imminence before I knew it was there. The baby would grow in the S-curve of a serpent, a sheathed continuity of form without limbs. Or it would grow from one end only, lower limbs and torso withered, birdlike or less, enormous swollen head pushing into my lungs until I gasped for breath. I imagined profound deformity drifting there in the dark, a clot of furled flesh. I dreamed and dreamed again of washing my blotted baby - no, sister, no, baby down the drain, methodically, every last bit, then realizing in heart-stopping horror what I had done. In and out of my dreams floated the grotesque image of an ant lion, that insect with a name like an oxymoron or an odd chimera from my childhood bestiary. In its adult form Myrmeleon immaculatus is lovely, long-bodied like a damselfly, with a pair of ethereal wings. Its larva, the ant lion, is a queer, wedge-shaped thing with long, bristling sickle-like jaws and a nasty habit of sucking the juice from its prey. Metamorphosis is the law of the universe. If an insect can contain both winged elegance and hideous nymph, why couldn't this germ of a baby?
I taunted myself with medieval ideas that pregnant women transmit marks of their fancies to the bodies of the children in their wombs, old notions that my dark thoughts themselves might unfavorably alter the form of the fetus. As antidote I thought of dancers and athletes. I summoned the nudes of Leonardo and Michelangelo, bodies in every conceivable attitude exploding with perfect proportion, form, and grace. Still, it took a wing stroke of will to loose the nightmares and find peaceful sleep.
In the immense animal pressure of labor, my fears evaporated, and when my daughter arrived, sweet and sound as a nut, I accepted her perfection as given.
During those nine months on guard came a day that was significant in the history of biology, because it was the day on which the discovery of a new creature was announced: a little organism never before seen, living on the lips of Norway lobsters. The new creature was called Symbion pandora: Symbion for its intimate life with the lobster; pandora for the bizarre box-within-a-box form it takes during one of its life stages. Less than a millimetre in length, it looks like a Lilliputian sac, its foot a sticky disk, its mouth a splendid circle of whirling cilia, with an anus just adjacent. So radically different in form is Symbion pandora from any other animal that science created a special phylum for it: Cycliophora.
The unearthing of a new species is not a rare event, of course. New kinds of insects, molluscs, even mammals are turning up everywhere we look. People living on the island of Panay in the central Philippines found new species of rodent, the Panay cloudrunner, an agile squirrel-like, nocturnal creature with the shrill cry of an insect. In the Annamite Mountair bordering Laos, a new species of striped rabbit was discovered, and from the underbrush along the backbone of the Ecuadorean Andes, a hitherto unknown bird, plump, long-legged, a kind of antpitta with a haunting, hollow, hiccuping call. Scientists exploring the sea's dim recesses lately clapped eyes on a giant squid, not a new species, but an animal so elusive as to disappear for centuries and reappear only in the guise of a monstrous tentacle washed ashore. Architeuthis, with eyes the size of dinner plates, looks like something etched in the sea foam on an ancient map. News of smaller but no less interesting quarry came not long ago when marine biologist plumbing a patch of deep-sea ooze off the coast of New Jersey hauled up buckets of worms, jellyfish, anemones, corals, snails - close to eight hundred species, more than half of them new to science.
But the discovery of a whole new phylum! The animal kingdom has only thirty-five basic phyla, each defined by the distinctive body plan of its members. Those hundreds of ooze-dwelling New Jersey creatures, stunning though they were, all fell neatly into known phyla with familiar body plans: nematodes, annelids, sipunculans. Symbion pandora, on the other hand, commanded its own singular category, a taxonomic Oscar. And it turned up not in a remote mountain spur or the abyssal depths of an oceanic trench or in the recesses of the last surviving rainforests but right under our noses.
In Linnaeus's day, the number of recognized species was about twelve thousand; today the number of named plants and animals is roughly two million. Into this vast, sprawling scheme has crept the fact of Symbion. The discovery of the little oddity from the lips of lobsters, which demands not just a place on an obscure branch in the tree of life but a whole new bough, pleases me immensely - more so than, say, the sighting of a new planet. Animals exert a special hold on the human mind. Studies of the brain suggest that special areas in our gray matter are highly sensitive to certain categories, chief among them animals. People who have suffered damage to a region of tissue at the back of the brain fail to recognize skunks, tigers, cats. Those with damage in a different patch can recognize the animals but can't recall their names. The theory goes that the two patches communicate. When we see a skunk, the cluster of brain cells specializing in animals may tweak the neurons in the brain region where the word "skunk" is retrieved. I like this notion that animals possess substantial niches in our cerebrum, one for themselves, one for the names we give them.
