I Have Landed (39 page)

One other group, until a discovery published in 1999, had also yielded no record within the Cambrian explosion, although late-Cambrian representatives
(well after the explosion itself) have been known for some time. But, whereas popular texts have virtually ignored the Bryozoa, the absence of this other group had been prominently showcased and proclaimed highly significant. No vertebrates had ever been recovered from deposits of the Cambrian explosion, although close relatives within our phylum (the Chordata), but not technically vertebrates, had been collected. (The Chordata includes three major subgroups: the tunicates,
Amphioxus
and its relatives, and the vertebrates proper.)

This absence of vertebrates from strata bearing nearly all other fossilizable animal phyla provided a ray of hope for people who wished to view our own group as “higher” or more evolved in a predictable direction. If evolution implies linear progression, then later is better—and uniquely later (or almost uniquely, given those pesky bryozoans) can only enhance the distinction. But the November 4, 1999, issue of
Nature
includes a persuasive article by D.-G. Shu, H.-L. Luo, S. Conway Morris, X.-L. Zhang, S.-X. Hu, L. Chen, J. Han, M. Zhu, Y. Li, and L.-Z. Chen (“Lower Cambrian vertebrates from South China,” volume 402, pages 42–46), reporting the discovery of two vertebrate genera within the Lower Cambrian Chengjiang Formation of South China, right within the temporal heart of the Cambrian explosion. (The Burgess Shale of Western Canada, the celebrated site for most previous knowledge of early Cambrian animals, postdates the actual explosion by several million years. The recently discovered Chengjiang fauna, with equally exquisite preservation of soft anatomy, has been yielding comparable or even greater treasures for more than a decade.)

These two creatures—each only an inch or so in length, and lacking both jaws and a backbone, in fact possessing no bony skeleton at all—might not strike a casual student as worthy of inclusion within our exalted lineage. But jaws and backbones, however much they may command our present focus, arose later in the history of vertebrates, and do not enter the central and inclusive taxonomic definition of our group. The vertebrate jaw, for example, evolved from hard parts that originally fortified the gill openings just behind, and then moved forward to surround the mouth. All early fishes lacked jaws—as do the two modern survivors of this initial radiation, the lampreys and hagfishes.

The two Chengjiang genera possess all defining features of vertebrates—the stiff dorsal supporting rod or notochord (subsequently lost in adults after the vertebral column evolved), the arrangement of flank musculature in a series of zigzag elements from front to back, the set of paired openings piercing the pharynx (operating primarily as respiratory gills in later fishes, but used mostly for filter-feeding in ancestral vertebrates). In fact, the best reconstruction of
branching order on the vertebrate tree places the origin of these two new genera after the inferred ancestors of modern hagfishes, but before the presumed forebears of lampreys. If this inference holds, then vertebrates already existed in substantial diversity within the Cambrian explosion. In any case, we now know two distinct and concrete examples of vertebrates all the way down. We vertebrates do not stand higher and later than our invertebrate cousins, for all “advanced” animal phyla made their debut in the fossil record at essentially the same time. The vaunted complexity of vertebrates did not require a special delay to accommodate a slow series of progressive steps, predictable from the general principles of evolution.

2.
An ultimate parasite, or “how are the mighty fallen.”
The phyla of complex multicellular animals enjoy a collective designation as Metazoa (literally, higher animals). Mobile, single-celled creatures bear the name Protozoa (or “first animals”—actually a misnomer, since most of these creatures stand as close to multicellular plants and fungi as to multicellular animals on the genealogical tree of life). In a verbal in-between stand the Mesozoa (or “middle animals”). Many taxonomic and evolutionary schemes for the organization of life rank the Mesozoa exactly as their name implies—as a persistently primitive group, intermediate between the single-celled and multicellular animals, and illustrating a necessary transitional step in a progressivist reading of life's history.

