The most obvious and serious caution in these data emerges from another general property of bacteria: their almost ineradicable ubiquity. How do we know that these bacteria, cultured from waters collected at depth, really live in these underground environments? Perhaps they were introduced into deeper waters by the machinery used to dig the oil wells and boreholes that provided sites for sampling; perhaps (with even more trepidation) they just represent contamination from ubiquitous and ordinary bacteria of our surface environments, stubbornly living in laboratories despite all attempts to carry out experiments in sterile conditions. (A fascinating, and very long, book could be written about remarkable claims for bacteria in odd places—on meteorites, living in geological dormancy within 400-million-year-old salt deposits—that turned out to be ordinary surface contaminants. I well remember the first "proven" extraterrestrial life on meteorites, later exposed as ordinary ragweed pollen. Ah-choo!)
This well-known possibility sends shivers down the spine of any scientist working in this area. I am no expert and cannot make any general statement. I would not doubt (and neither do the authors of these articles) that some reports may be based on contamination. But all known and possible precautions have been taken, and best procedures for assuring sterility have been followed. Most persuasively, many of the bacteria isolated from these deep environments are anaerobic hyperthermophiles (jargon for bacteria growing at very high temperatures in the absence of oxygen) that could thrive in subterranean conditions, and cannot be laboratory contaminants because they die in ordinary surface environments of "low" temperature and pressure and abundant oxygen.
Writing in _The New York Times_ on December 28, 1993, William J. Broad summarized the case nicely:
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Some scientists say the microbes may be ubiquitous throughout the upper few miles of the Earth's crust, inhabiting fluid-filled pores, cracks, and interstices of rocks while living off the Earth's interior heat and chemicals. Their main habitats would be in the hot aquifers beneath the continents and in oceanic abysses, fed perpetually by the nutrients carried by the slow circulation of fluids like oil and deep ground water.
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We might ask one further question that would clinch the case for underground ubiquity: Moving away from the specialized environments of deep-sea vents and oil reservoirs, do bacteria also live more generally in ordinary rocks and sediments (provided that some water seeps through joints and pore spaces)? New data from the mid-1990s seem to answer this most general question in the affirmative as well.
R. J. Parkes et al. (1994) found abundant bacteria in ordinary sediments of five Pacific Ocean sites at depths up to 1,800 feet. Meanwhile, the United States Department of Energy, under the leadership of Frank J. Wobber, had been digging deep wells to monitor contamination of groundwater from both inorganic and potentially microbial sources (done largely to learn if bacteria might affect the storage of nuclear wastes in deep repositories!). Wobber's group, taking special pains to avoid the risk of contamination from surface bacteria introduced into the holes, found bacterial populations in at least six sites, including a boring in Virginia at 9,180 feet under the ground!
William J. Broad wrote another article for the _Times_ (October 4,1994), this time even more excited, and justifiably so:
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Fiction writers have fantasized about it. Prominent scientists have theorized about it. Experimentalists have delved into it. Skeptics have ridiculed it. But for decades, nobody has had substantial evidence one way or another on the question of whether the depths of the rocky earth harbor anything that could be considered part of the spectacle of life—until now . . . . Swarms of microbial life thrive deep within the planet.
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Stevens and McKinley (1995) then described rich bacterial communities living more than three thousand feet below the earth's surface in rocks of the Columbia River Basalt in the northwestern United States. These bacteria are anaerobic and seem to get energy from hydrogen produced in a reaction between minerals in the basaltic rocks and groundwater seeping through. Thus, like the biotas of the deep-sea vents, these bacteria live on energy from the earth's interior, entirely independent of the photosynthetic, and ultimately solar, base of all conventional ecosystems. To confirm their findings in the field, Stevens and McKinley mixed crushed basalt with water free from dissolved oxygen. This mixture did generate hydrogen. They then sealed basalt together with ground waters containing the deep bacteria. In these laboratory conditions, simulating the natural situation at depth, the bacteria thrived for up to a year.
Following a scientific tradition for constructing humorous and memorable acronyms, Stevens and McKinley have named these deep bacterial floras, independent of solar energy, and cut off from contact with surficial communities, SLiME (for subsurface lithoautotrophic microbial ecosystem—the second word is just a fancy way of saying "getting energy from rocks alone"). Jocelyn Kaiser (1995), writing a comment for _Science_ magazine on the work of Stevens and McKinley. used a provocative title: "Can deep bacteria live on nothing but rocks and water?" The answer seems to be yes.
