Full House (22 page)

But the environment of the earth’s shallow interior—liquid flowing through cracks and pore spaces in rocks—may be quite common on other worlds, both in our solar system and elsewhere (frozen surfaces of distant planets will not permit life, but interior heat may produce liquid—and a possible environment for life at bacterial grade—within underground rocks). In fact, Gold estimates that "there are at least ten other planetary bodies [including several moons of the giant planets] in our solar system that would have had a similar chance for originating microbial life" because "the circumstances in the interior of most of the solid planetary bodies will not be too different from those at a depth of a few kilometers in the Earth."

Finally, we may need to make a complete reversal of our usual perspective and consider the possibility that our conventional surface life, based on photosynthesis, might be a very peculiar, even bizarre, manifestation of a common universal phenomenon usually expressed by life at bacterial grade in the shallow interior of planetary bodies. Considering that we didn’t even know only ten years ago such interior life existed, the transition from unknown to potentially universal must be the most astonishing promotion in the history of favorable revisions! Gold concludes:

The surface life on the Earth, based on photosynthesis, for its overall energy supply, may be just one strange branch of life, an adaptation specific to a planet that happened to have such favorable circumstances on its surface as would occur only very rarely: a favorable atmosphere, a suitable distance from an illuminating star, a mix of water and rock surface, etc. The deep, chemically supplied life, however, may be very common in the universe.

The modal bacter, in other words, may not only dominate, even by weight, on earth, but may also represent life’s only common mode throughout the universe.

No Driving to the Right Tail

A proper theory of morality depends upon the separation of intentions from results. Tragic deaths may occur as unintended consequences of decent acts—and we rightly despise the cold-blooded killer, while holding sympathy for the good Samaritan, even if an unnecessary death becomes the common result of such radically different intentions (the robber who shoots the store owner, and the policeman who kills the same owner because he fired at the robber and missed).

Similarly, any proper theory of explanation in natural history depends upon the distinction of causes and consequences. Darwin’s central theory holds that natural selection acts to increase adaptation to changing local environments. Therefore, features built directly by natural selection— the thick coat of the woolly mammoth in my example on page 139, for example—evolve for adaptive reasons by definite cause. But many features that become vital to the lives of their bearers may arise as uncaused (or at least indirectly produced) and "unintended" sequelae or side consequences. For example, our ability to read and write has acted as a prime mover of contemporary culture. But no one could argue that natural selection acted to enlarge our brains for this purpose—for Homo sapiens evolved brains of modern size and design tens of thousands of years before anyone thought about reading or writing. Selection made our brains large for other reasons, while reading and writing arose later as a fortuitous or unintended result of an enlarged mental power directly evolved for different functions.

Our intuitions tell us—quite rightly in this case, I believe—that this distinction between results directly caused and consequences incidentally arising is both important in explaining any particular feature of the organic world and fundamental to any general understanding of evolution. The main issue is not predictability—for a phenomenon may be predictable whether it arises directly for cause or incidentally as a consequence. The key question centers on the nature and character of explanation. The purposeful killer and the erring policeman produce the same result (and with equal predictability in the old-fashioned Newtonian sense of potential for deducing the outcome once we know the positions of all people, the sight line of the gun, the timings, etc.)—yet we yearn to judge the meaning differently based on the distinction between intention and accident.

In the same way, a right tail of increasing maximal complexity might arise on the bell curve of life either (as tradition has held) because evolution inherently drives life to higher levels of complexity or (as I argue in the major claim of this book) as an incidental side consequence of life’s necessary origin at the left wall of minimal complexity followed by successful expansion thereafter with retention of an unvarying bacterial mode. Our intuitions detect a radical difference in meaning between these two pathways to predictable production of the same result—and our intuitions are right again. We do, and should, care profoundly about the different meanings—for, in one case, increasing complexity is the driving raison d’être of life’s history; while, in the other, the expanding right tail is a passive consequence of evolutionary principles with radically different main results. In one case, progress rules and shapes the history of life as the central product of fundamental causes; in the other, progress is secondary, rare, incidental, and shaped by no cause working directly in its interest.