But the Mesozoa have always been viewed as enigmatic—primarily because they live as parasites within truly multicellular animals, and parasites often adapt to their protected surroundings by evolving an extremely simplified anatomy, sometimes little more than a glob of absorptive and reproductive tissue cocooned within the body of a host. Thus the extreme simplicity of parasitic anatomy could represent the evolutionary degeneration of a complex, free-living ancestor rather than the maintenance of a primitive state.

The major group of mesozoans, the Dicyemida, live as microscopic parasites in the renal organs of squid and octopuses. Their adult anatomy could hardly be simpler—a single axial cell (which generates the reproductive cells) in the center, enveloped by a single layer of ciliated outer cells, some ten to forty in number, and arranged in a spiral around the axial cell, except at the front end, where two circlets of cells (called the
calotte
) form a rough “mouth” that attaches to the tissues of the host.

The zoological status of the dicyemids has always been controversial. Some scientists, including Libbie Hyman, who wrote the definitive multivolume text on invertebrate anatomy for her generation, regarded their simplicity as primitive, and their evolutionary status as intermediate in the rising complexity of
evolution. She wrote in 1940: “their characters are in the main primitive and not the result of parasitic degeneration.” But even those researchers who viewed the dicyemids as parasitic descendants of more-complex, free-living ancestors never dared to derive these ultimately simple multicellular creatures from a
very
complex metazoan. For example, Horace W. Stunkard, the leading student of dicyemids in the generation of my teachers, thought that mesozoans had descended from the simplest of all Metazoa above the grade of sponges and corals—the platyhelminth flatworms.

Unfortunately, the anatomy of dicyemids has become so regressed and specialized that no evidence remains to link those creatures firmly with other animal groups, so the controversy of persistently primitive versus degeneratively parasitic could never be settled until now. But newer methods of gene sequencing can solve this dilemma, because even though visible anatomy may fade or transform beyond genealogical recognition, evolution can hardly erase all traces of complex gene sequences. If genes known only from advanced Metazoa—and known to operate only in the context of organs and functions unique to Metazoa—also exist in dicyemids, then these creatures should be interpreted as degenerated metazoans. But if, after extensive search, no sign of distinctive metazoan genomes can be detected in dicyemids, then the Mesozoa may well rank as intermediates between single and multicelled life after all.

In the October 21, 1999, issue of
Nature
, M. Kobayashi, H. Furuya, and P. W. H. Holland present an elegant solution to this old problem (“Dicyemids Are Higher Animals”). These researchers located a
Hox
gene—a member of a distinctive subset known only from metazoans and operating in the differentiation of body structures along the antero-posterior (front to back) axis—in
Dicyema orientale
. These particular
Hox
genes occur only in triploblastic, or “higher,” metazoans with body cavities and three cell layers, and not in any of the groups (such as the Porifera, or sponges, and the Cnidaria, or corals and their relatives) traditionally placed “below” triploblasts. Thus the dicyemids are descended from “higher,” triploblastic animals and have become maximally simplified in anatomy by adaptation to their parasitic lifestyle. They do not represent primitive vestiges of an early stage in the linear progress of life.

In short, if the traditionally “highest” of all triploblasts—the vertebrate line, including our exalted selves—appears in the fossil record at the same time as all other triploblast phyla in the Cambrian explosion; and if the most anatomically simplified of all parasites can evolve, as an adaptation to local ecology, from a free-living lineage within the “higher” triploblast phyla; then the biological, variational, and Darwinian meaning of “evolution” as unpredictable
and nondirectional gains powerful support from two cases that, in a former and now disproven interpretation, once bolstered an opposite set of transformational prejudices.