Symbion pandora. With a name like yours, you might be any shape, almost. Symbion looks for all the world like a toe loosed from the old life. It reminds me of those dreamlike creatures uncovered in the fossil beds of the Burgess Shale in the Canadian Rockies, eight thousand feet above sea level: five-eyed Opabinia; Anomalocaris, with a mouth like a circular nutcracker; Hallucigenia, its bulbous head and body suited to its name; flattened Amiskwia; elegant, segmented Pikaia. In these limestone remains of an ancient sea, life that turned to stone six hundred million years ago, is a fantastic diversity of forms: insects, earthworms, molluscs, which seem to have come into the world all at once. The generous bloom of new phyla in the Cambrian period has no equal, not even in the times following the mass extinction at the end of the Permian period, when 95 percent of all marine species disappeared, and there must have been a lot of empty niches to fill.
Just what sparked the explosion is still a matter of debate: a rise in atmospheric oxygen, shifts in ocean currents, the opening of warm, shallow seas, or, possibly, polar wander - a listing of the Earth, which sent continents soaring across its surface, turning topsy-turvy entire ecosystems and opening the door for new species. One of my favorite theories suggests that the Cambrian bloom had distinctly humble roots. At some point more than half a billion years ago, when organisms evolved a one-way digestive tract that expelled fecal pellets, the ocean was chemically transformed in a global rain of feces, which opened up the seas' depths to colonization by myriad organisms.
Whatever the reason, the Cambrian era produced a menagerie of forms, all the major body plans of today, even the weird plan of Symbion pandora. (Scientists have in their hands the fossil of an arthropod from the Cambrian period that looks as if it may have hosted like-bodied ancestors of Symbion.)
The world is full of creatures if not new in shape, then strange. I think of the ant lion, the hammerhead shark, the pangolin, with its serpentine tail bound to a body like an artichoke. Even within a single phylum, the possibility of form seems infinite in all directions. Vertebrates alone possess a profusion of shapes - hagfish, platypus, spiny anteater, slimy sculpin, star-nosed mole, with its radial snout of twenty-two fleshy rays that grope through dark soil for edible morsels. Arthropods, with their hundreds of thousands of species in marvellous shapes, put the rest of life in the shade. As Jorge Luis Borges wrote, "The zoology of dreams is far poorer than the zoology of the Maker." And when you consider that what lives today represents only a tenth of all known forms of life, when you take into account all that have vanished - animals with myriad wings, teeth, horns, and tails sculpted by natural selection - the panoply of living forms seems almost unimaginable.
And yet the basic body plans for all were present in the Burgess Shale. Not a single new one has appeared during the last half billion years, and some have gone the way of Opabinia, into evolution's dustbin. In fact, as Stephen Jay Gould has written, a cardinal feature of modern life is stereotypy, the cramming of millions of animal species into just a few basic anatomical plans.
What I had been wishing for during those worrying days of pregnancy, beyond full-size brain and sufficient limbs, was bilateral symmetry, one of those ancient anatomical plans, the body tidily divided into like halves along an axis from head to toe. There are other forms of architecture, of course. The radial elegance of the medusa bell, the cephalopod, the starfish with arms extending in all directions. Or the spiral, the circle unwound and set free: the sinistral spiral of a honeysuckle vine, the horns of wild sheep, the cochlea of an ear, the human umbilical cord, and, especially, the shells of certain molluscs, expanding geometric figures that sweep out in coiled edifices of exquisite beauty. Radial symmetry is an ancient form of organisation, one eminently sensible for the sessile creature, allowing it to feed in a circle and respond to danger from all directions. But the human notion of fine form is bound up with the bilateral, the symmetry of left and right, the pairing of limbs for efficient locomotion and for beauty, the balanced hips and legs, the eyes equidistant from the nose, the lips curling out from a precise central axis on the face.
