by David Toomey
Most scientists, but by no means all. Some suspect otherwise, and their reasoning is quite straightforward. Since, as the vast majority of biologists now believe, life is not a once-in-the-history-of-the-universe event, but a more or less inevitable by-product of physics and chemistry, it follows naturally enough that life on Earth may well have had more than one beginning. It follows further that if a second beginning had occurred under even slightly different circumstances, a different sort of life would have resulted.
This is the possibility described and explored in a 2009 article called “Signatures of a Shadow Biosphere.”2 Its six authors represent, as we might expect, a rather unusual collection of expertise. Four of the six have backgrounds in the life sciences. Two of them—perhaps the two who have worked hardest to bring the article’s provocative ideas to a wider audience—are cut from a rather different disciplinary cloth. Paul Davies trained as a mathematical physicist, and as recently as the 1990s his main work was in cosmology and quantum gravity. Of late he has widened his gaze considerably, becoming more interested in fundamental questions about the nature of scientific inquiry. Carol Cleland is a member of NASA’s Astrobiology Institute and—this belying any charges that NASA lacks a wider perspective—a professor of philosophy at the University of Colorado, Boulder. She is fond of quoting Thomas Kuhn (the historian of science we met in the previous chapter) and suspects that many scientists miss opportunities because they don’t think outside the box Kuhn calls the “reigning paradigm.” It is Cleland, along with microbiologist Shelley Copely, who coined the phrase shadow biosphere—a provocative and slightly unsettling reference to a hypothetical biosphere of microbial weird life that, like the realm of fairies and elves just beyond the hedgerow, may or may not impinge on our own.
Davies, Cleland, and the other authors of the article are excited by this possibility for two reasons. First, the discovery of such life would make it possible for biologists, by comparing the differences and commonalities of two examples of life, to begin to discover universal laws of biology much as physicists since Newton have discovered universal laws of physics. As a science, biology would have fully matured. Second, and more profoundly, the discovery of such life would settle the debate over life’s probability once and for all. It would mean that life in the universe is common and may arise anywhere conditions are right. Not incidentally, such a discovery would ripple far beyond biology into all realms of human experience, altering forever our understanding of our place in the universe.
But we’re getting ahead of ourselves. If a shadow biosphere of weird life exists, it would be prudent, before confronting questions of its larger meaning, to ask where and how it might have begun.
A SECOND GENESIS
The scarred and battered face of Earth’s Moon is the visible legacy of the violence of the early Solar System, a time some 4 billion years ago when asteroids and comets routinely struck all its larger worlds, including a molten, slowly cooling Earth. Heat and radioactivity from the planet’s core sent lava through cracks in the newly formed crust, and as the surface cooled, steam from the atmosphere condensed and fell as rains that, lasting for millennia, created the planet’s first, shallow oceans. Such an environment would seem inhospitable to life, yet the most necessary ingredients were there: complex carbon-based molecules and liquid water. Indeed, most scientists believe that this is the environment in which life gained its first foothold.
Almost all familiar life builds its proteins from the same twenty amino acids—the molecules that biology textbooks call life’s “building blocks.”** What’s interesting is that an organism would enjoy no particular advantage by limiting itself to these twenty, and many others might work as well. It seems that in its first incarnations, familiar life used the amino acids it used for no better reason than that they were available and nearby. In another part of the early Earth (and Earth, it is worth remembering, is a big place—still bigger if you’re a few complex organic molecules edging toward replication and self-organization), other amino acids would be available. And another set of complex organic molecules edging toward replication and self-organization might use them—the result being a second genesis, of another sort of life.
In 1988, Caltech geologists Kevin Maher and David Stevenson suggested that the standard picture of life’s beginning was too simple.3 Their point was that conditions suitable for life may have lasted many millions of years; there was world enough and time for many beginnings—but perhaps no more than beginnings. The reason for that last qualification is that the era in Earth’s history when life began overlapped with periods of “heavy bombardment” by meteors. Every so often—on average once in a half-million years—an unusually large meteorite—say, the size of Manhattan Island—would strike with such force that the oceans would boil, the atmosphere would be superheated, and the planet would be rendered all but sterile. The time between each of these armageddons might be just enough for life to begin all over again. But if one beginning during any particular respite is unlikely, two beginnings are improbable in the extreme. And for this reason we might conclude that there is little chance that at any given time two forms of life coexisted. We might conclude this, that is, except for the fact that no particular sterilization would be complete. After all, familiar life today survives and flourishes on the ocean floor and deep underground—both places well protected from any unpleasantness nearer the Earth’s surface. A robust sort of primitive organism might have done likewise.
