When I arrived on January 16, Kamoya’s team had just found a new and remarkably well-preserved ape skull (in a profession that usually works with fragments, mostly teeth, a skull more than half complete, and with a fully preserved dentition, is cause for rejoicing). The next day, we studied and mapped the geological context and then brought the specimen back to camp. I wrote in my field book: “Everyone is very excited because they have just found the finest Miocene ape skull known from Africa. It is quite new—with a long face, inflated nasal region, incisors worn flat with a diastema [gap] a finger wide to the massive canine—almost like a beaver among apes.”
The greatest of all fossil finders Kamoya Kimeu gathering material at West Turkana. Photograph by Delta Willis.
Research is a collectivity, and we all have our special skills. Kamoya’s workers are the world’s greatest spotters; Richard also has a hawk’s eye, the intuition of a geologist who has lived with his land, and the organizational skills of a Washington kingpin; his wife, Meave, has an uncanny spatial sense and can beat any jigsaw champ in putting fossil fragments together; yours truly, I fear, is good for one thing only—seeing snails.
All field naturalists know and respect the phenomenon of “search image”—the best proof that observation is an interaction of mind and nature, not a fully objective and reproducible mapping of outside upon inside, done in the same way by all careful and competent people. In short, you see what you are trained to view—and observation of different sorts of objects often requires a conscious shift of focus, not a total and indiscriminate expansion in the hopes of seeing everything. The world is too crowded with wonders for simultaneous perception of all; we learn our fruitful selectivities.
I couldn’t see bone fragments worth a damn—and Richard had to direct my gaze before I could even distinguish the skull from surrounding lumps of sediment. But could I ever see snails, the subject of my own field research—and no one else had ever found a single snail at that site. So I rest content with my minuscule contribution, made in character, to the collective effort. At the top right of page 143 in the November 13, 1986, issue of Nature—the article that describes the new skull—a few snails are included in the faunal list of the site, some added by my search image. (I also found, I believe, the first snails at the important South African hominid site of Makapansgat in 1984—where I also couldn’t see a bone. I think I am destined to be known in the circle of hominid exploration as “he who only sees the twisted one.”)
The traditions of nature writing demand that this personal narrative now continue at some length, with overwritten paeans to the wonder of this discovery, set in glowing clichés about the stark and fragile (two good adjectives) beauty of the countryside. But I desist. First of all, this isn’t my style; it also doesn’t match anything that actually happens in the field. People have varied reactions to such good fortune. Some may jump up and down, fall upon their knees to praise God, or wax eloquent about the new line wrested from nature’s complex book. Most people I know, certainly including Richard, Kamoya, and myself as outsider, do not have personalities that match these romantic stereotypes. The conversation may flow more happily at dinner; some kind of glow must form within. But you still have to make sure that the trucks have gas, that the water jugs are full—and you do have to get up at dawn the next day because it’s too hot to work in the afternoon. My favorite kind of excitement is quiet satisfaction.
Richard and Kamoya’s team found a second, smaller ape skull that field season at West Turkana. Both are new genera, not merely variants on familiar themes of the ape’s bush. Richard and Meave Leakey published two papers in the November 13, 1986, issue of Nature describing these new forms as Afropithecus (the one I witnessed) and Turkanapithecus. In the most interesting line of the Afropithecus paper, they write: “Afropithecus displays characters typical of a variety of Miocene hominoids combined in a single taxon.” In other words, this new genus represents a unique combination of features known to vary among early apes—as if we might shuffle the known variations into many more plausible combinations as yet undiscovered. The bottom line after all this exegesis is simplicity itself: We are not at, perhaps not even near, the asymptote for true diversity of apes at their flourishing beginning. If one field season in uncharted lands could yield two new genera, how many remain undiscovered in the hundreds of square miles still open for exploration? Apes were bushier than we had ever imagined during their early days; human evolution seems even more twiggy, more contingent on the fortunes of history (not enjoined like the successive rungs of a ladder), less ordained, and more fragile. Our vaunted march to progress, the standard iconography of our evolution, is just one more expression of life’s little joke.
I have consciously permitted a professional’s bias to permeate this essay so far. I have been equating “success” with numbers of branches on the bush—for paleontologists tend to view large-scale evolution as the differential birth and death of species, and we slip too easily into an equation of success with exuberance of branching. But, of course, we must also consider the quality of twigs, not merely their number. Homo sapiens is one small twig, holding with just a few others all the heritage of a group once far more diverse in branches. Yet our twig, for better or for worse, has developed the most extraordinary new quality in all the history of multicellular life since the Cambrian explosion. We have invented consciousness with all its sequelae from Hamlet to Hiroshima. Life’s little joke shows us our fragility, our smallness on the proper metaphor of the bush, but we have turned the joke upon itself with the power of one evolutionary invention.