The majority of animals are bilaterally symmetrical, including flat-worms - small brown fleshy blotches that look only one step up the evolutionary ladder from mud - as well as the wonders of the Burgess Shale, even Symbion pandora. We are not alone in relishing the trait. Many animals see the beauty of bilateral symmetry and prefer it: apes, dolphins, birds, even bees. Bucks with the biggest, most symmetrical antlers boast the biggest harems. Female swallows select mates with perfectly symmetrical forked tails. Japanese scorpion flies prefer the scent of a male with a symmetrical body. Lately, we've heard the somewhat disturbing news that symmetry may reflect the fundamental health of an individual, the strength of immune cells, the robustness of genes. One study showed a modest link between bodily asymmetry and lowered IQ; another went so far as to suggest that men with asymmetrical hands had low sperm counts and poor sperm motility. A developing organism stressed by poor nutrition, disease, inbreeding, or almost any genetic defect will exhibit some visible asymmetry. Perfect symmetry, in turn, may signal internal well-being. Despite my mother's arguments to the contrary, it seems, beauty is no weak guarantee.
And yet, in the throes of gestation I had also been wishing for asymmetry. Our insides hardly reflect a looking-glass world. Beneath the skin, symmetry vanishes. Heart, liver, spleen, pancreas, gut, all lie to one side of the body's midline, and those organs themselves are largely lopsided, the intestines tortuously looped and coiled to fit neatly in a small cavity, the heart divided into four irregular chambers and netted with a maze of curving blood vessels that send blood through it in swirling patterns. The inner body's extravagant asymmetry arises early in embryonic development; if it didn't, the outcome would be decidedly unlovely.
Early thinkers offered various explanations for how the body is shaped. An ancient Indian source declared the embryo to be fashioned from semen and blood, with the firm parts of the body coming from the father and the soft ones from the mother. In his Generation of Animals, Aristotle wrote that "the female provides the material, the male provides that which fashions the material into shape" in much the same way that a carpenter carves wood into a bedstead. Pliny (that Roman encyclopedist with the fatal desire to see the eruption of Mount Vesuvius) wrote this in the first century:
Bears couple at the beginning of winter, and not in the usual manner of quadrupeds but both lying down and hugging each other; afterwards they retire apart into caves, in which they give birth on the thirtieth day to a litter of five cubs at most. These are a white and shapeless lump of flesh, little larger than mice, without eyes or hair and only the claws projecting. This lump the mother bears slowly lick into shape.
(Natural History, Book VIII, p. liv)
At least Pliny gave credit to the bear. The medieval Women's Secrets presented the prevailing belief that heavenly bodies shaped human ones. In the third month of gestation, Mars divided the arms from the sides in the developing fetus, the neck from the arms, and formed the head. In the fourth month, the Sun created the heart. In the fifth, Venus perfected the ears, nose, mouth, and penis, and caused the separation of fingers and toes. Monsters of nature, caused by celestial influences, resulted either from too little heavenly matter or from too much; "in this way people are born with two heads or six fingers on one hand." (The book later notes that some monstrosities are caused by irregular positions during coitus: "I have heard tell that a man who was lying sideways on top of the woman during sexual intercourse caused the woman to produce a child with a curved spine and a lame foot.")
In the sixteenth century, certain natural philosophers claimed that they could detect in the head of a sperm a tiny person, a homunculus. This minute creature needed only to be transported into the woman's womb to grow. In their view, the first embryo of a species contained all future embryos - like those Russian dolls, ten wooden women in peasant dress, one within another, from the great bulbous Eve to a woman the size of a bean. Homunculi were nested inside homunculi ad infinitum, all the way back to the earliest embryo, a very small woman indeed. Development, then, was little more than a swelling up of the already complete being.
Paracelsus, the sagacious Swiss alchemist and philosopher of the great and small, held with the homunculus idea, and revealed his formula for creating one: "If the sperm, enclosed in a hermetically sealed glass, is buried in horse manure for about forty days and properly magnetized it begins to live and move. After such a time it bears the form and resemblance of a human being."
Over a century later, in 1677, Anton van Leeuwenhoek, who first saw sperm as parasites, seemed to accept this homunculus theory. Leeuwenhoek rarely described wonders that were not there, but one day, when he put a specimen of semen beneath the lens of a new and powerful microscope, he believed that he saw in the spermatozoon the outlines of a sort of embryo. A clamor of philosopher-scientists then claimed that they, too, had seen under the microscope minute forms of men in the semen of men, horses in the semen of horses, cocks in the semen of cocks. One went so far as to say that he had seen in a drop of donkey semen some very large ears.
