Leonardo’s Mountain of Clams and the Diet of Worms (44 page)

Suppose that the simplest kind of cellular life arises as a predictable result of organic chemistry and the physics of self-organizing systems wherever planets exist with the right constituents and conditions—undoubtedly a common occurrence in our vast universe, But suppose, in addition, that no predictable
directions exist for life’s later development from these basic beginnings.

Evolving life must experience a vast range of possibilities, based on environmental histories so unpredictable that no realized route—the pathway to consciousness in the form of
Homo sapiens
or Little Green Men, for example—can be construed as a highway to heaven, but must be viewed as a tortuous track rutted with uncountable
obstacles and festooned with innumerable alternative branches. Any reasonably precise repetition of our earthly route on another planet therefore becomes wildly improbable even in a trillion cases. (Since the universe must contain millions of appropriate planets, consciousness in some form—but not with the paired eyes and limbs, and the brain built of neurons in the only example we know—may
evolve frequently. But if only one origin of life in a million ever leads to consciousness, then Martian bacteria most emphatically do not imply Little Green Men.)

In other words, I think that we have traditionally made the wrong division in a sequence of three steps: an appropriate planet without life (1), the origin of simplest cellular life at bacterial grade (2), and the evolution of consciousness
(3). The traditional view, based on our arrogant assumption that life reaches a necessary apotheosis in creatures like us, assumes a wondrous specialness for life of any kind, and an inevitable evolution toward consciousness thereafter. Thus, the transition from 1 to 2 must be rare and onerous, but the passage from 2 to 3 easy and predictable.

Only under this false view can I understand the thrill
felt by so many Earthlings at the recent report of Martian fossils. These people jumped to the false conclusion that a reported step 2, read as a near miracle of improbable advance from step 1, required nothing more than ample time to reach a fully predictable step 3—so that finding bacteria becomes tantamount to positing Little Green Men (a consummation never reached on Mars only because conditions
changed, water disappeared, and “ample time” therefore failed to accrue). The only real difference between this common view and Lowell’s canals lies in our improved knowledge of Martian geological history. Lowell thought that Martian conditions, while constantly deteriorating, had remained sufficiently hospitable to permit a predictable passage to step 3—while we now know that Mars dried
up far earlier, with life still caught at step 2.

But I regard this division of the three steps as deeply erroneous, and based only on our prejudice for regarding the origin of life as special, with consciousness as a guaranteed climax thereafter. Surely the easy and constantly repeated passage lies between steps 1 and 2—representing the ordinary operation of physics and chemistry under appropriate
conditions—while a transition from step 2 to step 3 faces the overwhelming improbability of any particular historical path among millions of equally attainable alternatives. Life of bacterial grade may arise almost everywhere, and then usually proceed nowhere in particular, if anywhere at all—a perfectly splendid outcome since bacteria dominate nearly all environments of life on earth even
today. We now live, as Earth always has (see my 1996 book
Full House
), in an Age of Bacteria. These simplest organisms will dominate our planet (if conditions remain hospitable for life at all) until the sun explodes. During our current, and undoubtedly brief, geological moment, they watch with appropriate amusement as we strut and fret our hour upon the stage. For we are, to them, only transient
and delectable islands ripe for potential exploitation.

If we could make this readjustment to view
Homo sapiens
as an ultimate in oddball rarity, and life at bacterial grade as the common expression of a universal phenomenon, then we could finally ask the truly fascinating question raised by the prospect of Martian fossils. If life originates as a general property of the material universe under
certain conditions (probably often realized), then how much can the basic structure and constitution of life vary from place to independent place? We simply cannot answer this question from the only “sample” we know—life on earth—and for an interesting reason arising from the core of scientific method.