Sheltered locales on and in Earth are not the only places such an organism might have weathered the storm. Davies has suggested another, rather more distant, refuge. A meteor striking Earth with sufficient force might launch fragments of rock into orbit around the Sun. Some meteors might hold microbes or spores that could lie dormant for thousands or even millions of years, until the moment when the orbit of the rock fragments and the orbit of Earth happened to intersect, and the fragments would fall back to Earth. Some would split open on impact, and their microbial passengers—any that survived, that is—would wake to a world once again fit for life, and perhaps already harboring another kind of life, one that had appeared in the half-million years during which they were away. Like Homer’s Odysseus, the microbes would have returned home after a voyage of many years, to find strangers living there. It would be a space odyssey on a microbial scale—and an interplanetary one.
As mentioned earlier, life on Earth might not only have returned from somewhere else. It might have begun somewhere else. Four billion years ago, the planet Mars had a thick atmosphere of carbon dioxide, with rainfall, and streams and rivers of liquid water coursing through valleys and emptying into lakes and shallow seas. In short, it was a congenial abode for life. Like Earth of the period, Mars was also pummeled with meteors, and some struck with enough force to launch rock fragments into orbit around the Sun. After thousands or millions of years, some of the fragments intersected Earth’s orbit and fell to Earth. In fact, scientists have found at least twenty-eight of them, one of which is ALH84001, made famous in 1996 when David McKay, chief scientist for astrobiology at NASA’s Johnson Space Center, and his research group suggested that it bore evidence of life. Although their conclusions remain controversial, it seems clear that early in their history, Earth and Mars traded material, and some of that material may have contained microbes. Davies and others believe it barely possible that life on Earth—familiar, weird, or both—has a Martian ancestry.
None of these ideas are proof that weird life, let alone a shadow biosphere of weird life, exists. But collectively, they make a case that weird life had ample time to arise on Earth and several means by which to do it. Suppose then, that it did arise. An obvious question presents itself: Wouldn’t we have noticed by now? And if it were microbial, wouldn’t microbiologists have noticed? The answer, interestingly enough, is: not necessarily.
WHAT WE DON’T KNOW
Those of us who take our science news from Discover magazine and nature shows are regularly and properly astound
ed by what biologists and microbiologists know. If we were to learn what they don’t know, we might be just as astounded, for what they don’t know is a great deal. Take, for example, the answer to the straightforward question, How many species are there? The difficulty here is simply that there is no reliable way to determine that number, or even to estimate it, except perhaps to replicate work performed in 1981 by Terry Erwin of the Smithsonian Institution.
Erwin wanted a census of the number of the world’s arthropod species—insects, spiders, crustaceans, centipedes, and the like. He and his team arranged a grid of specimen bottles, with 1-meter-wide funnels affixed to each, beneath a tree in Panama. With the air calm, they sprayed insecticide into the canopy, and some hours later they collected and began to classify the thousands of arthropods that had fallen through the funnels and into the bottles. Erwin counted 163 species of beetles known to live exclusively in the species of tree they had fallen out of, multiplied that number by the number of tropical tree species known, and concluded that beetle species numbered more than 8 million (thus incidentally supplying quantitative evidence for British geneticist J. B. S. Haldane’s possibly apocryphal remark that the Creator must have “an inordinate fondness for beetles”4). Since beetles are known to represent 40 percent of all arthropods, Erwin assumed the same proportion in the tree whose denizens were under study, and after a number of other calculated guesses, he concluded that the number of arthropod species worldwide might be as high as 30 million.5 But no one, including Erwin, thinks this number definitive, and other estimates vary wildly.
Bear in mind, too, that this is only what we don’t know about just one phylum. Our ignorance of the rest of the natural world is proportionately greater. In 2002 the famed entomologist Edward O. Wilson estimated that 1.5–1.8 million species have been identified and catalogued, but well-reasoned guesses of the actual number lay within a stunningly wide range: 3.6–100 million.6 The full meaning of these numbers is so dumbfounding as to bear restating: for every species known to science, there is at least one that is unknown, and there may be as many as fifty.
Since Erwin’s work, several international programs have begun to catalogue biodiversity. The Census of Marine Life, a decade-long project to make a comprehensive tally of life in Earth’s oceans, found 5,000 previously unknown species, including an animal that lives without oxygen, several species believed to be extinct since the Jurassic period, and 600-year-old tube worms. The ongoing International Barcode of Life project identifies species with only a snippet of their DNA, and has so far assigned bar codes to more than 100,000 species. Coordinated with both projects and with several zoological organizations is the Encyclopedia of Life (mentioned earlier), now at half a million pages and growing.
Of species still undiscovered, it is possible that some are quite large. As recently as the mid-1990s, scientists were astonished to discover a 200-pound animal inhabiting the mountains shared by Vietnam and Laos. It looks part antelope and part cow but, now classified as the only member of the genus Pseudoryx, is neither. Most unknown species, though, are likely to be small—and many are no doubt microscopic. The 1989 edition of Bergey’s Manual of Systematic Bacteriology lists roughly 4,000 species of bacteria,7 but microbiologists, using several ingenious and indirect measurements, have inferred that the true number may be in the millions.