The notion that a whole being - avian, equine, human - may be tucked into the head of a sperm, and, if lucky enough, may find an empty egg, climb in, lock the door behind it, and grow, is so appealing that it is easy to see why people were loath to abandon it. This was a generation that believed that mice arose from piles of old clothes, that geese sprang from barnacles, and that lower forms of life issued out of meat, mud, or slime. One could accept such magic in a self-contained universe - where the Earth was but a cup, the sky a cover, plants and animals frozen into their existing shapes; where the size of a creature had no lower limit. One could take life as it hatched.
To us, the analogies by which these early philosopher-scientists understood the birth of body shape are farfetched. Yet are they more so than the astonishing leap of self-organization that is the real genius of the embryo?
In the 1980s biologists zeroed in on some of the genes critical to this feat. One class, the Hox genes, shape the head-to-tail pattern of the body in the first few days of development, organizing it into front, middle, and hind region, determining the location of head, chest, and lower body, the general placement of limbs, digits, and organs.
The secret of Hox genes was revealed through a set of experiments with mutant fruit flies. The fruit fly, Drosophila melanogaster, originally came from a tropical region of Africa but has been cosmopolitan for some time, spoiling the bananas and peaches on kitchen counters and buoying the studies of biologists everywhere. The tradition of fly genetics goes back a hundred years to the discovery in Thomas Hunt Morgan's laboratory of a spontaneous mutant with white eye color. Morgan saw the fly, with its lightning-quick life cycle and prolific offspring, as a means for testing Mendel's laws of genetics. Morgan and his flies revealed the secret that genes are arranged on chromosomes in linear fashion, and that their order can be mapped by tracking the pattern in which genetic traits are inherited.
To understand how genes work in development, scientists expose their subject to X-rays or harsh chemicals that cause mutations and then note the effect on the descendants - how the progeny of the exposed creature grow, what sort of anomalies appear, such as normal body pieces growing in decidedly abnormal places or one organ changed into another. By looking at the irregularities in the offspring, they can deduce what the genes were supposed to do, and thereby determine their role in normal development. In the fruit fly, for instance, mutations in the Antennapedia gene cause legs to grow in place of antennae. Mutations in Proboscipedia make legs develop in place of probosces. These genes were called "homeotic" (from the Greek homeo, meaning "alike") because of their ability, when mutated, to transform one body part into the likeness of another. A mutation in one of these genes can cause radical change in a creature and spontaneous abortion.
When biologists studied these homeotic genes in detail they found, tucked within each, an identical fragment of DNA, sharply defined, as though enclosed in a box. They called the fragment a homeobox; the developmental genes that harbored a homeobox were called Hox genes. Now it's understood that Hox genes belong to a family of hundreds of different kinds of genes, all possessing a homeobox - a snippet of DNA that encodes a protein with one of those ancient keystone shapes, a homeodomain. This little "helix-turn-helix" twist can literally grab hold of the DNA inside a cell and control its fate.
As an embryo takes shape, each cell must know where to go and what to be, when to make specific chemicals - head-forming proteins, for instance - and when to shut down the production. When Hox genes switch on inside a developing cell, they help shape its identity. The protein they make screws itself into a groove in the double helix and switches on other genes that participate in the cell's development, setting off a cascade of biochemical activity that ultimately directs the body plan.
Since the discovery of Hox genes in fruit flies in 1984, more than a hundred such genes have surfaced in a broad variety of animals. Biologists "fish" for them, using the fruit fly genes as probes. To their astonishment, they have found Hox genes in sea urchins, worms, mice, birds, cows, humans, suggesting that a Hox gene or gene cluster existed in primitive form in an ancestor common to all living animals. (The genes have even surfaced in plants, although they're not thought to shape body plan the way they do in animals.)
The number of Hox genes differs from creature to creature. Humans have thirty-nine, most of them clustered in four sets on four different chromosomes; invertebrates such as fruit flies possess just one cluster of eight. But so close are these genes in form and function that when scientists performed the ultimate Frankensteinian experiment, inserting a human Hox gene into the embryo of a developing fruit fly, the human gene made a perfect little fly body.
And here's a surprise to vindicate those early philosopher-scientists and their beliefs about the origin of form. It turns out that Hox genes line up along the chromosome in roughly the same order as the parts of the body whose development they affect. Genes at one end of the line control the emerging head; genes in the middle, the abdominal segments; genes at the other end, the hind region. In a sense, then, the Hox genes represent a kind of biochemical homunculus tucked into the nucleus of early embryonic cells.