All life on earth—everything from bacteria to mushrooms to hippos—shares an astonishing range
of detailed biochemical similarities, including the structure of heredity in DNA and RNA, and the universal use of ATP as an energy-storing compound. Two possible scenarios, with markedly different implications for the nature of life, might explain these regularities: either all earthly life shares these features because no other chemistry can work, or these similarities only record the common
descent of all organisms on earth from a single origin that happened to feature this chemistry as one possibility among many. In the first case, life on other worlds will independently evolve the same chemistry as a sole viable choice; in the second case, other living systems may feature a wide range of alternate chemistries.

We cannot ask a more important question about the nature of life. But,
ironically, we also cannot begin to answer this question with the data now at our disposal. Above all, experimental science requires repetition to test the predictability of outcomes. If a phenomenon happens only once, we simply cannot know whether the properties we observe must exist as we find them, or whether other “replays” might yield markedly different results.

Unfortunately, all life on
earth—the only life we know—represents, for all its current variety, the results of a
single experiment
, for every earthly species evolved from the common ancestry of a single origin. We desperately need a
repetition
of the experiment (several would be even better, but let’s not be greedy!) in order to make a judgment.

Mars represents our first real hope for a
second experiment
—the sine qua non
for any proper answer to the question of questions. Unless earthly and Martian life share a single origin by seeding from a common source—an obvious possibility if Martian fossils can reach Earth by meteoritic impact!—then any life on Mars fills the holy grail of our ultimately precious second experiment.

Ancient Martian fossils will not yield the required evidence, for we need living matter
with intact biochemistry, ripe for reading either as DNA, or as a workable alternative as yet unimagined by students of earthly life.

The Martian surface may now be cold, dry, and dead. But, on our planet, bacteria can live in pore spaces within rocks several miles below the earth’s surface, so long as water percolates through. A similar subterranean environment on Mars may still feature water
in liquid form. Thus, if life at bacterial grade ever evolved on Mars, these organisms almost surely disappeared from the Martian surface long ago, but may still live within the more hospitable environment of subsurface rocks. The putative fossils from Mars provide our greatest reason for hope that the second experiment still lies buried, but very much alive, beneath the surface of our sister planet.

So let us send forth our robots, and perhaps (eventually) even our persons, to look, find, and return—for this experiment can be done! Forget those Little Green Men, those nonexistent canal builders, those fantasies recently unleashed under the false belief that ancient bacteria imply the eventual evolution of consciousness. The simplest life may pervade the cosmos, and a second independent sample
may answer the riddle of the ages. Let us use our distinct and
oddball
intelligence to track down any direct evidence for the range of life’s
universal
structure. The next step from our sun—the most accessible of all other planets—may yield a ready answer. The host of the cosmic bacterial manifold, the dominant beings who mocked Lowell for thinking that his kind rather than their kind might pervade
the universe, may then smile with satisfaction and say, “So you finally understand; well done, thou good and faithful servant.”

TRIUMPH OF THE ROOT-HEADS

I
AM NOT MUCH OF A BETTING MAN
. F
OR ME, A
M
AN O
’ W
AR IS AN OLD
British fighting ship, and a Native Dancer inhabits Tahiti, wears grass skirts, and gyrates on the beach for Fletcher Christian and Captain Bligh in various Hollywood versions of
Mutiny on the Bounty.
Nonetheless, if compelled to put up or shut up, I would make an unconventional wager on the controversial
subject of progress in evolution.

In our culture’s focal misunderstanding of evolution, most people assume that trends to increasing complexity through time must impart a primary and predictable direction to the history of life. But Darwinian natural selection only yields adaptation to changing local environments, and better function in an immediate habitat might just as well be achieved by greater
simplicity in form and behavior as by ever-increasing complexity. Thus, one might predict that cases of evolutionary simplification will be just about as common as increases in complexity.

But I would be tempted to bet on culture’s underdog, and to suspect that examples of simplification might actually hold a small overall edge. I hazard this unconventional proposition because a common lifestyle
assumed by tens to hundreds of thousands of animal species—namely, parasitism—usually involves evolutionary simplification of adult form in comparison with free-living ancestors. Since I know of no comparable phenomenon that could supply a countervailing bias for complexity, a compendium of all cases might produce a majority for simplification—as natural selection in free-living forms imparts
no bias in either direction, while parasitism gives a clear edge to simplification.

