Eight Little Piggies (32 page)

2.
The evolution of complex cells
. Many biologists would place nature’s fundamental distinction not between plants and animals, or even between unicellular and multicellular organisms, but at a division within unicellular creatures. The structurally simple prokaryotes, bacteria and cyanophytes, have no organelles within their cells—no nucleus or chromosomes, no mitochondria. The complex eukaryotes have evolved the array of internal structures that grace (or disfigure according to your view or status) nearly every high school biology final with its inevitable question: Label all the parts of the cell and state their functions.

This increment of complexity from prokaryote to eukaryote is deemed fundamental, in part because we view eukaryote organization as an absolute prerequisite to the later evolution of multicellular organisms. (To cite just one standard argument: Darwinian evolution of complexity requires copious variation to fuel natural selection; most variation arises from the mixture, via sexual reproduction, of two differing genetic systems in each offspring; sexual reproduction requires a mechanism for exact division of genetic material so that 50 percent of each parent reconstitutes the needed 100 percent in offspring; meiosis by reduction division of paired chromosomes is the biological invention that secured equal separation; prokaryotes, lacking chromosomes and other organelles, cannot produce an exact genetic halving.)

We now encounter the same conundrum faced in the last example: We can see why multicellular life required the evolution of organelles, but eukaryotic cells arose at least 800 million years before the origin of multicellular animals—so progress to multicellular complexity cannot be the reason why organelles evolved.

A favored theory for the origin of some organelles (the mitochondrion and chloroplast but not, alas, the nucleus, for which no good theory now exists) invokes the process of symbiosis. Mitochondria and chloroplasts look uncannily like entire prokaryotic organisms (they have their own DNA and are the same size as many bacteria). Almost surely, they began as symbionts within cells of other species and later became more highly integrated to form the eukaryotic cell (so that each cell in our body has the evolutionary status of a former colony). Now, one can argue that the wedge drove the ancestors of mitochondria to a life of symbiosis. These bacteria, gaining protection or whatever, did not enter the primordial eukaryote in order to provide an opportunity for multicellular complexity a billion years down the road. Symbiosis occurred for immediate Darwinian reasons; then the wheel turned and the rubber made for symbiosis put the first footprints on the path of multicellular complexity.

3.
The basic features of human consciousness
. The wheel and the wedge then interact for more than half a billion years to the separation of our lineage from the ancestry of chimpanzees some six to eight million years ago. The wedge produces some forward motion (and more blind alleys of overspecialization), but the wheel inaugurates each domain of change—the limbs on an odd group of fishes, by an unusual arrangement of fin bones, can bear the body’s weight on land (see Essay 4); mammals get a chance after 100 million years in the backwaters because dinosaurs succumb in a mass extinction.

Australopithecus
now begins the process that textbooks used to call, before we reformed our language to include all people, the “ascent of man.” Doesn’t the wedge finally prevail? Isn’t the unreversed trend of increasing brain size, from
Australopithecus
to
Homo habilis
to
Homo erectus
to us, driven by ordinary natural selection working on the advantages of superior cognition? Let me take the most conservative argument of the wedge (not my actual view) and reply: Yes, fine; I agree. The human brain got large because natural selection directly favored some traits of cognition that gave bigger-brained people advantages in competition.

Does such an admission imply that the foundations of human cognition, the universal traits that we define as “humanity” or “human nature,” were built directly by the wedge? Of course not—and no argument is more important for our understanding of human nature, yet less widely appreciated, than this. Yes, the brain got big by natural selection. But as a result of larger size, and the neural density and connectivity thus imparted, human brains could perform an immense range of functions quite unrelated to the original reasons for increase in bulk. The brain did not get big so that we could read or write or do arithmetic or chart the seasons—yet human culture, as we know it, depends upon skills of this kind. If you label me as a hopelessly parochial academic for citing only the skills of an intellectual elite, I reply that the fortuitous side consequences of large brains include the defining activities of all people. What about language, the most widely cited common denominator and distinguishing factor of humanity? And I don’t mean using sound or gesture for communication, as many complex animals do. I refer to the unique syntax and underlying universal grammar of all languages. I can’t prove that language was not the selected basis of increasing brain size, but the universals of language are so different from anything else in nature, and so quirky in their structure, that origin as a side consequence of the brain’s enhanced capacity, rather than as simple advance in continuity from ancestral grunts and gestures, seems indicated. (I lay no claim to originality for this argument about language. The reasoning follows directly as an evolutionary reading for Noam Chomsky’s theory of universal grammar.)

To cite another example, consider Freud’s argument on the origin of religion—or at least of a belief in some form of persistence after death as a common feature of this institution. Freud held that all religions maintain some belief in personal persistence after death—whether in heaven, by reincarnation, in a universal soul, or merely by continuity of tradition. This belief marks the common basis of religion because our large brains “forced” us to learn and acknowledge the fact of personal mortality (a concept not clearly grasped by any other animal). Now you cannot argue that our brains became large so that we would appreciate the fact of our death; knowledge of mortality is an inevitable (and largely unfortunate) side consequence of mental power evolved for other reasons. Yet this unwanted knowledge forms the basis of an institution often regarded as the most fundamental consequence of human nature.

If such features as language and the basis of religion are side consequences of the wheel, not direct gifts of the wedge, then is human nature a predictable product of organic improvement, honed in the fires of competition, or a set of oddly cobbled side consequences rooted in an unparalleled neural complexity built for other reasons? We are a bit of both—though more, I suspect, quirks of the wheel than boons of the wedge—and in this mixture lies our hope and our destiny.

If I have upset your equanimity by attributing the genuine complexity of human cognition to fortuity piled upon fortuity (with a little yardage for predictability after each spin of the wheel), then I must apologize for one further disturbance in conclusion. We talk about the “march from monad to man” (old-style language again) as though evolution followed continuous pathways of progress along unbroken lineages. Nothing could be further from reality. I do not deny that, through time, the most “advanced” organism has tended to increase in complexity. But the sequence from protozoan to jellyfish to trilobite to nautiloid to armored fish to dinosaur to monkey to human is no lineage at all, but a chronological set of termini on unrelated evolutionary trunks. Moreover, life shows no trend to complexity in the usual sense—only an asymmetrical expansion of diversity around a starting point constrained to be simple. Let me explain that last cryptic remark: For reasons of organic chemistry and the physics of self-organizing systems, life arose at or very near the lower limit of preservable size and complexity in the fossil record. Since diversity, measured as number of species, has increased through time, extreme values in the distribution of complexity can move in only one direction. No species can become simpler than the starting point, for life arose at the lower limit of preservable complexity. The only open direction is up, but very few species take this route. Increasing complexity is not a purposeful trend of an unbroken lineage but only the upper limit of an expanding distribution as overall diversity increases. We focus on this upper tail and call its expansion a trend because we crave some evolutionary rationale for our perception of ourselves as a predictable culmination.

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:

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?

SAMUEL TAYLOR COLERIDGE
, in a reverie laced with laudanum, presented an image of striking incongruity in describing the pleasure palace of Kubla Khan:

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, would 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 Tommotian to middle Cambrian age in Asia, Europe, North and Central America, and Australia.