Hox genes don't act alone in determining body shape. Chemicals called morphogens, "makers of structure," working in the early stages of development, ooze slowly through the embryo, helping to establish compass points, tell head from tail, up from down, left from right. These morphogens are part of that complex system of communication among developing cells in the fetus. Some are the products of Hedgehog genes, so named because of the prickly look of fruit flies with mutated forms of these genes. (The names of developmental genes are nothing if not metaphoric, and create a rich taxonomic tree in my head: late bloomer, hunchback, hairy, deadpan, daughterless, frizzled, wingless, lunatic fringe.)
Hedgehog genes, like Hox genes, are common among different animals. The morphogens they make work along a gradient, diffusing as they ooze. Developing cells read the strength of these chemicals to determine where they are and what they should become - leg, wing, fin, brain. Biologists still have only a ghost of an idea how morphogens nudge a cell toward one fate or another, but they suspect the proteins stimulate other genes. One morphogen playfully named Sonic Hedgehog is secreted on the left side in a clump of embryonic cells. With the help of other whispering genes - nodal, lefty, activin, snail - it induces the left-right asymmetry of the vertebrate heart. One new theory suggests that this process is helped along by cilia, those whiplike hairs on the outer membrane of certain cells. The theory goes that the counterclockwise twirling of cilia creates a flow of morphogens that tells the embryo its left from its right.
Together, the proteins made by Hox, Hedgehog, and other genes create a significant chemical inequality that divides the embryo into front and back, bottom and top, left and right, limbs and head. It is this splendid chemical injustice, this early asymmetry, that we must thank for all the pattern we possess.
Imagine if one had to take conscious charge of this task. I'd run too many veins, drop whole skeins of nerves. I'd be asking whether this thimbleful of cells we had packed at the start were enough to make a whole limb. I'd worry that the yaw of that lung, the twist of this blood vessel, would get out of hand; shouldn't we shift that finger a touch to the left? No sculpture would be harder to shape than this.
Fortunately, an embryo generates the laws of its own becoming. With the help of a stable, conservative set of genes, the transformation from single cell to fully shaped being repeats itself precisely, generation after generation, in species after species, from worm to human. In fact, it is the rock solid reliability of Hox genes that makes possible life's running sea of forms. If each new species had to reinvent the basic mechanism to control body pattern, there would be no time to evolve novel features, like wings, hooves, hammerheads, sicklelike jaws, serpentine tails, circles of whirling cilia. Nature improvises on a common theme.
The proliferation of primordial Hox genes half a billion years ago - or perhaps changes in the way they were used - may have sparked the explosion of complex anatomies in the Cambrian age. The genes must have evolved about the time body plans were diversifying, and their sequences have since held ground, like the body plans they help to create.
Some biologists suspect that a random mutation in the DNA of one of our ancient relatives half a billion years ago gave the creature a second set of Hox genes. The first set kept to its original role in shaping the body; the second set allowed the creature to evolve a fancier head, one packed with paired sensory organs and a complex brain - the forerunner, presumably, of our own massive lump of grey matter, with its specialized niche of tissue for animals, its need for nomenclature, its love of music.
For a long time it was believed that we vertebrates were born with our full complement of brain cells, and those that died were lost forever. (I've always thought it a delicious justice that birds are an exception to this rule. As they learn the songs of their species, new brain cells continue to bloom.) But lately has come promising news that our brains may indeed grow new cells throughout life, in the hippocampus and even in the neocortex, the part of the brain associated with learning and memory. Running boosts the growth of new nerve cells, at least in mice. And listen to this: rats engaged in the act of mothering sprout abundant new brain cells and do better than virgin females in tests of learning and memory, the result of hormones, perhaps, or the sheer metamorphic experience of motherhood. And, of course, while our neurons live, they go on making new connections or rearranging old circuits in response to experience. I like to imagine the molecular network created as I contemplate tiny, peninsular Symbion or consider the oozing diffusion of morphogens. Or when I look at my daughter's hands spread like starfish as she nurses, the twinned pairing of her limbs, bowed out in relaxation, the soft contours of her fat little symmetrical face.
(c) Jennifer Ackerman 2001