As a final thought to contrast the predictable unfolding of stellar “evolution” with the contingent nondirectionality of biological “evolution,” I should note that Darwin's closing line about “this planet . . . cycling on according to the fixed law of gravity,” while adequate for now, cannot hold for all time. Stellar “evolution” will, one day, enjoin a predictable end, at least to life on Earth. Quoting one more time from the
Britannica
article on stellar evolution:

The same predictability also allows us to specify the timing of this catastrophe—about 5 billion years from now! A tolerably distant future to be sure, but consider the issue in comparison with the very different style of change known as biological evolution. Our planet originated about 4.6 billion years ago. Thus, half of the earth's potential history unfolded before contingent biological evolution produced even a single species with consciousness sufficient to muse over such matters. Moreover, this single lineage arose within a marginal group of mammals (among two hundred species of primates amid only four thousand or so species of mammals overall; by contrast, the world holds at least half a million species of beetles alone among insects). If a meandering process consumed half of all available time to build such an adaptation even once, then mentality at a human level certainly doesn't seem to rank among the “sure bets,” or even mild probabilities, of history.

We must therefore contrast the good fortune of our own “evolution” with the inexorable “evolution” of our nurturing sun toward a spectacular climax that might make our further evolution impossible. True, the time may be too distant to inspire any practical concern, but we humans do like to ponder and to wonder. The contingency of our “evolution” offers no guarantees against the certainties of the sun's “evolution.” We shall probably be long gone by then, perhaps taking a good deal of life with us, and perhaps leaving those previously indestructible bacteria as the highest mute witnesses to a stellar expansion leading to unicellular Armageddon as well. Or perhaps we, or our successors, will
have colonized the universe by then, and will only shed a brief tear for the destruction of a little cosmic exhibit titled “the museum of our geographic origins.” Somehow, I prefer the excitement of wondering and cogitation—not to mention the power inherent in action upon things that
can
be changed—to the certainty of distant dissolution.

T
HE
COMPARISON
OF
THE
HUMAN
BODY
AND
THE
UNI
verse—the microcosm with the macrocosm—has served as a standard device for explicating both the factuality and the meaning of nature throughout most of Western history. When Leonardo da Vinci, for example, likened our bodily heat, breath, blood, and bones to the lavas of volcanic eruptions, the effusions of interior air in earthquakes, the emergence of streams from underground springs, and the rocks that build the earth's framework—and then interpreted these sequences as particular expressions of the four Greek elements of fire, air, water, and earth—he did not view his argument as an excursion into poetry or metaphorical suggestion, but as his best understanding of nature's actual construction.

We now take a more cynical, or at least a more bemused, view of such analogistic reveries—for we recognize that the cosmos, in all
its grandness, does not exist for us, or as a mirror of our centrality in the scheme of universal things. That is, we would now freely admit that most attempts to understand such geological or astronomical scales of size and time in terms of comfortable regularities noted in our short life spans or puny dimensions can only represent, in the most flattering interpretation, an honorable “best try” within our own mental and perceptual limits or, at worst, yet another manifestation of the ancient sin of pride.

As a striking example, however unrecognized by most people who could scarcely avoid both walking the walk and talking the talk, the recent fuss over our millennial transition cannot be entirely ascribed to modern commercial hype because the taproot of concern draws upon one of the oldest surviving arguments about deep and meaningful coincidence between the human microcosm and the surrounding macrocosm of universal time and space—in this case, an explicit comparison of human secular calendars to the full sweep of the creation and subsequent history of the earth and life. By this reckoning, January 1, 2000, should have marked the termination of the old order, and the inception of something new and at least potentially glorious. This momentous turning of calendrical dials should therefore have inspired our attention for reasons almost immeasurably deeper than the simple visual attraction of changing all four markers from 1999 to 2000—the “odometer rationale,” if you will. (Of course, the vast majority of people, in our secular and technological age, have forgotten this old, and factually discarded, Christian argument for the significance of millennial turnings. But vestiges of these historical claims still affect both our calendars and our discourse. Moreover, and with potentially tragic results, the vestiges of a majority persist as literal portents for a few “true believers,” leading, in the most extreme case, to the suicide of thirty-nine members of the Heaven's Gate cult in 1997.)