The Panda’s Thumb (22 page)

This simple interpretation, as Woese and his group realize, is not the only possible reading of their results. We may propose two other perfectly plausible hypotheses: (1) The three monerans that they used may not represent the entire group very well. Perhaps the RNA sequences of other monerans will differ as much from the first three as all the methanogens do. We would then have to include the methanogens with all monerans in a single grand group. (2) The assumption of nearly constant evolutionary rates may not hold. Perhaps the methanogens split off from one branch of monerans long after the main groups of monerans had branched from their common ancestor. These early methanogens may then have evolved at a rate far in excess of that followed by moneran groups in diverging from each other. In this case, the great difference in RNA sequence between any methanogen and any moneran would only record a rapid evolutionary rate for early methanogens, not a common ancestry with monerans before the monerans themselves split into subgroups. The gross amount of biochemical difference will accurately record time of divergence only if evolution proceeds at reasonably constant biochemical rates.

But one other observation makes Woese's hypothesis attractive and inspires my own strong rooting for it. The methanogens are anaerobic; they die in the presence of oxygen. Hence, they are confined today to unusual environments: muds at the bottom of ponds depleted of oxygen or deep hot springs in Yellowstone Park, for example. (The methanogens grow by oxidizing hydrogen and reducing carbon dioxide to methane—hence their name.) Now, amidst all the disagreement that afflicts the study of our early earth and its atmosphere, one point has gained general assent: our original atmosphere was devoid of oxygen and rich in carbon dioxide, the very conditions under which methanogens thrive and for which the earth's original life might have evolved. Could modern methanogens be remnants of the earth's first biota, originally evolved to match its general condition, but now restricted by the spread of oxygen to a few marginal environments? We believe that most free oxygen in our atmosphere is the product of organic photosynthesis. The Fig Tree organisms were already indulging in photosynthesis. Thus, the golden age of methanogens may have passed long before the advent of Fig Tree monerans. If this reverie be confirmed, then life must have originated long before Fig Tree times.

In short, we now have direct evidence of life in the oldest rocks that could contain it. And, by reasonably strong inference, we have reason to believe that a major radiation of methanogens predated these photosynthesizing monerans. Life probably arose about as soon as the earth became cool enough to support it.

Two closing thoughts, admittedly reflecting my personal prejudices: First, as a strong adherent to exobiology, that great subject without a subject matter (only theology may exceed us in this), I am delighted by the thought that life may be more intrinsic to planets of our size, position, and composition than we had ever dared to imagine. I feel even more certain that we are not alone, and I hope that more effort will be directed toward the search for other civilizations by radio-telescope. The difficulties are legion, but a positive result would be the most stupendous discovery in human history.

Secondly, I am led to wonder why the old, discredited orthodoxy of gradual origin ever gained such strong and general assent. Why did it seem so reasonable? Certainly not because any direct evidence supported it.

I am, as several other essays emphasize, an advocate of the position that science is not an objective, truth-directed machine, but a quintessentially human activity, affected by passions, hopes, and cultural biases. Cultural traditions of thought strongly influence scientific theories, often directing lines of speculation, especially (as in this case) when virtually no data exist to constrain either imagination or prejudice. In my own work (see essays 17 and 18), I have been impressed by the powerful and unfortunate influence that gradualism has exerted on paleontology via the old motto
natura non facit saltum
(“nature does not make leaps”). Gradualism, the idea that all change must be smooth, slow, and steady, was never read from the rocks. It represented a common cultural bias, in part a response of nineteenth-century liberalism to a world in revolution. But it continues to color our supposedly objective reading of life's history.

In the light of gradualistic presuppositions, what other interpretation could have been placed upon the origin of life? It is an enormous step from the constituents of our original atmosphere to a DNA molecule. Therefore, the transition must have progressed laboriously through multitudes of intervening steps, one at a time, over billions of years.

But the history of life, as I read it, is a series of stable states, punctuated at rare intervals by major events that occur with great rapidity and help to establish the next stable era. Prokaryotes ruled the earth for three billion years until the Cambrian explosion, when most major designs of multicellular life appeared within ten million years. Some 375 million years later, about half the families of invertebrates became extinct within a few million years. The earth's history may be modelled as a series of occasional pulses, driving recalcitrant systems from one stable state to the next.

Physicists tell us that the elements may have formed during the first few minutes of the big bang; billions of subsequent years have only reshuffled the products of this cataclysmic creation. Life did not arise with such speed, but I suspect that it originated in a tiny fraction of its subsequent duration. But the reshuffling and subsequent evolution of DNA have not simply recycled the original products; they have produced wonders.

OBLIVION, NOT INFAMY
, is the usual fate of a crackpot. I shall be more than mildly surprised if any reader (who is not a professional taxonomist with a special attachment to sponges) can identify Randolph Kirkpatrick.

