by David Toomey
The episode became something of an embarrassment for all involved—NASA, whose Astrobiology Institute had supported the work of some of the paper’s authors and had sponsored the news conference, the journal Science (whose peer reviewers had recommended publication), and of course, the authors themselves. Wolfe-Simon, far from backing away from her claims, has welcomed the critiques as part of the way good science is conducted. As of this writing, the one attempt by other researchers to reproduce Wolfe-Simon’s results has failed.19
2-Methylamine acid and substitutions (like arsenic for phosphorus) in DNA are only two of the possibilities—informed guesses, as it were—of what to look for if we are seeking weird life in an ecologically separate biosphere. No doubt there are many others, most of them thus far unimagined.
Weird life that is ecologically integrated or biochemically integrated with our own biosphere would prefer less extreme conditions, and so might be more difficult to isolate, but there are ways. Davies suggests we might look for a difference more fundamental than any we’ve discussed thus far—that presented by the “handedness” of molecules. You can’t fit your right hand comfortably into a left-handed glove because the form of the glove is the form of the hand turned inside out, and vice versa. A biologist would say that the two gloves have different chirality (from the Greek for “hand”). Large molecules like amino acids and sugars also have chirality, and if they are to fit together to make still-larger molecules, like proteins and DNA, they must have the same chirality.
MIRROR, MIRROR
As it happens, every one of the amino acids used by proteins in familiar life is left-handed, and every sugar in DNA is right-handed. Things didn’t have to be this way. Right-handed amino acids would have worked just as well, as long as all were right-handed, and left-handed sugars would have worked just as well, as long as all were left-handed. Things are this way because 3.5–3.8 billion years ago, some complex self-organizing prebiotic molecules needing an amino acid happened to use a left-handed one, and some needing a sugar happened to use a right-handed one. The first stitch set the pattern, and it has been followed ever since.
Suppose, however, that in any of the “second genesis” scenarios posited a few pages back, another set of complex self-organizing prebiotic molecules went right where the first had gone left, or went left where the first had gone right. The result might be a sort of weird life that is nearly identical to familiar life but, being made of molecules with mirror chirality, would be unable to interact with it biochemically. How might we find it? In fact, two scientists have already tried.
In 2006, acting on a suggestion from Davies, astrobiologist Richard Hoover and microbiologist Elena Pikuta put out bait. They began with a standard culture medium, a sort of smorgasbord for microbes, and switched some of its nutrients for their mirror counterparts. Then they took extremophile microbes retrieved from Mono Lake and introduced them to the medium. The researchers expected that, if mirror microbes were living among the extremophile microbes, they would make their presence known by eating the mirror nutrients. Soon enough, something began to eat the nutrients, and after a moment of cautious excitement, Hoover and Pikuta identified that something not as a mirror microbe, but as a heretofore unknown bacterium of the familiar sort, possessed of an unusual ability to chemically alter the mirror nutrients so that it could better digest them. It was a bit of biochemical sleight of hand that Hoover and Pikuta now suspect is owed to certain enzymes. The finding came as a small disappointment, but it was only the first attempt of its kind, and the bacterium that they named Anaerovirgula multivorans (roughly, “little rod that will eat anything”) was another reminder that a lot of nature was left to discover.20
There is another way that weird life might have escaped our attention: by being very, very small.
SIZE MATTERS
Every living organism known is made of cells. Although some cells are quite large (the Gargantua of celldom, Thiomargarita namibiensis, is the size of the period at the end of this sentence), most are best measured on the scale of nanometers—a nanometer being one-billionth of a meter. The lower limit for a cell’s size seems to be set by ribosomes, the (relatively) large molecules of proteins and RNA that work inside all cells to link amino acids and make new proteins. If they are to squeeze ribosomes inside themselves, cells must be a least a few hundred nanometers across. It is for this reason that most microbiologists think that smaller cells are impossible. Most microbiologists—but not all. Benner, for one, has suggested that cells might be much smaller if they made proteins not with ribosomes but with RNA.21
There have been at least three reports of very, very small things that—to their discoverers at least—seemed to be living or once living. In 1990, Robert Folk, an emeritus professor at the University of Texas at Austin, discovered in sedimentary rocks tiny structures that he took to be tiny fossils, the calcified remains of organisms a mere 30 nanometers across.22 He has since found similar structures in other sediments and in meteorites. Some of his peers have been intrigued, and a few have pointed to Folk’s findings as evidence that the tiny wormlike formations in the Martian meteorite ALH84001, while much smaller than bacteria, are not too small to have once been living. In 1996, Australian geologist Philippa Uwins was studying sandstone bore samples from a deep-ocean borehole off the coast of western Australia. She and her colleagues found tiny filaments that under an electron microscope looked like blobs in a lava lamp. By an ingenious method, Uwins was able to show that there was DNA inside the structures (not just on their surfaces)—evidence that they might be living, or at least might once have been living.23 In 1988, Finnish biochemist Olavi Kajander was examining cells with an electron microscope and found within them tiny particles some 20 nanometers across.24 Believing them to be living, he called them “nanobacteria.” Of the three discoveries, Kajander’s may be the strangest—and the most unsettling—because the particles are found in human tissue.