The prophet Micah caught both sides of this tension with great understanding when he wrote that fragility and size of origin imply little about ultimate effect: “But thou, Bethlehem Ephrathah, though thou be little among the thousands of Judah, yet out of thee shall he come forth unto me that is to be ruler in Israel” (Micah 5:2). If we could merge the two themes, and if rulers could learn humility and respect from our common origins as fragile twigs on the bush of life, then we might break the equation between ability and right to dominate and might even fulfill that most famous of Biblical prophecies, which is, after all, about the proper nurturing of trees and bushes—“and they shall beat their swords into plowshares, and their spears into pruning hooks.”
6 | Grand Patterns of Evolution
Two Steps towards a General Theory of Life’s Complexity
21 | The Wheel of Fortune and the Wedge of Progress
CHARLES DARWIN was a master of metaphor, and much of his success may be attributed to his uncanny feel for timely comparisons that virtually compel understanding. We all know the two metaphors that Darwin invoked to define his theory: natural selection and the struggle for existence. We might also consider Darwin’s three principal descriptions of nature, each wonderfully apt and poetic, and each a metaphor.
The tangled bank: To stress the intricacy of relationships among organisms as arising, somewhat paradoxically, by planless evolution. Darwin begins the last paragraph of the Origin of Species:
It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us.
The tree of life: Borrowed from other contexts to be sure (Proverbs 3:18, for example), but used brilliantly by Darwin to express the other form of interconnectedness—genealogical rather than ecological—and to illustrate both success and failure in the history of life. Darwin placed this famous passage at a crucial spot in his text—the very end of chapter four, marking the conclusion of his argument for natural selection (the rest of the book discusses problems and examples):
As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by gene
ration I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications.
The face of nature (and the darkness behind): To argue that apparent balance and harmony arise from the struggle and death of individuals:
We behold the face of nature bright with gladness, we often see superabundance of food; we do not see, or we forget, that the birds which are idly singing round us mostly live on insects or seeds, and are thus constantly destroying life; or we forget how largely these songsters, or their eggs, or their nestlings, are destroyed by birds and beasts of prey.
But if Darwin relied on metaphors to enlighten his readers, he also followed this good strategy in his private quest for understanding. Darwin’s notebooks are, if anything, more awash in metaphor than his published works. I believe, along with many Darwin scholars (see, in particular, Ralph Colp’s 1979 article in bibliography), that one metaphor stands out among all others in Darwin’s own struggle to formulate the principles of natural selection—the metaphor of the wedge. At the very least, this comparison holds pride of place as the first image invoked by Darwin to explicate his initial statement of natural selection.
Great ideas, like species, do not have “eureka” moments of sudden formulation in all their subtle complexity; rather, they ooze into existence along tortuous paths lined with blind alleys (to invoke a metaphor). Still, not all moments are equal, and some may even be judged crucial. Darwin, at least, claimed that September 28, 1838, had been the key day for natural selection, and that his reading of Thomas Malthus’s Essay on the Principles of Population had enabled him to put the disparate pieces of his puzzle together, as he recalled Malthus’s argument that growth in population, if unchecked, must quickly outstrip food supply, leading to inevitable struggle for limited resources and death for losers. He wrote Malthus’s principle into his notebook and appended, directly thereafter, his very first metaphor for his new theory of evolution:
One may say there is a force like a hundred thousand wedges trying [to] force every kind of adapted structure into the gaps in the economy of nature, or rather forming gaps by thrusting out weaker ones.
Darwin honed and sharpened this metaphor throughout the next twenty years, as he prepared for the storm of publishing his ideas about evolution. He eventually settled upon the image of a surface absolutely chock-full with wedges, representing species in an economy of nature sporting a No Vacancy sign. Evolutionary change can only occur when one species manages to insinuate itself into this fullness by driving (wedging) another species out. Darwin developed this metaphor most fully in his manuscript for the long, unpublished version of the Origin of Species (a compressed account appears on page 67 of the shorter book that he eventually produced in 1859 under the pressure of A. R. Wallace’s independent formulation of natural selection):
Nature may be compared to a surface covered with ten thousand sharp wedges, many of the same shape and many of different shapes representing different species, all packed closely together and all driven in by incessant blows: the blows being far severer at one time than at another; sometimes a wedge of one form and sometimes another being struck; the one driven deeply in forcing out others; with the jar and shock often transmitted very far to other wedges in many lines of direction.
I have focused upon Darwin’s commitment to the metaphor of the wedge because this unappreciated core belief, more than the notion of natural selection itself, shaped Darwin’s conventional view about progress and predictability in the long-term history of life. Darwin, like any honest man in a world of such inordinate complexity, struggled hard but failed to resolve several crucial issues in the interpretation of nature. No question troubled him more than the common assumption, so crucial to Victorian Britain at the height of industrial and imperial success, that progress must mark the pathways of evolutionary change.
