Metamorphosis, p.20
Metamorphosis, page 20
But I believe that sea squirts are wonderful. Their tunics can look like grapes or peaches or barrels or bottles. They come in luscious red or bright orange, technicolor, fluorescent, or translucent. Some are hard as a butterfly’s carapace and some even softer to the touch than a slug. And since the cellulose in the tunic can be converted into ethanol, sea squirts might prove a surprising source of biofuel one day. Catching a ride on the hull of ships, or in ballast water or on the shells of mollusks, far-flung species have colonized new pastures from the Mediterranean to the Pacific Northwest. Sea squirts seem completely harmless. Nothing about them gives away just how wild their lives really are.
When a sea squirt egg begins to grow, it first turns into a baby larva that looks a lot like a tadpole. It has an eye and a mouth, and a tapering tail with which it swims. It’s also built around a spine: Precisely 215 cells build a structure called a notochord, one that scientists study today to figure out the origin of vertebrate brains. Within hours or days, depending on the species, the larva ends the charade of apparent independence. Swimming away from the light, it heads downward, where it latches onto a coral or rock. In a matter of minutes the tail will be consumed from within, melting away like an ice pop. The notochord is destroyed, its contents devoured by special digesting cells in the adult. Soon the brain is also ravaged. All the cells that can be reused will be retooled and recycled, to serve the growth and form of the adult. Ready to begin its tethered, plankton-filtering life, the headless, eyeless, limbless sea squirt now emerges, victorious. Only barely noticeable teeth within its syphon give any hint that it ever was a completely different creature. The larva has “sacrificed” itself for its grown-up self, disappearing as if in an act of sorcery. Harboring a dark secret, silently, the sea squirt sways gently in the current.
The situation begs a psychoanalyst, as much as an embryologist: In human terms, it would be like a baby eating itself to become an adult. Except that even that impossible scenario doesn’t do justice to the weirdness of the ascidians. This baby is absolutely nothing like the adult: It looks like a tadpole, with a tapering tail and a head and a face. Most astonishingly, the adult sea squirt is an invertebrate, but the sea squirt larva is a chordate, like us humans.
People think sea squirts are boring, but their life cycle is otherworldly. Not that fate is always entirely determined; as in axolotls, there are species of ascidians who forego their final metamorphosis while remaining free-swimming, sexually reproducing larvae their whole lives.i It’s as if evolution has let them off the hook, given them a pass. Still, I can’t help but wonder: As its tail undulates downward, heading away from the light, is the sea squirt tadpole an independent entity? When its tail begins to shrivel up and its proto-spine to crack, what’s going on in the brain it’s about to lose: Could it be contemplating the meaning of its sacrifice? The adult sea squirt and its tadpole larva have the exact same genome in each of their cells, but they’re neither brothers nor sisters, nor parent and child. What are they then: A saint? A schizophrenic? Attempting to answer the question challenges our ideas of agency and selfhood. As much as we’d like to feel the sea squirt from within, we just don’t have the language to translate its experience into our own lives.
A frog larva compared to a sea squirt larva. Credit: Inna Gertsberg.
Leaving aside the mystery of identity, we are also faced with an evolutionary question: Why should ascidians have participated in what looks like a dramatic reversal? Why start life out as a chordate to become an invertebrate in the shape of a sack? The turn-of-the-twentieth-century taxonomist and fish specialist David Starr Jordan thought he had the perfect answer: The sea squirt had once been a “higher” fish, which, due to its own vices, had unfortunately been “degraded.” The culprit was laziness, or what Jordan and his cowriters called a combination of “dependence,” “inactivity,” and “idleness,” one imagines with a sneer. Alas, “bad habits” are no longer fashionable as explanations for the fortunes of wild animals. So what is going on?