I regard this argument as impeccable—in its own restricted way. But nature scorns such crimping limits imposed by frailties of human cognition upon her wonderful and multifarious variety. This argument about parasites only works under the aegis of another bias almost as serious as our equation of evolution with
progress: our prejudice for regarding adult anatomy as
the
organism, and our failure to consider entire life cycles and complexities of physiological function.

Consider one of the standard “laments” or “stories of wonder” in conventional tales of natural history: the mayfly that lives but a single day (a sadness even recorded in the technical name for this biological group—Ephemoptera). Yes,
the adult fly may enjoy only one moment in the sun, but we should honor the entire life cycle and recognize that the larvae, or juvenile stages, live and develop for months. Larvae are not mere preparations for a brief adulthood. We might better read the entire life cycle as a division of labor, with larvae as feeding and growing stages, and the adult as a short-lived reproductive machine. In this
sense, we could well view the adult fly’s day as the larva’s clever and transient device for making a new generation of truly fundamental feeders—the insect equivalent of Butler’s famous quip that a chicken is merely the egg’s way of making another egg.

This essay treats the most celebrated story of extreme simplification in an adult parasite—in the interests of illuminating, reconciling, and,
perhaps, even resolving two major biases that have so hindered our understanding of natural history: the misequation of evolution with progress, and the undervaluing of an organism by considering only its adult form and not the entire life cycle.

The adult of
Sacculina
, the standard representative of a larger group with some two hundred species, the Rhizocephala, could hardly be more different
from its barnacle ancestors—or more simplified in anatomy and appearance. The two names accurately record this dramatic evolutionary change—for
Sacculina
is a Latin “little sac,” while
Rhizocephala
is a Greek “root-head.” As we shall see, the rhizocephalans are clearly barnacles by ancestry, but the adult preserves not a hint of this crustacean past. Rhizocephalans are parasites upon other crustaceans,
and nearly all infest decapods (crabs and their relatives). The adult consists of two parts with names that (in a refreshing change from usual practice) almost count as vernacular expressions, rather than jargon. From the outside, a human observer sees only a formless sac (called the
externa
) attached to the underside of the crab’s abdomen. The sac is little more than a reproductive device, containing
the ovary and a passageway for introduction of males and their sperm. The externa contains no other differentiated parts—no appendages, no sense organs, no digestive tract, and no sign of segmentation at all. The fertilized eggs develop within the externa (which then operates as a brood pouch).

But how can the externa function without any evident source of nutrition? Closer examination reveals
a stalk that pierces the crab’s abdomen and connects the externa to an elaborate network of roots (called the
interna
). These roots may pervade the entire body of the crab. They penetrate through the hemocoelic spaces (the analogs of blood vessels) and invest many of the crab’s internal organs. They provide nutrition to the parasite by absorption from the crab’s vital fluids. In some species,
roots are restricted to the abdomen, but in
Sacculina
they may run through the entire body, right to the ends of the appendages. (This system is not so grisly—in inappropriate human terms—as first glance might suggest. The parasite does not devour the host, but rather maintains the crab as a “life support” system.) The name
Sacculina
(for the most common genus) honors the externa, while the designation
of the entire group—Rhizocephala, or root-head—recognizes the interna.

These barnacle parasites have been known to zoologists since the 1780s (though the first recorder, correctly observing the release of crustacean larvae from the externa, misinterpreted the sac as an organ of the crab, induced by the parasite much as some insect larvae can commandeer a plant to grow a protective gall). Ever
since this early discovery, rhizocephalans have played a classic role in conventional natural history as the standard example of maximal degeneration in parasites. Many of the foremost zoologists of Darwin’s generation highlighted
Sacculina
as one of evolution’s primary marvels.