But consider the system of variation as a whole, rather than focusing upon a few species at the right tail. What has ever changed besides overall diversity? The modal organism on earth is now, has always been, and probably will always be, a prokaryotic cell. There are more bacteria in the gut of each person reading this essay than there are humans on the face of the earth. And who has a better hope for long-term survival? We might do ourselves in by nuclear holocaust, but prokaryotes will probably hang tough until the sun explodes.
Progress as a predictable result of ordered causes therefore becomes a double delusion—first because we must seek its cause more in the quirkiness of the wheel, turning tires into sandals and big brains toward fear of death, than in the plodding predictability of the wedge, propelling monkeys into men; and secondly, because the supposed sweep of life toward progress only records our myopic focus on the right tail of a distribution whose mode has never moved from a prokaryotic cell.
Our reasons for profound unwillingness to abandon a view of life as predictable progress have little relation to truth, and all to do with solace. Ironically, while using the wedge to supply ultimate solace in his claim that “all corporeal and mental endowments will tend to progress towards perfection” (from the concluding section of the Origin of Species), Darwin also recognized a challenge in the bloodthirsty character of unrelenting battle. He therefore concluded chapter 3 of the Origin with one of the few soft statements of a very tough-minded thinker:
When we reflect on this struggle, we may console ourselves with the full belief, that the war of nature is not incessant, that no fear is felt, that death is generally prompt, and that the vigorous, the healthy, and the happy survive and multiply.
Our chances of understanding nature would improve so immensely if we would only shift our search for solace elsewhere. (Solace will always be a desperate need in this vale of tears, but why should the facts of our belated evolution be pressed into such inappropriate, if noble, service?) Perhaps I am just a hopeless rationalist, but isn’t fascination as comforting as solace? Isn’t nature immeasurably more interesting for its complexities and its lack of conformity to our hopes? Isn’t curiosity as wondrously and fundamentally human as compassion?
New Discoveries in the Earliest History of Multicellular Life
23 | Defending the Heretical and the Superfluous
SAMUEL TAYLOR COLERIDGE, in a reverie laced with laudanum, presented an image of striking incongruity in describing the pleasure palace of Kubla Khan:
It was a miracle of rare device,
A sunny pleasure-dome with caves of ice!
This vision of tropical languor mixed with arctic sternness recalls a juxtaposition of similar disparity from my own education—Marco Polo in Chinese summer palaces and Eric the Red conning settlers by describing inhospitable arctic real estate as “Greenland.” This odd matching of China with Greenland records a key episode of “white man’s history,” taught as universal by New York City public schools in the late 1940s.
The history of civilization, we learned, is centrifugal—a process of outward expansion from European or near Near Eastern centers. Heroes of this process were called “explorers”—and they “discovered” land after land, despite the nagging admission that all these places featured indigenous cultures often more complex and refined than the European “source” (Kubla Khan vs. the Doge of Venice).
We worked through the panoply of explorers in strict chronology. Eric the Red, a tenth-century Norseman, came first, moving northwest into bleakness and chill. Marco Polo, Kubla Khan’s most famous visitor (and Coleridge’s source), followed, moving southeast into exotic splendor and warmth. (Eric’s son Leif might have merited a chapter in between, especially since he reached North America several hundred years before the official date for “discovery” of our well-populated continent. But, remember, I grew up in New York, not Lake Wobegon, and the Knights of Columbus had effectively put the kibosh on any Viking claims. Leif Ericson and Vineland ranked with Odin and Thor in the category of Scandinavian mythology.)
Thus, Greenland and China—lands of nearly maximal disparity in climate and geography—have always stood together in my mind as the one-two punch of initial discovery. And now, some forty years later, my own profession of more ultimate origins has juxtaposed these incongruous places again, this time in the legitimate service of discovery about true beginnings. During the last year, fossil finds in China and Greenland have penetrated the terra incognita of animal origins with an éclat to match the deeds of any old-time explorer.
