The Panda's Thumb: More Reflections in Natural History

by Stephen Jay Gould

  The mayfly lives but a day as an adult. It may, for all I know, experience that day as we live a lifetime. Yet all is not relative in our world, and such a short glimpse of it guarantees distortion in interpreting events ticking on longer scales. In a brilliant metaphor, the pre-Darwinian evolutionist Robert Chambers wrote in 1844 of a mayfly watching the metamorphosis of a tadpole into a frog:

  Suppose that an ephemeron [a mayfly], hovering over a pool for its one April day of life, were capable of observing the fry of the frog in the waters below. In its aged afternoon, having seen no change upon them for such a long time, it would be little qualified to conceive that the external branchiae [gills] of these creatures were to decay, and be replaced by internal lungs, that feet were to be developed, the tail erased, and the animal then to become a denizen of the land.

  Human consciousness arose but a minute before midnight on the geologic clock. Yet we mayflies try to bend an ancient world to our purposes, ignorant perhaps of the messages buried in its long history. Let us hope that we are still in the early morning of our April day.

  30 | Natural Attraction: Bacteria, the Birds and the Bees

  THE FAMOUS WORDS “blessed art thou among women” were uttered by the angel Gabriel as he announced to Mary that she would conceive by the Holy Spirit. In medieval and Renaissance painting, Gabriel bears the wings of a bird, often elaborately spread and adorned. While visiting Florence last year, I became fascinated by the “comparative anatomy” of Gabriel’s wings as depicted by the great painters of Italy. The faces of Mary and Gabriel are so beautiful, their gestures often so expressive. Yet the wings, as painted by Fra Angelico or by Martini, seem stiff and lifeless, despite the beauty of their intricate feathering.

  But then I saw Leonardo’s version. Gabriel’s wings are so supple and graceful that I scarcely cared to study his face or note the impact he had upon Mary. And then I recognized the source of the difference. Leonardo, who studied birds and understood the aerodynamics of wings, had painted a working machine on Gabriel’s back. His wings are both beautiful and efficient. They have not only the right orientation and camber, but the correct arrangement of feathers as well. Had he been just a bit lighter, Gabriel might have flown without divine guidance. In contrast, the other Gabriels bear flimsy and awkward ornaments that could never work. I was reminded that aesthetic and functional beauty often go hand in hand (or rather arm in arm in this case).

  In the standard examples of nature’s beauty—the cheetah running, the gazelle escaping, the eagle soaring, the tuna coursing, and even the snake slithering or the inchworm inching—what we perceive as graceful form also represents an excellent solution to a problem in physics. When we wish to illustrate the concept of adaptation in evolutionary biology, we often try to show that organisms unconsciously “know” physics—that they have evolved remarkably efficient machines for eating and moving. When Mary asked Gabriel how she could possibly conceive, “seeing I know not a man,” the angel replied: “For with God nothing shall be impossible.” Many things are impossible for nature. But what nature can do, she often does surpassingly well. Good design is usually expressed by correspondence between an organism’s form and an engineer’s blueprint.

  I recently encountered an even more striking example of good design: an organism that builds an exquisite machine directly within its own body. The machine is a magnet; the organism, a “lowly” bacterium. When Gabriel departed, Mary went to visit Elizabeth, who had also conceived with a bit of help from on high. Elizabeth’s babe (the future John the Baptist) “leaped in her womb” and Mary pronounced the Magnificat, including the line (later set so incomparably by Bach) et exaltavit humilis—“and he hath exalted them of low degree.” The tiny bacteria, simplest in structure among organisms, inhabitants of the first rung on traditional (and fallacious) ladders of life, illustrate in a few microns all the wonder and beauty that some organisms require meters to express.

  In 1975, University of New Hampshire microbiologist Richard P. Blakemore discovered “magnetotactic” bacteria in sediments near Woods Hole, Massachusetts. (Just as geotactic organisms orient toward gravitational fields and phototactic creatures toward light, magnetotactic bacteria align themselves and swim in preferred directions within magnetic fields.) Blakemore then spent a year at the University of Illinois with microbiologist Ralph Wolfe and managed to isolate and culture a pure strain of magnetotactic bacteria. Blakemore and Wolfe then turned to an expert on the physics of magnetism, Richard B. Frankel of the National Magnet Laboratory at M.I.T. (I thank Dr. Frankel for his patient and lucid explanation of their work.)

  A magnetotactic bacterium with its chain of tiny magnets (X 40,000)

  D. L. BALKWILL AND D. MARATEA

  Frankel and his colleagues found that each bacterium builds within its body a magnet made of twenty or so opaque, roughly cubic particles, measuring about 500 angstroms on a side (an angstrom is one ten-millionth of a millimeter). These particles are made primarily of the magnetic material Fe304, called magnetite, or lodestone. Frankel then calculated the total magnetic moment per bacterium and found that each contained enough magnetite to orient itself in the earth’s magnetic field against the disturbing influence of Brownian motion. (Particles small enough to be unaffected by the gravitational fields that stabilize us or by the surface forces that affect objects of intermediate size are buffeted in a random manner by thermal energy of the medium in which they lie suspended. The “play” of dust particles in sunlight provides a standard illustration of Brownian motion.)