It’s a lazy Saturday, and we’re folding laundry and doing our best to smile. The last few weeks have been tough: Yaeli aching, the kids complaining, and I—nearing my second half century—more engrossed with my Rumi music and myself than I care to admit. A shrink friend of mine tells me that at my age many men go through a kind of disassembling, that their ideal self is slowly replaced with a more accurate self-understanding, and that seeing yourself more clearly isn’t always that much fun.
Later that day I learn that it was a Russian named Alexander Kovalevsky who first connected between the free-swimming tadpole ascidian and the sessile sea squirt, affixed to coral rock. This was in 1866, and both Ernst Haeckel and Charles Darwin were astounded. In evolution ascidians were ancient, going back 540 million years, to the Cambrian. “Thus, if we may rely on embryology, ever the safest guide in classification,” Darwin wrote in his Descent of Man, “it seems that we have at last gained a clue to whence the Vertebrata have derived.”
The next day, at the beach, I come across a starfish. Maybe it’s their gently tapering arms that make them lovable to me, or maybe it’s their proud silence that sways my heart. Maybe I sense in them a kindred spirit, the five appendages splayed in every direction, as if hoping to give the world a hug. My daughter Shaizee loves them too. But I’m pretty sure she’d be shocked to learn the truth about the one we just encountered. Not because this velvety bottom-dweller of the northern seas and Mediterranean is a ruthless predator. Not because it eats members of its own species in the dead of night. What would truly flip Shaizee out about Luidia sarsii is a mind-bending fact about this and other starfish: It can remain a child while simultaneously becoming an adult.
Like any other creature, the starfish embryo begins to develop, one cell dividing into two, then four, then eight, then sixteen. At thirty-two they turn into a little ball, called a blastocoel. In conventional development, the blastula stage is followed by a gastrula, a ball with cells three layers deep. Each layer is destined to give rise to different parts of the developing animal: The heart and muscles will develop from the mesoderm, the skin and nerves from the ectoderm, the endoderm will eventually produce the lining of the gut. Now cascading molecular signals will direct each cell in each layer to migrate in the embryo until everything is in place, and—presto—a creature will be born.
Except that in the starfish it doesn’t work like that. When the embryo reaches the stage of the gastrula, instead of one plan continuing to unfurl on the way to becoming one animal, two plans start to unfold side by side. The “fate maps” in one of these embryos begin to sculpt an incipient larva, while the “fate maps” in the other go about producing a young adult. The description may sound simple enough, but consider what just happened: A fertilized egg that somehow contains not one, but two, developmental programs, has kicked both into gear at the very same time. And while the larva initially grows at a faster pace than the adult, the very same creature goes about building two brains, two hearts, and two anuses.
In the meantime the larvae, called bipinnarians, grow to be an inch and a half long. On one end of their body are two angelic flappy wings, fleshy and transparent; these help them dive down into streams during the day, and glide to eat algae on the surface at night. The other end of the body is a tangled mess of arms that look like the roots of an upturned vegetable. It may not seem that way at first, but as its name suggests, the larva is now a bilaterian: With a front and a back side, a head and tail and left and right, its body plan is symmetrical, just like ours.
Except all the while, inside itself, this tiny larva has been growing its adult. From a cluster of special stem cells in its inner cavity, an alien has been sprouting within. Slowly, a star emerges, with its own brain and heart, appearing on the tangled roots—salmon pink, or bright orange, a tiny splash of color on the pellucid stalk—like a jewel. Rather than bilateral symmetry, it has radial symmetry—the symmetry of a starfish. Gradually, the five arms elongate, with a fringe of white spines. And in a moment of poetic grace—a truly immaculate conception—the adult detaches from the roots and falls, sinking in the water column, like a rock.
This is what’s incredible: Unlike the sea squirt whose larva “sacrifices” itself for the adult, here, the two part ways, each to lead its separate life. The colorful, sexual starfish settles on the sea floor where it will grow a hundredfold, as the angelic inch-and-a-half-long translucent larva just resumes its virginal pelagic life. We’re accustomed to thinking that adulthood always follows youth, and that youth precedes adulthood. But starfish offer a radical alternative. Incredibly, in a single organism, the two can persist, side by side.
