I Have Landed (47 page)

On the same theme of power in exceptions, and to make a somewhat ironic point in closing, Linnaeus's hope that he had discovered a fully universal basis
(God's own rules of creation) for classifying all natural objects has recently suffered another fascinating blow—but this time from inside (that is, from the world of organisms). Science had already denied Linnaeus's universality more than two hundred years ago by accepting his procedures only for historically generated genealogical systems based on a geometry of branching (the evolution of organisms as a primary example), and rejecting his binomial schemes for rocks, diseases, and other systems based upon different theoretical foundations of order. (In fact, Linnaeus did try to establish a binomial classification of human diseases as well—in one of his least successful treatises,
Genera morborum
, published in 1736 and immediately forgotten!) But now, one of the most important biological discoveries of our age has also challenged the universal application of Linnaean taxonomy in the world of organisms as well.

We need not fret for fat, furry, multicellular creatures—the plants, fungi, and animals of our three great multicellular and macroscopic kingdoms of life. For evolution, in this visible world of complex creatures, does follow the Linnaean topology nearly all the time. That is, the basic structural rule that validates the binomial system works quite satisfactorily at this level—for branches never join once they have separated, and each species therefore becomes a permanently independent lineage, forging no further combinations with others after its origin. Evolution cannot make a nifty new species of mammal by mixing half a dolphin with half a bat to generate an all-purpose flyer and swimmer.

Until a few years ago, we thought that this rule of permanent separation also applied to the world of unicellular bacteria—the true dominators of earth and rulers of life in my opinion (see my book
Full House
, 1996). In other words, we assumed that the bacterial foundation of the tree of life grew in a fully Linnaean manner, just like the multicellular section. (Actually, and to emphasize the importance of the discovery described below, the bacterial domain occupies most of life's tree because the three multicellular kingdoms sprout as three terminal twigs on just one of the tree's three great limbs, the other two being entirely bacterial.)

And, apparently, we were wrong. By a set of processes collectively called “lateral gene transfer” (LGT for short), individual genes and short sequences of genes can move from one bacterial species to another. For two reasons, these transfers may challenge Linnaean logic in a serious way. First, LGT does not seem to respect taxonomic separation. That is, genes from genealogically distant bacterial species seem to enter a host species with no greater difficulty than genes from closely related species. Second, the process occurs frequently enough to preclude dismissal as a rare and peculiar exception to the prevailing
Linnaean rule of strict branching with no subsequent amalgamation. (If only a percent or two of bacterial genomes represented imports from distant species by LGT, we could view the phenomenon as a fascinating anomaly that does not degrade the primary signal of Linnaean reality. But, at least for some species, LGT may be sufficiently common to flash a primary signal of its own. In
E. coli
, the familiar bacillus found in all human guts, for example, 755 of 4,288 genetic units [about 18 percent of the entire genome] record at least 234 events of lateral genetic transfer during the last 100 million years.)

Professional evolutionary biologists have been puzzled and excited by these discoveries about LGT. But the word has hardly filtered through to the interested public—an odd situation given the status of LGT as a challenge to one of our most basic assumptions about the nature and fundamental topology of evolution itself, not to mention the foundation of Linnaean logic as well! Perhaps most of us just don't care about invisible bacteria, whereas we would sit up and take notice if we heard that LGT played a major role in the evolution of animals. Or perhaps the issue strikes most people as too abstract to command the same level of attention that we heap and hype upon such events of minimal theoretical interest as the discovery of a new carnivorous dinosaur larger than
Tyrannosaurus
. But I would not so disrespect the concerns of public understanding. Properly explained, the theoretical challenge of LGT to some truly fundamental views about the nature of evolution and classification should be fascinating to all people interested in science and natural history.

To put the matter baldly, if LGT plays a large enough role in bacterial evolution to overcome the Linnaean signal of conventional branching without subsequent joining, then binomial logic really doesn't work. An honest diagnosis could not then recommend a simple remedy of some minor repairs or small plaster patches—for the Linnaean system would truly be broken by the collapse of this central theoretical prerequisite. The hierarchical basis of Linnaean logic demands that life's history develop as a tree, without amalgamation of branches once a lineage originates in independence. But if LGT dominates the composition of bacterial genomes, then trees cannot express the topology of evolutionary relationships, because the pathways of life then form a meshwork, as bacteria evolve by importing genes from any position, no matter how evolutionary distant, on the genealogical net.

But don't just trust the words of this expert on land snails (in the realm of fat, furry things) and diligent essayist on subjects beyond his genuine expertise. Consider these measured words from a technical article by the leading researcher on the subject (W. Ford Doolittle of Dalhousie University, Halifax, writing in the June 25, 1999, issue of
Science
, a special report on evolution from
America's leading journal for professional scientists). Doolittle wrote, in an article titled “Phylogenetic classification and the universal tree”:

Do not lament for the spirit of Linnaeus. Yes, his dreams about the discovery of a universal system suffered two sequential blows—first, soon after his death, when scientists recognized that his logic could only work for organisms, and not for rocks and all the rest of the natural world. And, second, as discovered only in the last decade, when Linnaean taxonomy encountered a strong biological challenge from the frequency of lateral gene transfer, the ultimate tree-buster, in the substantial domain of bacteria, albeit not so strongly expressed in our own world of multicellular life (although the sequence of the human genome, published in February 2001, does reveal some important bacterial “immigrants” as well).