I have written many essays and an entire book on the origin of multicellular animals. Yet, from a dominant perspective in evolutionary thought, such a subject should not exist at all, at least in the sense of “first” items that an explorer might discover. We inhabit a world of graded continuity, and transformation of single-celled microscopic ancestors to multicellular animals of modern design should occur by smooth transition over such a long time that no single organism or species should qualify as an unambiguous “first.”
Life is continuous in the crucial sense that all creatures form a web of unbroken genealogical linkage. But connectivity does not imply insensible transition. Nothing breaks the continuity between caterpillar and butterfly, but stages of development are tolerably discrete. Similarly, the origin of animals reminds us, in outline, of an old quip about the life of a soldier—long periods of boredom punctuated by short moments of terror. In the evolution of multicellular animals, nothing much happens for very long periods of time, while everything cascades in brief geological moments. We can talk meaningfully about “firsts,” and discoveries in Greenland and China qualify for this category of ultimate importance. A quick review of basic information will set a proper context:
Life on earth is as old as it could be—a striking fact that, in itself, points to chemical inevitability in origination (given proper conditions that may be improbable in the universe). Paleontological discoveries, starting in the mid-1950s, have shattered the previous consensus—never more than a sop to our hopes for uniqueness—that life is exceedingly improbable and only arose because so much geological time provided such ample scope for the linking of unlikely events (given enough trials, you will eventually flip thirty heads in a row). Under this discredited view, life arose relatively late in the earth’s history, following a long geological era called “Azoic” (or lifeless, and representing the time needed for all those trials before the thirty fortunate successes).
But fossils of simple unicellular creatures have now been found in appropriate rocks of all ages, including the very oldest that could contain evidence of past life. The earth is 4.5 billion years old, but heat generated from two major sources—the decay of short-lived radioisotopes and bombardment by cosmic debris that pervaded the inner solar system during its early history—melted the earth’s surface some 4 billion years ago. All rocks must therefore postdate this early liquefaction. The oldest known rocks on earth are a bit older than 3.8 billion years, but they have been so altered by heat and pressure that no fossils could have survived. The oldest rocks that could contain preserved organic remains are 3.5 to 3.6 billion years old from Australia and South Africa—and both deposits do feature fossils of single-celled creatures similar to modern bacteria. Hints and indications are not proofs, but I don’t know what message to read in this timing but the proposition that life, arising as soon as it could, was chemically destined to be, and not the chancy result of accumulated improbabilities.
But if origination bears a signature of chemical inevitability, the pattern of later history tells a story of historical contingency dominated by portentous but unpredictable events. (I find nothing strange or unlikely in such a model of historical chanciness for subsequent pattern following a substrate of initial necessity. One might argue, for example, that the origin of speech and writing follows predictably from the evolved cognitive structure of the human mind. But the actual languages that developed, their timings and their interrelationships, wou
ld never unfold in the same way twice.)
Yet whatever attitude we adopt towards the total pattern, we must at least admit that one key event—the origin of multicellular animals—carries no prima facie signature of stately inevitability. If multicellular complexity is a predictable advance upon unicellular existence, then this salutary benefit surely took its time arising, and certainly burst upon the scene with unseemly abruptness by quirky and circuitous routes.
Nearly five-sixths of life’s history is the story of single-celled creatures (with some amalgamation, towards the end to threads, sheets, and filaments of algal grade—an event entirely separate from the origin of animals in any case). Then, about 650 million years ago, the first multicellular assemblage appears in rocks throughout the world. This fauna, named Ediacara for an Australian locality, consists entirely of soft-bodied creatures with anatomical designs strikingly different from all modern animals (flattened disks, ribbons, and pancakes composed of strips quilted together). Some paleontologists have suggested that the Ediacara animals bear no relationship to modern creatures, and represent a separate, but failed, experiment in multicellular life.