On the surface, Kirkpatrick fit the stereotype of a self-effacing, mild-mannered, dedicated, but slightly eccentric British natural historian. He was the assistant keeper of “lower” invertebrates at the British Museum from 1886 until his retirement in 1927. (I have always admired the English penchant for simple, literal terms—lifts and flats for our elevators and apartments, for example. We use the Latin
curator
for guardians of museum collections; the British call them “keepers.” We, however, have done better in retaining “fall” for their “autumn.”) Kirkpatrick trained as a medical student, but decided on a “less strenuous career” in natural history after several bouts with illness. He chose well, for he traveled all over the world searching for specimens and lived to be eighty-seven. In the last months of his life, in 1950, he continued to pedal his bicycle through London's busiest streets.

Early in his career, Kirkpatrick published some sound taxonomic work on sponges, but his name rarely appears in scientific journals after the First World War. In an obituary note, his successor attributed this halt in mid-career to Kirkpatrick's behavior as “an ideal public servant.” “Unassuming to a fault, courteous and generous, he would spare no effort to help either a colleague or a visiting student. It was in all probability his extreme willingness to interrupt whatever he was doing to help others that prevented his completing his work.”

Kirkpatrick's story, however, is by no means so simple and conventionally spotless. He did not stop publishing in 1915; instead, he shifted to private printing for a series of works that he knew no scientific journal would touch. Kirkpatrick spent the rest of his career developing what has to be the nuttiest of crackpot theories developed in this century by a professional natural historian (and keeper at the staid British Museum, no less). I do not challenge this usual assessment of his “nummulosphere” theory, but I will stoutly defend Kirkpatrick.

In 1912, Kirkpatrick was collecting sponges off the island of Porto Santo in the Madeira group, west of Morocco. One day, a friend brought him some volcanic rocks collected on a peak 1,000 feet above sea level. Kirkpatrick described his great discovery: “I examined them carefully under my binocular microscope and found to my amazement traces of nummulitic disks in all of them. Next day I visited the place whence the fragments had come.”

Now
Nummulites
is one of the largest forams that ever lived (forams are single-celled creatures related to amoebas, but they secrete shells and are commonly preserved as fossils).
Nummulites
looks like the object that provided its name: a coin. Its shell is a flat disk up to an inch or two in diameter. The disk is built of individual chambers, one following the next and all wound tightly into a single coil. (The shell looks much like a coil of rope, appropriately scaled down.) Nummulites were so abundant in early Tertiary times (about 50 million years ago) that some rocks are composed almost entirely of their shells; these are called “nummulitic limestones.” Nummulites litter the ground around Cairo; the Greek geographer Strabo identified them as petrified lentils left over from rations doled out to slaves who had built the Great Pyramids.

Kirkpatrick then returned to Madeira and “discovered” nummulites in the igneous rocks there as well. I can scarcely imagine a more radical claim about the earth's structure. Igneous rocks are the products of volcanic eruption or the cooling of molten magmas within the earth; they cannot contain fossils. But Kirkpatrick argued that the igneous rocks of Madeira and Porto Santo not only included nummulites but were actually made of them. Therefore, “igneous” rocks must be sediments deposited at the ocean bottom, not the products of molten material from the earth's interior. Kirkpatrick wrote:

Nothing could stop Kirkpatrick now. He returned to London, itching to examine igneous rocks from other areas of the world. All were made of nummulites! “I annexed in one morning for
Eozoon
volcanic rocks of the Arctic and in the afternoon of the same day those of the Pacific, Indian and Atlantic oceans. The designation
Eozoon orbis-terrarum
then suggested itself.” Finally, he looked at meteorites and, yes, you guessed it, all nummulites:

Kirkpatrick did not shy away from the evident conclusion:—all rocks on the earth's surface (including the influx from space) are made of fossils: “The original organic nature of these rocks is to me self-evident, because I can see the Foraminiferal structure in them, and often very clearly indeed.” Kirkpatrick claimed that he could see the nummulites with a low-power hand lens, although no one ever agreed with him. “My views on igneous and certain other rocks,” he wrote, “have been received with a good deal of skepticism, and this is not surprising.”

I hope I will not be dismissed as an establishment dogmatist if I state with some assurance that Kirkpatrick had somehow managed to delude himself. By his own admission, he often had to work very hard in toeing his own line: “Sometimes I have found it necessary to examine a fragment of rock with the closest scrutiny for hours before convincing myself that I have seen all the above-mentioned details.”

But what version of the earth's history would yield a crust made entirely of nummulites? Kirkpatrick proposed that nummulites had arisen early in the history of life as the first creatures with shells. Hence, he adopted for them the name
Eozoon
, first proposed in the 1850s by the great Canadian geologist Sir J. W. Dawson for a supposed fossil from some of the earth's oldest rocks. (We now know that
Eozoon
is an inorganic structure, made of alternating white and green layers of the minerals calcite and serpentine—see essay 23.)

In these early times, Kirkpatrick speculated, the ocean bottom must have accumulated a deep deposit of nummulitic shells over its entire surface, for the seas contained no predators to digest them. Heat from the earth's interior fused them together and injected them with silica (thus solving the vexatious problem of why igneous rocks are silicates, while true nummulites are made of calcium carbonate). As the nummulites were squeezed and fused, some were pushed upward and tossed out into space, later to descend as nummulitic meteorites.