Our human notions of identity have once again met their limit. The larva gave birth to the adult, but having sprung from the very same fertilized egg, it is not its parent. It is not its brother, either, or a cousin. The genes in each and every one of the cells of the two separate creatures are identical, but two separate developmental programs have produced a relation that we can’t quite wrap our heads around. Looking at the two creatures, it’s easy to see that they show no interest in one another. If we encountered both together, we’d have no idea they had the same origin. But they are, in a very real sense, the self-same creature. If I didn’t know better, I’d turn to Rumi for answers.
“Hungry, you’re a dog,” the thirteenth-century mystic wrote, “angry and bad-natured. Having eaten your fill, you become a carcass; you lie down like a wall, senseless. At one time a dog, at another time a carcass, how will you run with lions, or follow the saints?” Rumi’s answer to his own question was that one needed to ween oneself of the ego. To do that, rather than resisting change, it was necessary to embrace it. Even small change could lead to great diversity of spirit, and therefore great resplendence. Even change that was based on persistence. Change, and the change of change, and its persistence, were the lifeblood of the soul.
There was a reason why my ear had begged for music inspired by Rumi. His poetry was the mystical complement to a new modern science. Gradually, I began to perceive, like a dog rising from its slumber, that it was this science that provided the key to understanding metamorphosis.
Footnote
i Members of this class are called larvaceans, and are transparent planktonic creatures less than half an inch long.
4.
EVO-DEVO
Evolutionary developmental biology, or “evo-devo” for short, is a field in biology that compares the development of different creatures to infer how development evolved. Another way to express this is that it’s concerned with how changes in embryonic development during single generations relate to the evolutionary changes that occur between generations. Under this name it’s a new field, just a generation or two old, but it’s preoccupation is ancient. We won’t go all the way back, but instead start shortly after Darwin, with a student of Haeckel named Hans Driesch.
Driesch is remembered as the first non-Jewish professor to be removed from his academic post by the Nazis; he was an outspoken pacifist and came to the defense of Jewish colleagues in the face of nationalist discrimination. He’s also famous for moving from embryology to natural theology. Known in his day as a brilliant scientist, he became a philosopher, and in later years devoted his energies to studying what he called “the science of the super-normal.” Driesch was a genius who, according to some, had lost his marbles. And what led him down this path was a tiny sea urchin egg.
Back in 1891, Driesch discovered that when he placed a fertilized sea urchin egg in a vial of water and shook it vigorously right after its first cleavage, the two individual cells, called blastomeres, separated from one another, and each continued to grow. The same was true for the four and then eight cells separated by shaking in the next cleavages. Amazingly, in many instances, each of the dislodged cells developed into perfectly healthy larvae, though at a slower pace than normal and usually reaching only half the normal size.
The reason Driesch was spending his time shaking cells apart was to disprove the theory of a man from his alma mater. Wilhelm Roux, the son of a fencing master, had taken a frog’s egg and, when it divided, killed one of the blastomeres by pricking it with a heated needle. The surviving blastomere, he found, developed into half an embryo. Roux was sure this could mean only one thing: that the nucleus of the cell contained only half the determinants of an embryo. From the very beginning, there had to be factors of future development laid out in fixed positions within the fertilized egg. How it worked would go something like this: After one division, factors determining the left and right side of the embryo would divide into two separate cells; with the next cleavage, factors determining the head and tail would be divided up into the four top and bottom blastomeres, followed by factors determining the heart, brain, and limbs in the next cleavages, followed by factors determining specific heart chambers, brain structures, toes, and so on after them. Roux called this his “mosaic theory of development,” and ruled out any influence of the outside environment. By an intricate, unfolding tapestry, creatures were determined from within.