Multicellular animals of modern design—and with hard parts readily preservable as fossils—first appear, also with geological alacrity, in an episode called the “Cambrian Explosion” some 550 million years ago. Trilobites, a group of fossil arthropods beloved of all collectors, provide the principal signature for this first fauna of modern design. The full flowering of this initial fauna reaches its finest expression in the exquisite, soft-bodied fossils of the Burgess Shale, subject of my recent book, Wonderful Life.
This basic pattern has been well publicized and is now known to most nonprofessionals with strong interests in the history of life: a long period of unicellular creatures only; followed by a rapid appearance of the Ediacara fauna, perhaps with no relationship to living animals; and the final, equally quick, origin of modern anatomical designs in the Cambrian Explosion, with maximum expression soon thereafter in the Burgess Shale.
Less well known is the fine-scale geological anatomy of the Cambrian Explosion itself. Trilobites do not appear in the earliest Cambrian strata with hard-bodied fossils; they enter the geological record in the second phase of the Cambrian, called Atdabanian. The initial phase, called Tommotian after a Russian locality, contains a fauna with an interesting balance of the familiar and the decidedly strange. The new discoveries in China and Greenland give us our first decent insight into the anatomical character of the strange component—hence the great importance of these new finds, for we cannot grasp the ordinary (so designated only because they survived to yield modern descendants) without the surrounding context of creatures that left no progeny and therefore appear to us like products of a science fiction novel.
The earth’s first hard-bodied fauna of the Tommotian does include several fossils of modern design—sponges, echinoderms, brachiopods, and mollusks, for example. It also features an outstanding group of large, reef-building creatures that died out well before the end of the Cambrian. These enigmatic animals, called archaeocyathids, resemble a two-layered cone. Put one ice-cream cone within another, leave a small space between, and you have a reasonable anatomical model for an archaeocyathid. The affinities of archaeocyathids have been debated for more than a century, with uncertain results. Most paleontologists would probably vote for a position near sponges, but scientific issues are not settled at the ballot box, and other opinions enjoy strong minority support.
But by far the most enigmatic, and most mind-boggling, component of the Tommotian faunas includes a set of bits and pieces with a catch-all name that spells frustration. These tiny spines, plates, caps, and cups tell us so little about their origin and affinity that paleontologists dub them the “small, shelly fauna,” or SSF for short. “Small shellies” may be a charming phrase, when issued from the mouth of a professional who usually spouts incomprehensible Latin jargon, but please remember that this name conveys ignorance and frustration rather than delight.
We may envision two obvious potential interpretations for the SSF. Perhaps they are the coverings of tiny, entire organisms, a diminutive fauna for a first try at modernity. But perhaps—and this second alternative has always seemed more likely to paleontologists—they are bits and pieces representing the disarticulated coverings of larger multicellular organisms studded with hundreds or thousands of these SSF elements. This second position certainly makes sense. We can easily imagine that the ability to secrete hard skeletons had not fully developed in these earliest days, and that many of the first skeletonized organisms did not bear a discrete, fully protective shell, but rather a set of disconnected, or poorly coordinated fragments that only later coalesced to complete skeletons. These fragments, disarticulating after death, would form the elements of the SSF.
If this second interpretation prevails, then paleontologists are in deep trouble, and well up the proverbial creek named for the droppings of these and all later creatures. For how can we possibly reconstruct a complete animal from partial fragments that didn’t even form a coherent skeleton, and that clothed a creature of entirely unknown shape and form? Yet we can obtain no real insight into the full nature of this crucial, first Tommotian fauna until we can reanimate these most important components of the SSF. Jigsaw puzzles are hard enough when we have all the pieces and their ensemble forms a picture that can guide us as we assemble the parts. But the SSF fragments set a daunting and almost hopeless task, for they probably represent pieces from one hundred different jigsaw puzzles all mixed together. The pieces contain no pictures, and we probably have less than one piece in ten of the total covering for each frame. Moreover, to make matters even worse, we don’t know the sizes or shapes of the frames.