Our ignorance of the microbial realm is disquieting—or should be—not merely because there is so much of it (microbes compose as much as 80 percent of the Earth’s biomass and 10 percent of your dry body weight), but because it is the realm from which our own “macrobial” realm originated and upon which it still depends. Microorganisms act as the basis of all food chains and work to regulate the chemistry of Earth’s atmosphere and oceans. In fact, if the Gaia hypothesis of British inventor and scientist James Lovelock and American biologist Lynn Margulis has any validity, then Earth’s climate has for billions of years been held in delicate equilibrium by oceanic phytoplankton and other microorganisms working, one must note, without committees, treaties, or international protocols. Their other achievements are similarly impressive. They originated all the chemical systems upon which life depends, systems we cannot yet replicate and do not fully understand. And they have adapted to the most extreme of Earth’s environments—environments in which we, without artificial means anyway, could not survive. Microbes were the first organisms on Earth, and given their record of success, they will surely be the last.
The reason we have so little knowledge of the microbial world lies in the limitations of the instruments and techniques we have available to explore it. Under a microscope, that most time-honored of scientific tools, a given species from the domain Archaea and a given species from the domain Bacteria may be indistinguishable, even though they have less in common with each other than you have with, say, a soft-shell crab. Most bacteria and archaea look like spheres or rods. Microbiologists can enhance the view and identify parts of any given cell with “staining,” but the parts might represent only some differences, and not necessarily the most important or fundamental ones.
Microbiologists who want to study a microbe thoroughly and over time will “culture” it—that is, introduce a sample of the microbe to nutrients in standard culture dishes, and wait until the sample proliferates into a colony containing enough individual microbes that they can be sorted and analyzed. This is not as easy as you might expect. While certain species, most famously Escherichia coli, grow so readily that laboratory biologists call them “weeds,” the fact is that most single-celled organisms don’t survive long in captivity. Many a microbe that thrives in a puddle or pond, when carefully removed, carefully transported, and carefully placed in a culture dish, will shrivel up and die. To a nonscientist, it may come as a shock to learn that biologists have been able to culture less than 1 percent of the microbes they have seen, as it were, in the wild.8
Not that they know all that much about the wild. With humility that one can only call admirable, the NRC report of 2007 notes, “It is clear that little or nothing is known of the physiological diversity of most microorganisms in most Earth environments.”9 This includes environments that are nearby. As Wilson observes, a pinch of soil from any forest floor, no more than can be held between thumb and forefinger, is likely to contain thousands of bacterial species, many of them unknown.10
All this is by way of saying that the fact that we have not found microbial weird life should not lead us to conclude that it doesn’t exist. As English Astronomer Royal Martin Rees observed, with regard to another scientific mystery, “Absence of evidence is not evidence of absence.”11 Or is it?
I add a dollop of doubt because we might easily imagine a second objection to the notion of weird life. Familiar life is successful, as mentioned earlier, because it is resilient, tenacious, aggressive, and inventive. Suppose that at some moment in the roughly 4-billion-year reign of familiar life, a sort of weird life did emerge. Might we assume that it would have lost any and all competition for resources, and that almost immediately after its appearance, familiar life would have pushed it into extinction? The answer, again, is: not necessarily. According to Davies, Cleland, and their colleagues, there are at least three ways weird life might have managed, and might manage still: as ecologically separate from familiar life, ecologically integrated with it, or biochemically integrated with it.
THREE TYPES OF SHADOW BIOSPHERE
One way weird life could manage is by moving into places that no familiar life, not even extremophiles, wants. There are many such places—the core of Chile’s Atacama Desert,12 ice sheet plateaus, hydrothermal vents with temperatures above 400°C, and high-brine liquid water at temperatures below –30°C. Weird life in any of these places would likely be part of a biosphere ecologically separate from our own—and these are phenomena known to exist. Since 1990, scientists have discovered several ecosystems of extreme familiar life that are separated from the rest of the biosphere. There is a microbial community beneath the Columbia River in Washing
ton State composed of bacteria that live inside basalt rock, another in the Twin Falls area of Idaho, still another near a gold mine in South Africa.13 Each is remarkable for its source of energy: chemosynthesis in the first two cases, and radioactive decay in the third.
There is also the possibility that weird microbes, while greatly outnumbered by familiar microbes, are living among them. Molecular biologist Mitch Sogin, a coauthor of the 2007 NRC report, called the diversity of most microbial communities “staggering,” and noted that most of the diversity was owed to a small number of individual microbes.14 In other words, few microbes of each species, but an enormous number of species nonetheless. It is possible that weird life is present and unaccounted for in many microbial communities, keeping its profile low and, since it is weird, consuming what no one else wants and excreting what no one else is bothered by. Such weird life would compose a biosphere ecologically integrated with our own.