This issue of directly caused results versus incidental consequences has reverberated throughout the history of evolutionary thought. A large literature, both scientific and philosophical, has been devoted to explicating these vital distinctions. A daunting and somewhat jargony terminology has arisen (some, I confess, of my own construction) to carry this debate in the technical literature—adaptations versus exaptations, aptations versus spandrels, selection versus sorting (see Sober, 1984; Gould and Lewontin, 1979; Gould and Vrba, 1982; Vrba and Eldredge, 1984). We will stick to the vernacular here, and make our main distinction between intended results and incidental consequences.

As the main claim of this book, I do not deny the phenomenon of increased complexity in life’s history—but I subject this conclusion to two restrictions that undermine its traditional hegemony as evolution’s defining feature. First, the phenomenon exists only in the pitifully limited and restricted sense of a few species extending the small right tail of a bell curve with an ever-constant mode at bacterial complexity—and not as a pervasive feature in the history of most lineages. Second, this restricted phenomenon arises as an incidental consequence—an "effect," in the terminology of Williams (1966) and Vrba (1980), rather than an intended result—of causes that include no mechanism for progress or increasing complexity in their main actions.

At most, one might advance Thomas’s (1993) claim that "progressive emergence of increasing complexity over the long term is the main effect of evolution. As such, it compels our attention." In other words, Thomas admits that increasing complexity is an incidental consequence, an effect rather than a main result of causes framed in its interest. He holds, however, that progress still compels our attention as the "main" effect among all of evolution’s incidental consequences. But what possible criterion can validate this claim beyond the parochial and subjective desire to designate as primary an effect that both led to human life and placed us atop a heap of our own definition? I think that any truly dominant bacterium would laugh with scorn at this apotheosis for such a small tail so far from the modal center of life’s main weight and continuity. I do realize that bacteria can’t laugh (or cogitate)—and that philosophical claims for our greater importance can be based on the consequences of this difference between them and us. But do remember that we can’t live on basalt and water six miles under the earth’s surface, form the core of novel ecosystems based on the earth’s interior heat rather than solar energy, or serve as a possible model for cosmic life in most solar systems.

In other words, progress as a purely incidental consequence (and limited to a small right tail) just won’t do as a validation for our traditional hopes about intrinsic human importance—the spin-doctoring that prevents the completion of Darwin’s revolution in Freud’s crucial sense of pedestal smashing (see chapter 2). I think that virtually every evolutionist who has ever considered the issue in the terms of this book (that is, as a history of variation in all life
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the full house—rather than as a tale told by abstracted means or extreme values only) has come to the conclusion that the appearance of progress as an expanding right tail must arise as an incidental consequence, not as a main result.

The traditional hope for intrinsic progress as an explicit result must therefore rest upon a fallback position—not nearly so grand as the original formulation, but a source of some potential solace nonetheless. Even if we must admit that an expanding right tail arises as an incidental consequence of origin at a left wall with subsequent proliferation, could we not also hold that other forces operate as well on life’s bell curve—and that some of these other forces do include an intrinsic and predictable drive to progress?

As stated in point 6 of my epitome (see page 173), such an argument could be true, would take the following form, and can be tested empirically: life as a whole begins at the left wall and is therefore free to expand in only one direction. Therefore we cannot use life-as-a-whole to test for drives to progress—because upward movement of the mean must, in part, reflect the left wall’s constraint, not any potential drive. But if we could study the history of smaller lineages with founding members far from the wall—and therefore free to vary in either direction—then we could devise a clear test for general progress. Do such "free" lineages show a tendency for increases in complexity to be more frequent, or greater in effect, than decreases? If most free lineages show a trend to increasing complexity, then we could assert a general principle of progress as a main result for its own sake. The full phenomenon of life’s expanding right tail would then arise by two separate and reinforcing processes: an incidental consequence based on constraints of origin at the left wall, and a direct result of intrinsic bias to greater complexity in lineages free to vary in both directions.

This conjecture is logically sound but, by all evidence so far in hand, empirically wrong. I would raise two arguments against intrinsic progress, the first briefly and subjectively, the second at greater length and based upon some compelling recent evidence.