In this light, the reanimation of a complete SSF animal from preserved skeletal fragments seems truly hopeless—and so it has been, as two decades of work have produced no plausible reconstructions. We must adopt another strategy—unfortunately passive in one sense, though active in another. We must hope to discover a different kind of fossil—not the common disarticulated bits that cannot be reassembled, but a rare preservation of an entire SSF organism with all its elements in place. I call such a change in focus passive because we must wait for the discovery of a basically soft-bodied creature with its covering bits of shell still in place—and soft-bodied preservation is rare in the fossil record. But this strategy is also active because we now have good guidelines for exploration; we now know where and how to look for soft-bodied fossils.
The discoveries in Greenland and China can now be placed into proper context and excitement in a single sentence: They represent the first remains of entire SSF organisms, preserved with full coverings of their separated skeletal elements. The second interpretation of the SSF has prevailed. These cups, caps, cones, and spines are bits and pieces of incomplete skeletons upon larger organisms—and we finally have some insight into the nature of these important creatures; the dominant component of the earth’s first skeletonized fauna. (The SSF elements arise in the earliest Tommotian beds, but persist into subsequent Cambrian strata. The SSF animals of China and Greenland were found in later rocks containing trilobites as well, but their SSF elements are identical with those found in earliest Tommotian sediments, so the two organisms are true representatives of this heretofore mysterious first fauna.)
Microdictyon is a classic element of the SSF. The hard parts, and previously only-known components, are round to oval, gently convex, phosphatic caps, no more than 3 mm in diameter. Each cap is a meshwork of hexagonal cells with round holes in the center of each cell (see figure). How could a paleontologist possibly move from this limited morphology to a reconstruction of the animal that secreted these partial coverings?
Since scientists, having no access to divine inspiration or the magical arts, cannot make such a move, Microdictyon has simply stood as a stratigraphic marker of its time and a complete mystery in anatomical terms. Microdictyon has been found worldwide in rocks of To
mmotian to middle Cambrian age in Asia, Europe, North and Central America, and Australia.
A plate of Microdictyon showing the characteristic meshwork. Acta Palaeontologica Sinica, Vol. 28, No. 1, p. 5.
In 1989, three Chinese colleagues from the Nanjing Institute of Geology and Paleontology—Chen Jun-yuan, Hou Xianguang, and Lu Haozhi—published a remarkable article in volume 28 of the Acta Paleontologia Sinica. (I remain profoundly grateful for the international character and cooperative traditions of paleontological work. Our science is global, and we would be stymied if we lost access to information from selected parts of the world. I thank both Drs. Chen and Hou for sending me reprints of their work along with letters providing further valuable data about their discoveries.)
Drs. Chen, Hou, and Lu have been working with the remarkable Chengjiang fauna of south-central China, an equivalent in age and soft-bodied preservation of the famous Burgess Shale in western Canada. Among other stunning creatures of unknown affinity, they discovered several specimens of a worm-like animal, some 8 cm in length.
Restoration of Microdictyon sinicum showing the paired side plates. Acta Palaeontologica Sinica, Vol. 28. No. 1, p. 5.
This creature (see figure) bore ten thin pairs of leglike appendages, generally decreasing in strength from front to back. Traces of a simple, tubular gut can be seen on most specimens. But, most remarkable of all, this animal carried pairs of rounded phosphatic caps, inserted in pairs on the body sides, just above the joining points of the legs with the trunk. Each pair of legs, in other words, sports a corresponding pair of caps on the trunk above. These caps, mirabile dictu, are the elements previously named Microdictyon, but known only from the uninterpretable hard-part dabs. Their discovery on the Chinese animal not only adds a fascinating and mysterious creature to the roster of earliest animals, but also confirms our long-held suspicions about the SSF. Microdictyon, at least, is just an element covering a much larger body. Moreover, the hard parts enclose only a small portion of the body and do not articulate with each other (the space between pairs of caps is about double the diameter of the caps themselves). How could we possibly have inferred the character of the animal from the caps alone?
Eight Little Piggies Page 32