Kirkpatrick still was not satisfied. He thought he had discovered something even more fundamental. Not content with the earth's crust and its meteorites, he began to see the coiled form of nummulites as an expression of life's essence, as the architecture of life itself. Finally, he broadened his claim to its limit: we should not say that the rocks are nummulites; rather, the rocks and the nummulites and everything else alive are expressions of “the fundamental structure of living matter,” the spiral form of all existence.

Nutty, yes (unless you feel that he had intuited the double helix). Inspired, surely. A method to his madness, yes, again—and this is the crucial point. In framing his nummulosphere theory, Kirkpatrick followed the procedure that motivated all his scientific work. He had an uncritical passion for synthesis and an imagination that compelled him to gather truly disparate things together. He consistently sought similarities of geometric form among objects conventionally classified in different categories, while ignoring the ancient truth that similarity of form need not designate common cause. He also constructed similarities out of his hopes, rather than his observations.

Still, an uncautious search for synthesis may uncover real connections that would never occur to a sober scientist (although he may be jostled to reflect upon them once someone else makes the initial suggestion). Scientists like Kirkpatrick pay a heavy price, for they are usually wrong. But when they are right, they may be so outstandingly right that their insights beggar the honest work of many scientific lifetimes in conventional channels.

The cover to Kirkpatrick's privately published Nummulosphere. Of it, he writes: “The design on the cover represents Neptune on the globe of waters. On one of the prongs of his trident is a piece of volcanic rock in the shape of a nummulitic disk, and in his hand is a meteorite. These emblems signify that Neptune's domain is enlarged not only at the expense of nether Jove, but also at that of high Jove whose supposed emblem of sovereignty—the thunderbolt—really belongs to the Sea God…Neptune's bolt is poised ready to be hurled at rash and ignorant mortals of the type of the a priori would-be refuter, daring to dispute the validity of his title-deeds.”

Let us return then to Kirkpatrick and ask why he was on Madeira and Porto Santo in the first place when he made his fateful discovery in 1912. “In September 1912,” he writes, “I journeyed to Porto Santo via Madeira, in order to complete my investigation of that strange organism, the sponge-alga
Merlia normani
,” In 1900, a taxonomist named J. J. Lister had discovered a peculiar sponge on the Pacific islands of Lifu and Funafuti. It contained spicules of silica, but had an additional calcareous skeleton bearing a striking resemblance to some corals (spicules are the small, needle-like elements forming the skeleton of most sponges). A sober man, Lister could not accept the “hybrid” of silica and calcite; he conjectured that the spicules had entered the sponge from elsewhere. But Kirkpatrick collected more specimens and correctly concluded that the sponge secretes the spicules. Then, in 1910, Kirkpatrick found
Merlia normani
on Madeira, a second sponge with siliceous spicules and a supplementary calcareous skeleton.

Inevitably, Kirkpatrick unleashed his passion for synthesis upon
Merlia
. He noticed that its calcareous skeleton resembled several problematic groups of fossils usually classified among the corals—the stromatoporoids and the chaetetid tabulates in particular. (This may seem like a small issue to many, but I assure you that it is a major concern of all professional paleontologists. Stromatoporoids and chaetetids are very common as fossils; they form reefs in some ancient deposits. Their status lies among the classical mysteries of my field, and many distinguished paleontologists have spent entire careers devoted to their study.) Kirkpatrick decided that these and other enigmatic fossils must be sponges. He set out to find spicules in them, a sure sign of affinity with sponges. Sure enough; they all contained spicules. We may be quite sure that Kirkpatrick had deluded himself again in some cases, for he included among his “sponges” the undoubted bryozoan
Monticulipora
. In any case, Kirkpatrick soon became preoccupied with his nummulosphere theory. He never published the major treatise that he had planned on
Merlia
. The nummulosphere made him a scientific pariah, and his work on coralline sponges was pretty much forgotten.

Kirkpatrick worked the same way in studying both nummulospheres and coralline sponges: he invoked a similarity of abstract, geometric form to infer a common source for objects that no one had thought to unite, and he followed his theory with such passion that he eventually “saw” the expected form, even where it manifestly did not exist. Yet, I must note one major difference between the two studies: Kirkpatrick was right about the sponges.

During the 1960s, Thomas Goreau, late of the Discovery Bay Marine Laboratory in Jamaica, began to explore the cryptic environments of West Indian reefs. These cracks, crevices, and caves contain a major fauna, previously undetected. In one of the most exciting zoological discoveries of the last twenty years, Goreau and his colleagues Jeremy Jackson and Willard Hartman showed that these habitats contain numerous “living fossils.” This cryptic community seems to represent an entire ecosystem literally overshadowed by the evolution of more modern forms. The community may be cryptic, but its members are neither moribund nor uncommon. The linings of caves and crevices form a major part of modern reefs. Before the advent of scuba diving, scientists could not gain access to these areas.