But Driesch showed that Roux had been sloppy: He’d failed to separate the remnants of the blastomeres he destroyed with his heated needle, which, it turned out, inhibited the regeneration of all the parts in the adjacent surviving cell. Had he simply separated the cells entirely, as Driesch himself did by shaking, he’d have gotten not half-embryos but entire, half-sized ones. Driesch concluded that the “mosaic theory” was mistaken. To describe what was really going on, he cribbed an ancient word from Aristotle: entelechy. What it meant was that each cell somehow had the ability to become an entire organism. How this internal “life force” worked could not be accounted for by the cutting-edge developmental mechanics (Entwickelungsmechanik) that both he and Roux had pioneered. And it drove Driesch, a die-hard rationalist, to vitalism, the “super-normal,” and God.
Roux and Driesch’s rivalry goes back to Ernst Haeckel. Both had been his students at Jena, and it was Haeckel who performed the experiments on which theirs were based. Rather than sea urchin or frog eggs, Haeckel had chosen the eggs of siphonophores, the delicate order of colonial hydrozoa he’d found on the Canary island of Lanzarote, and considered as beautiful as bouquets of flowers. Using a fine needle and a microscope, he divided two-day-old embryos of siphonophores into either two, three, or four groups of cells, showing that each group eventually developed into complete siphonophores. Yes, they were smaller than normal, and it usually took them more time, but nonetheless the magic occurred.
Haeckel performed his experiments thirty years before Driesch, and today they are largely forgotten. The reason for this is partly because Haeckel did not draw Driesch’s dramatic conclusion that every cell in an embryo was totipotent, meaning that it harbored within it the ability to develop into a full-blown organism. Instead, Haeckel focused on what development could teach us about evolution: Playing around with environmental conditions like water temperature, abundance of light, salinity, and movement, he found that some of the manipulated cells, incredibly, would take on the forms of related species rather than their own. Using the biogenetic law, he sketched out evolutionary trees detailing the relationships. All siphonophores descended from a simple medusa that budded off a hydroid colony, he thought, a theory embraced today for the entire phylum of cnidarians. Haeckel was delighted. Siphonophores, he wrote to Darwin, due to their many forms and shapes, provided “the most excellent demonstration of descent theory.”
What Haeckel had done was to think of development and evolution together: How a life-form grows from a single cell to an entire organism could harbor secrets about where it had come from in the deep past. But as embryology became more professional, it tore itself away from the more speculative theory of descent; Haeckel’s “apostate students” were more interested in the mechanical How than the evolutionary Why. Roux ended up rejecting his teacher’s biogenetic law; and as we’ve seen, Driesch (whom Haeckel thought might benefit from time in a sanitarium) turned to vitalism, and became a kind of mystic.
In the following decades, genetics and evolution by natural selection became wedded. This neo-Darwinian evolutionary synthesis, as it was called, did a lot to boost biology, placing it on a par with the “harder” sciences of chemistry and physics. But the price that was paid was that development and evolution were completely torn asunder. Treating individuals like gas molecules in a flask, the heroes of the synthesis were not embryologists, but pen-and-pencil geneticists.i Calculating gene frequencies and selection pressures, rather than peering carefully down a microscope at the ectoderm or mesoderm, they paid no attention to the development of individual organisms. To them evolution was not a private affair, but a population phenomenon. And while their equations showed that it could all work—evolution progressing gradually based on the selection of random mutations—they ventured no answers to the question of how a particular creature actually gained its form.
The neo-Darwinians had made a separation where their hero had perceived a unity. Darwin would spend his life debating the relative importance of variation and selection: If there was a whole lot of random variation, selection would need to be like an all-powerful sculptor, giving nature its form; but if variation was not entirely random, if, for example, variation was sometimes a response to the organism’s needs in a particular environment, then selection wouldn’t need to do quite as much of the work. Darwin swayed in later years toward this second idea, but one thing was clear to him: No evolution could be possible without there first being changes from one individual to the next. It made no sense to him—and clearly not to Haeckel—to separate ontogeny from phylogeny. Development and evolution were two sides of the same coin.