Darwin clearly understood that the basic mechanics of natural selection implied no statement about progress, for the theory only speaks of local adaptation to changing environments. But Darwin, as an eminent, if critical, Victorian himself, could not let go of progress so cleanly. He wished to validate predictable advance as a major theme of the fossil record, but knew that the bare bones of natural selection could not rationalize such a belief. How, then, could progress be affirmed as a fact of life’s history if the fundamental theorem of organic change—natural selection—did not imply progress by itself?
To resolve this troubling discordance between the mechanics of his basic theory and his fundamental impression of pattern in life’s history, Darwin called upon a second, basically ecological principle encompassed by the metaphor of the wedge. Nature, Darwin believed, is full of species (“a surface covered with ten thousand sharp wedges…all packed closely together”). All potential addresses are occupied, but new challengers continually arrive to compete for space. They can succeed in a full world only by driving other species out in overt competition for limited resources (“the one driven deeply in forcing out others”). It’s a tough, crowded world out there; successful creatures claw their way to the top and remain there by constant vigilance and conquest. The Origin of Species contains several passages about progress in the history of life, and all are validated, not by the bare bones mechanism of natural selection, but by the second principle of the wedge, the vision of a full world ruled by overt competition among organisms:
The more recent forms must, on my theory, be higher than the more ancient; for each new species is formed by having had some advantage in the struggle for life over other and preceding forms…. I do not doubt that this process of improvement has affected in a marked and sensible manner the organization of the more recent and victorious forms of life, in comparison with the ancient and beaten forms.
But what actual evidence do we have that long-term trends in the history of life arise by continuous and intense biological competition in a perennially crowded world? (Other models for the operation of natural selection are surely plausible. Perhaps most species do not fall victim to overt wedging by superior competitors but to changes in the physical environment that are too rapid or extensive to elicit an adaptive response.) I am persuaded that some cases in Darwin’s preferred mode of organic competition have been documented. Biologist Geerat Vermcij (who lays out the brief for Darwin’s view most elegantly in his book Evolution and Escalation), for example, has demonstrated a geological trend for thicker and stronger crab claws matched by ever more efficient defenses (spines, knobs, and thick shells) in the snails that crabs love to eat. I accept the interpretation of this lock-step escalation as an “arms race.”
But just as we needed so great a scholar as Aristotle, in so weighty a place as the Nicomachean Ethics, to teach us that one swallow does not make a spring (yes, he said spring, not summer), a case or two in the fossil record does not establish a pattern. Directional trends produced by wedging do occur, but they scarcely cry for recognition from every quarry and hillslope. The overwhelming majority of paleontological trends tell no obvious story of conquest in competition. Why did the large and gorgeously complex ammonites crash to oblivion some 65 million years ago, while their rarer and apparently simpler closest cousins, the nautiloids, survived to our own day as the “ship of pearl” immortalized by Oliver Wendell Holmes (and Eugene O’Neill) as the builder of “more stately mansions.” Why did all members of three great groups of crinoids die 225 million years ago, leaving all subsequent time to a fourth group that survives today but sports no feature ever identified as an improvement over the three losers? Why did a similar replacement of one group by another occur in reef-building corals at the same time, and why can no theme of improvement in competition be discerned here either? Why, for that matter, did dinosaurs die and mammals prevail after 100 million years of reptilian success and mammalian marginality? (If mammals were competitively superior, they certainly displayed no hurry in wedging out their “predecessors in the race for lif
e,” for 100 million years is almost two-thirds of mammalian history. Moreover, recent views on the sleekness and anatomical sufficiency of dinosaurs speak strongly against any notion of wedging by rat-sized mammals.)
I chose these four cases for two reasons. First, I think that they are far more characteristic of the fossil record—more numerous and more significant in import—than putative examples of progress by wedging. Second, they represent the kind of event that most directly challenges the metaphor of the wedge and therefore the favored rationale for progress by rigorous competition among organisms—“changing of the guard” across episodes of mass extinction. In fact, these four cases provide a small sample of events, a pair for each, in the two most celebrated mass extinctions: the Permian debacle that may have wiped out more than 95 percent of marine invertebrate species some 225 million years ago, and the Cretaceous event that removed remaining dinosaurs and gave mammals a chance some 65 million years ago.
Any reader with a good feel for the history of paleontology must now be intensely puzzled. How can I be suggesting that a study of mass extinction will alter our view about directionality in the history of life when widespread and coincident death of species is, perhaps, our oldest discovery about the stratigraphic record? The boundaries of the geological time scale, the alphabet of my profession, are defined by events of mass extinction. The two episodes cited in my cases separate the three great eras of life’s multicellular history: the Permian extinction between the Paleozoic and the Mesozoic, and the Cretaceous extinction between the Mesozoic and the Cenozoic. How can something so canonical, and so long appreciated, now threaten to disrupt another cherished view about biologically based competition leading to progress in a crowded world?
Eight Little Piggies Page 29