As a truly great scientist, Linnaeaus understood the central principle that honorable error, through overextension of exciting ideas, “comes with the territory”—and that theories gain both strength and better definition by principled limitations upon their realm of legitimate operation. Moreover, as the modern founder of the truly noble science of taxonomy, Linnaeaus also understood that all classifications must embody passionate human choices about the causes of order—in short, theories that must be subject to continuous revision and correction of error—and cannot only be conceived as passive descriptions of objective nature on the philatelic model.

Thus, taxonomies must express both concepts and percepts—and must therefore teach us as much about ourselves and our mental modes as about the structure of external nature.
¹⁴
Surely Linnaeaus, of all people, comprehended this fundamental and ineluctable interrelationship of mind and nature, for
when he composed, at the very beginning of
Systema Naturae
, his formal description of his newly crowned species,
Homo sapiens
, he linked us (in various editions)—in only one case correctly, as we now know—with three other mammals: monkeys, sloths, and bats. For each of these three, Linnaeus penned a conventional and objective description in terms of hairiness, body size, and number of fingers and toes. But for
Homo sapiens
, he chose the path of terseness and wrote just the three Latin words of another familiar motto. Not
natura non facit saltum
this time, but the foremost intellectual challenge of classical wisdom:
Nosce te ipsum
—Know thyself.

R
EVOLUTIONS
CANNOT
BE
KIND
TO
PROMINENT
AND
UNRE
-constructed survivors of a superseded age. But the insight and dignity of vanquished warriors, after enough time has elapsed to quell the immediate passions of revolt, often inspire a reversal of fortune in the judgment of posterity. (Even the most unabashed northerner seems to prefer Robert E. Lee to George McClellan these days.)

This essay details a poignant little drama in the lives of three great central European scientists caught in the intellectual storm of Darwin's
Origin of Species
, published in 1859. This tale, dormant for a century, has just achieved a vigorous second life, based largely on historical misapprehension and creationist misuse. Ironically, once we disentangle the fallacies and supply a proper context for understanding, our admiration must flow to Darwin's two most prominent opponents from a dispersed and defeated conceptual world: the Estonian (but ethnic German) embryologist and general naturalist
Karl Ernst von Baer (1792–1876), who spent the last forty years of his life teaching in Russia, and the Swiss zoologist, geologist, and paleontologist Louis Agassiz (1807–1873), who decamped to America in the 1840s and founded Harvard's Museum of Comparative Zoology, where I now reside as curator of the collection of fossil invertebrates that he began.

Meanwhile, our justified criticism must fall upon the third man in this topsy-turvy drama, the would-be hero of the new world order: the primary enthusiast and popularizer of Darwin's great innovation, the German naturalist Ernst Haeckel (1834–1919). Haeckel's forceful, eminently comprehensible, if not always accurate, books appeared in all major languages and surely exerted more influence than the works of any other scientist, including Darwin and Huxley (by Huxley's own frank admission), in convincing people throughout the world about the validity of evolution.

I willingly confess to hero-worship for the raw intellectual breadth and power of three great men: Darwin, who constructed my world; Lavoisier, because the clarity of his mind leaves me awestruck every time I read his work; and Karl Ernst von Baer, who lived too long and became too isolated to win the proper plaudits of posterity. But T. H. Huxley, who ranks fourth on my personal list, regarded von Baer as the greatest pre-Darwinian naturalist of Europe, and I doubt that any expert with the detailed knowledge to render judgment about general brilliance and specific accomplishments would disagree.

As the leading embryologist of the early nineteenth century, von Baer discovered the mammalian egg cell in 1827 and then, in 1828, published the greatest monograph in the history of the field:
Entwickelungsgeschichte der Thiere
(The developmental history of animals). He then suffered a mental breakdown and never returned to the field of embryology. Instead he moved to St. Petersburg in 1834 (a common pattern for central European scientists, as Russia, lacking a system of modern education, imported many of its leading professors in scientific subjects from abroad). There he enjoyed a long and splendid second career as an Arctic explorer, a founder of Russian anthropology, and a geomorphologist credited with discovering an important law relating the erosion of riverbanks to the earth's rotation.

Von Baer's theories of natural history allowed for limited evolution among closely related forms, but not for substantial transformation between major groups. Moreover, he held no sympathy for Darwin's mechanistic views of evolutionary causality. Darwin's book shook the aged von Baer from decades of inactivity in his old zoological realm—and this great man, whom Agassiz, in his last (and posthumously published) article of 1874, would call “the aged
Nestor of the science of Embryology,” came roaring back with a major critique entitled
Über Darwins Lehre
(On Darwin's Theory).

In a second article written to criticize a brave new world that often disparaged or even entirely forgot the discoveries of previous generations, von Baer made a rueful comment in 1866 that deserves enshrinement as one of the great aphorisms in the history of science. Invoking Louis Agassiz, his younger friend and boon companion in rejecting the new theory of mechanistic evolution, von Baer wrote:

Ernst Haeckel (1834–1919), with his characteristic mixture of gusto and bluster, fancied himself a Darwinian general, embattled in Agassiz's first two stages and unfurling the new evolutionary banner not only for biological truth, but for righteousness of all stripes. In 1874 he wrote in his most popular book,
Anthropogenie
(The Evolution of Man):