First, if I were a betting man, I would wager a decent sum (but not the whole farm) on a small natural preference for decreasing complexity within lineages, and not for the traditional increase, if any general bias exists at all. I make this surprising claim because natural selection, in its purest form, only yields adaptation to changing local environments. These changes should be effectively random (with respect to "progress"), for fluctuations in climate show no temporal trend. A bias for or against increasing complexity therefore requires a general advantage for one direction as life plays its Darwinian game. I can think of a reason why a bias for decreasing complexity might exist, but I cannot defend any corresponding preference for increases. Hence I would bet that a slight overall bias for decreasing complexity might well prevail in the aggregate of all lineages.

I have long been entirely underwhelmed by the standard arguments for general advantages of increasing complexity in the Darwinian game— adaptive benefit of more elaborate bodily form in competition for limited resources, for example. Why should more complex conformations generally prevail? I can imagine such an argument for mammalian brains—if complexity translates to rising flexibility and computing power. But I can envisage just as many situations where more elaborate forms might be a hindrance—more parts to fail, less flexibility because all parts must interact with precision.

But one common mode of Darwinian success (local adaptation) does entail an apparent preference for substantial decreases in complexity— namely, the lifestyle of parasites. We are not speaking here of an organic rarity, but of a mode of life evolved by probably hundreds of thousands of species—a substantial percentage of all living forms. Not all parasites gain adaptive benefit through simplification, but one large group of species certainly does—those that live deep within the bodies of their hosts, permanently attached and receiving all their nutrition by commandeering the blood supply, or some of the food already digested by the host. Such species require neither organs of locomotion nor digestion, and natural selection favors their loss. One or a few novel organs might evolve for special needs—hooks for attaching to the host, or suction devices to drain off food, for example—but these elaborations are more than offset by a far greater number of lost organs.

Often these immobile parasites become little more than bags or tubes of reproductive tissue—simple machines for propagation attached to the internal organs of their host. Sacculina, the famous barnacle parasite of crabs and other crustaceans, consists of a formless sac (acting as a brood pouch) attached to the crab’s abdomen, with a stalk protruding inside to a system of roots that drain food from the crab’s blood spaces. A twenty-foot-long tapeworm in a human intestine may contain of hundreds of sections (strobilae), each little more than a simple sac containing members of the next generation. The entire phylum Pentastomida, parasites of the respiratory tract of vertebrates, builds an elaborate organ for sucking blood, but no internal parts for locomotion, respiration, circulation, or excretion.

Thus, if "standard" natural selection on free-living creatures produces no bias in either direction, and if parasites tend to become simplified while no countervailing bias toward greater complexity exists, then a small overall tendency toward decreasing complexity may characterize the history of most lineages (as their parasitic species simplify, while their free
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living species show no trend). Please note that the right tail for the full bell curve of life will still expand through time—even if a bias toward decreasing complexity operates in most lineages. For species moving left to less complexity enter a domain already inhabited, while rarer species moving right may enter a previously unoccupied realm of complexity. The drunkard will end up in the road even if, for some reason, he moves more often toward the wall than toward the gutter—for he bounces off the wall but falls prostrate (and permanently) in the gutter. An entire system can extend its extreme in one direction even if individual lineages have a bias for excursions in the other direction.

But I can also think of an argument against my own claim for parasites. Adult forms do indeed tend to evolve toward greater simplicity, but when we confine our attention to adults, we fall into another conventional bias (not as general or pervasive, no doubt, as our preferences for progress, but a seriously distorting limitation nonetheless). A human being is not defined by the nongrowing form of adult years; kids are people too. Evolution shapes a full life cycle, not only an adult body. The immobile blood-sucking or food-draining adult parasite may have evolved toward greater simplicity compared with free-living ancestors, but full parasitic life cycles often change in the other direction toward great elaboration, sometimes with adaptation to two or three different hosts in the course of a full ontogeny.

The adult Sacculina may be an external blob attached to some internal roots, but the larval life cycle is astonishingly complex (see Gould, 1996)—several free-living planktonic forms, followed by a settling phase that cements to the crab, grows a dart that pierces the crab’s body, and then injects the few cells that eventually grow into the adult blob and roots. Similarly, pentastome larvae first bore through the gut of an initial host. When a vertebrate eats its first home, the matured pentastome moves to the respiratory tract either by crawling from the vertebrate’s stomach to the esophagus and then boring through, or by tunneling through the intestinal wall and into the bloodstream. The pentastome then attaches to its final site by means of complex hooks surrounding the mouth.