  The magnetotactic bacteria have built a remarkable machine, using virtually the only configuration that could work as a compass within their tiny bodies. Frankel explains why the magnetite must be arranged as particles and why the particles must be about 500 angstroms on a side. To work as an efficient compass, magnetite must be present as so-called single domain particles, that is, as bits with a single magnetic moment, containing opposite north- and south-seeking ends. The bacteria contain a chain of such particles, oriented with their magnetic moments north pole to the next south pole along the row—“like the elephants head to tail in a circus finale,” as Frankel states. In this way, the entire chain of particles operates as a single magnetic dipole with north- and south-seeking ends.

  If the particles were a bit smaller (less than 400 angstroms on a side), they would be “superparamagnetic”—a big word indicating that thermal energy at room temperature would cause internal reorientation of the particle’s magnetic moment. On the other hand, if particles were greater than 1,000 angstroms on a side, separate magnetic domains pointing in different directions would form within the particle. This “competition” would reduce or cancel the particle’s overall magnetic moment. Thus, Frankel concludes, “the bacteria have solved an interesting problem in physics by producing particles of magnetite of just the right size for a compass, of dimension 500 angstroms.”

  But evolutionary biology is preeminently the science of “why,” and we must ask what such a small creature could possibly do with a magnet. Since a bacterium’s cruising range is probably a few inches for the few minutes of its existence, I find it hard to believe that oriented motion in a north or south direction can play any role in its repertoire of adaptive traits. But what preferred direction of motion might make a difference? Frankel suggests, quite plausibly in my view, that an ability to move down might be crucial for such a bacterium—for down is the direction of sediments in aquatic environments, and down might lead to a region of preferred oxygen pressure. In this instance, “them of low degree” might wish to debase themselves even further.

  But how does a bacterium know which way is down? With the smug prejudices of our enormous selves, we might think the question inane for its obvious answer: all they have to do is stop whatever they are doing and fall. Not at all. We fall because gravity affects us. Gravity—the standard example of a “weak force” in physics—influences us only because we are large. We live in a world of competing forces, and the relative strength of
these forces depends primarily upon the size of objects affected by them. For familiar creatures of macroscopic dimensions, the ratio of surface area to volume is crucial. This ratio decreases continually as an organism grows, since areas increase as length squared and volumes as length cubed. Small creatures, insects for example, live in a world dominated by forces acting on their surfaces. Some can walk on water or hang upside down from a ceiling because surface tension is so strong and the gravitational force that might pull them down so weak. Gravitation works on volumes (or, to be more precise, upon masses that are proportional to volumes in a constant gravitational field). Gravitation rules us with our low ratio of surface to volume. But it troubles an insect very little—and a bacterium not at all.

  The world of a bacterium is so unlike our own that we must abandon all our certainties about the way things are and start from scratch. Next time you see Fantastic Voyage on the tube, take your eyes off Raquel Welch and the predaceous white blood corpuscle long enough to ponder how the miniaturized adventurers would really fare as microscopic objects within a human body (they behave just like regular folks in the film). They would, first of all, be subject to shocks of the Brownian motion, thus making the film something of a random blur. Also, as Isaac Asimov pointed out to me, their ship could not run on its propeller, since blood is too viscous at such a scale. It should have, he said, a flagellum—like a bacterium.

  D’Arcy Thompson, premier student of scaling since Galileo, urged us to set aside our prejudices if we would understand the world of a bacterium. In his masterpiece, Growth and Form (published in 1942 but still in print), he ends his chapter “On Magnitude” in his incomparable prose:

  Life has a range of magnitude narrow indeed compared to that with which physical science deals; but it is wide enough to include three such discrepant conditions as those in which a man, an insect and a bacillus have their being and play their several roles. Man is ruled by gravitation, and rests on mother earth. A water-beetle finds the surface of a pool a matter of life and death, a perilous entanglement or an indispensable support. In a third world, where the bacillus lives, gravitation is forgotten, and the viscosity of the liquid, the resistance defined by Stokes’s law, the molecular shocks of the Brownian movement, doubtless also the electric charges of the ionized medium, make up the physical environment and have their potent and immediate influence upon the organism. The predominant factors are no longer those of our scale; we have come to the edge of a world of which we have no experience, and where all our preconceptions must be recast.

  So how does a bacterium know which way is down? We use magnets for horizontal orientation so exclusively that we often forget (in fact, I suspect many of us do not know) that the earth’s magnetic field also has a vertical component, its strength depending upon latitude. (We damp out the vertical deflection in building compasses because it doesn’t interest us. As large creatures ruled by gravitation, we know which way is down. Only at our scale could folly be personified as not knowing “which way is up.”) A compass needle follows the earth’s lines of force. At the equator, these lines are horizontal to the surface. Toward the poles, they dip more and more strongly into the earth. At the magnetic pole itself, the needle points straight down. At my latitude in Boston, the vertical component is actually stronger than the horizontal. A bacterium, swimming north as a free compass needle, also swims down at Woods Hole.