My colleague Tom Gold of Cornell University may be one of America's most iconoclastic scientists. (One prominent biologist, who shall remain nameless, once said to me that Gold ought to be buried deep within the earth along with all his putative bacteria.) But no one sells him short or refuses to take him seriously—for he has been right far too often (we only threaten to bury alive the people we fear).
In a remarkable article entitled "The deep, hot biosphere" and published in the prestigious _Proceedings of the National Academy of Sciences_ in 1992, Gold set out the full case (truly universal, or at least potentially so) for the importance of bacterial biotas deep within the earth. (He did this, characteristically, a few years before firm data existed for rich bacterial communities in ordinary subsurface rocks. But he was right again, in this factual claim at least, if not necessarily in all his implications. Gold began his case by asking, "Are the ocean vents the sole representatives of this [deep bacterial life], or do they merely represent the examples that were discovered first?")
Of all living things that might expand the range of life beyond conventional habitats of land and oceans, bacteria are the obvious candidates. They are small enough to tit nearly anywhere, and their environmental range vastly exceeds that of all other organisms. Gold writes: "Of all the forms of life that we now know, bacteria appear to represent the one that can most readily utilize energy from a great variety of chemical sources."
Gold then makes a key estimate—for my argument about domination of the modal bacter, at least—of possible bacterial biomass, given the vast expansion of range into rocks and fluids of the earth's interior. Gold's effort is, of course, another back-of-the-envelope calculation, and must be treated with all the caution always accorded to this genre (but remember that the estimates may also be too low, rather than inflated). A large number of assumptions must be made: How deep do bacteria live? At what temperatures? How much of rock volume consists of pore space where bacteria may live in percolating waters? How many bacteria can these waters hold? Since we do not know the actual values for any of these key factors, we must make a "most reasonable" estimate. If actual values differ greatly from the estimate (as they may well do), then the final figure may he very far wrong. (I trust that nonscientific readers will now grasp why, in this enterprise, we are satisfied with "ballpark" estimates that might be "off" by even an order of magnitude or two.)
In any case, Gold based his number for total bacterial biomass on reasonable, even fairly conservative, estimates for key factors—so if most rocks permeable by water do contain bacteria, then his figure is probably in the right ballpark. Gold assumes an upper temperature range of 230° to 300°F and a depth limit of three to six miles. (If bacteria actually live deeper, their biomass might be much higher.) He calculates the mass of water available for bacterial life by assuming that about 3 percent of rock volume consists of pore spaces. Finally, he estimates that bact
erial mass might equal about 1 percent of the total mass of available underground water.
Putting all these estimates together. Gold calculates a potential mass of underground bacteria at 2 X 10^14 tons. This figure, he writes, is equivalent to a layer five feet thick spread out over the earth's entire land surface—an amount of biomass, Gold states, that would "indeed be more than the existing surface flora and fauna." As a cautious conclusion to his calculation of underground bacterial biomass, Gold writes:
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We do not know at present how to make a realistic estimate of the subterranean mass of material now living, but all that can be said is that one must consider it possible that it is comparable to all the living mass at the surface.
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When one considers how deeply entrenched has been the dogma that most earthly biomass lies in the wood of our forest trees, this potentially greater weight of underground bacteria represents a major revision of conventional biology—and quite a boost for the modal bacter. Not only does the earth contain more bacterial organisms than all others combined (scarcely surprising, given their minimal size and mass); not only do bacteria live in more places and work in a greater variety of metabolic ways; not only did bacteria alone constitute the first half of life's history, with no slackening in diversity thereafter; but also, and most surprisingly, total bacterial biomass (even at such minimal weight per cell) may exceed all the rest of life combined, even forest trees, once we include the subterranean populations as well. Need any more be said in making a case for the modal bacter as life's constant center of maximal influence and importance?
But Gold does take one further, and equally striking, step. We are now fairly certain that ordinary life exists nowhere else in our solar system—for no other planetary surface maintains appropriate conditions of temperature and liquid water. Moreover, such earthly surface conditions are probably rare in the universe, making life an unusual cosmic phenomenon.
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:
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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.
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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 die 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—evoke 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 or 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 cur
ve 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.
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