  This putative function for a bacterial compass is pure speculation at the moment. But if these bacteria use their magnets primarily to swim down (rather than to find each other, or to do Lord knows what, if anything, in their unfamiliar world), then we can make some testable predictions. Members of the same species, living in natural populations adapted to life at the equator, will probably not make magnets, for here a compass needle has no vertical component. In the Southern Hemisphere, magnetotactic bacteria should display reversed polarity and swim in the direction of their south-seeking pole.

  Magnetite has also been reported as a component of several larger organisms, all of which perform remarkable feats of horizontal orientation—the conventional use of a compass for familiar creatures of our scale. Chitons, eight-plated relatives of clams and snails, live primarily on rocks near sea level in tropical regions. They scrape food from the rocks with a long file called a radula—and the tips of the radular teeth are made of magnetite. Many chitons make substantial excursions from a living site, but “home” back to the precise spot thereafter. The idea that they might use their magnetite as an orienting compass suggests itself, but the evidence so far offers no support. It is not even clear that chitons have enough magnetite to perceive the earth’s field, and Frankel tells me that their particles are mostly above the single domain limit.

  Some bees have magnetite in their abdomens, and we know that they are affected by the earth’s magnetic field (see article by J. L. Gould, no relation, J. L. Kirschvink, and K. S. Defeyes in bibliography). Bees do their famous dance on the vertical surface of their honeycomb by converting the orientation of their flight to food in relation to the sun into an angle danced with respect to gravity. If the comb is turned so that bees must dance on a horizontal surface, where they cannot express direction in gravitational terms, they become disoriented at first. Finally, after several weeks, they align their dances to the magnetic compass. Moreover, a swarm of bees, placed into an empty hive without cues for orientation, build their comb in the magnetic direction it occupied in their parental hive. Pigeons, certainly no duffers at homing, build a structure made of magnetite between their brain and skull. This magnetite exists as single domains and can therefore function as a magnet (see C. Walcott et al. in bibliography).

  The world is full of signals that we don’t perceive. Tiny creatures live in a different world of unfamiliar forces. Many animals of our scale greatly exceed our range of perception for sensations familiar to us. Bats avoid obstacles by bouncing sound off them at frequencies that I cannot hear, although some people can. Many insects see into the ultraviolet and follow the “invisible” nectar guides of flowers to sources of food for them and pollen that they will carry to the next flower for fertilization (plants build these orienting color streaks for their own advantages, not to convenience the insects).

  What an imperceptive lot we are. Surrounded by so much, so fascinating and so real, that we do not see (hear, smell, touch, taste) in nature, yet so gullible and so seduced by claims for novel power that we mistake the tricks of mediocre magicians for glimpses of a psychic world beyond our ken. The paranormal may be a fantasy; it is certainly a haven for charlatans. But “parahuman” powers of perception lie all about us in birds, bees, and bacteria. And we can use the instruments of science to sense and understand what we cannot directly perceive.

  Postscript

  In asking why bacteria might build magnets within their bodies, Frankel speculated cogently that swimming north could make little difference to such a tiny creature, but that swimming down (another consequence of life around a compass at mid to high latitudes in the northern hemisphere) could be very important indeed. This led me to predict that if Frankel’s explanation be valid, magnetic bacteria in the Southern Hemisphere should swim south in order to swim down—that is, their polarity should be reversed relative to northern hemisphere relatives.

  In March, 1980, Frankel sent me a preprint of a paper with colleagues R.P. Blakemore and A.J. Kalmijn. They travelled to New Zealand and Tasmania in order to test the magnetic polarity of southern hemisphere magnetic bacteria. Indeed, they all swam south and down—an impressive confirmation of Frankel’s hypothesis and the basis of my essay.

  They also performed an interesting experiment, providing further confirmation of another kind. They collected magnetic bacteria at Woods Hole, Massachusetts, divided the sample of north-swimming cells into two parts. They cultured one subsample for several generations in a chamber of normal polarity, but grew the other in a chamber of reversed polarity to simulate Southern Hemisphere conditions. Sure enough, after severa
l weeks, north-swimming cells continued to predominate in the chamber of normal polarity. But in the chamber with reversed polarity, south-swimming cells now formed a majority. Since bacterial cells do not change polarity during their lifetimes, this dramatic change is probably the result of strong natural selection for the ability to swim down. Presumably, both north and south-swimming cells originate in each chamber, but selection quickly weeds out individuals that cannot swim down.

  Frankel tells me that he is now off to the geomagnetic equator to see what happens where the magnetic field has no downward component at all.

  31 | Time’s Vastness

  2:00 A.M., Jan. 1, 1979

  I WILL NEVER forget Toscanini’s last concert—the night that the greatest maestro of them all, the man who held all Western music in his infallible memory, faltered for a few seconds and lost his place. If heroes were truly invulnerable, how could they compel our interest? Siegfried must have a mortal shoulder, Achilles a heel, Superman kryptonite.