At present, the preponderance of evidence is that none of these findings is an organism, living or once living. In 2003 a research group concluded that what Folk had found were probably nothing more than by-products of bacteria with rather more typical dimensions.25 Recent research suggests that Uwins’s filaments are calcium carbonate and organic material that at some stage of development had encapsulated pieces of DNA.26 And a National Institutes of Health (NIH) study published in 2000 threw serious doubt on Kajander’s claims, which had already come under fire from several quarters. It should be said that Kajander himself continues to believe his “nanobacteria” are living, and his second thoughts are limited to his choice of nomenclature; he recently said that he probably should have given his findings a less provocative name, like (this is his phrase) “calcifying self-propagating nanoparticles.”27
Most microbiologists have given these findings a wide berth. One reason is that the work done so far—Kajander’s in particular—has generated controversies that give pause to scientists concerned for their careers and reputations. Nanoparticles, it seems, fall into a disciplinary no-man’s-land between chemistry and biology. Microbiologist John Cisar, who led the NIH study that countered Kajander, noted, “I’m not saying there’s nothing there. It’s just that we were looking at it from a microbiologist’s perspective. And when we didn’t find any signs of life, we moved on.”28 These findings may represent a class of forms somewhere between nonlife and life, forms unknown to science. But whatever they are—weird life, unusual chemistry, or something in between—very small weird life remains a real possibility. As David McKay noted of Uwins’s discovery (and it might easily apply to the others), “It’s something that shows that we just do not understand the small end of the spectrum.”29
As should by now be obvious, the big challenge facing seekers of weird life is that it could be weird in any number of ways, most of which we haven’t thought of. It is for this reason that Davies has suggested that the strategy with the best chance of success is simply to broaden our gaze and look for things that are unex
plained. One such thing, of particular interest to Carol Cleland, has been unexplained for a very long time.
DESERT VARNISH
In 1832, a young Charles Darwin was acting as assistant to the captain and unofficial naturalist aboard the HMS Beagle. As the ship was anchored off the coast of South America near San Salvador, Darwin explored the shore, where he was intrigued by rock outcroppings that glittered in the sunlight and seemed “burnished.” He deduced that the rocks shone because of a coating of thin layers of metallic oxides, but he could not explain how it might have been made.30
Geologists have since found the same coating—now called “desert varnish”—in many locales. Although they are no more certain of its provenance than was Darwin, they have ideas. They also have two observations that give reason to suspect that this substance may be a product of biology. The first is that the thin layers of minerals and chemicals that desert varnish reveals in cross section resemble the layers found in “stromatolites.” These are the mineral formations that in Shark Bay, Australia, look like half-submerged tortoise shells and in upstate New York look like fossilized cauliflower. Stromatolite layers are formed by generations of bacteria that, like a medieval city built and rebuilt on its own ruins, lived and died one atop another. The layering of desert varnish, so the thinking goes, might result from a similar biological process. The second observation is that many of the chemicals in desert varnish layers, most notably manganese and iron, are not in the rocks (like sandstone) that desert varnish typically varnishes, but are in fact produced by known organisms.
Even together, though, these observations are a long way from a clear-cut case for life. No laboratory microbiologist has been able to coax bacteria or algae to make desert varnish. And just as discouraging (to those who might wish it to be a product of organisms, that is), bacteria found in in situ desert varnish (the bacteria that might reasonably be expected to produce it) are of many varieties—too many, microbiologists think, to turn out the same product so consistently.
It is possible—and this is the prevailing view—that the stuff that intrigued Darwin and so many after him is the end result of some very complex chemistry. But no one has been able to reproduce that either. And so we have it: a natural phenomenon that exists in plain sight, and that after nearly two centuries of study remains a mystery. It is, so Cleland thinks, a fair candidate for weird life.
So far, we’ve learned of extremophiles that live at the outer boundaries of life as we know it. We’ve also learned of possibilities for life on Earth that, by subtle and not-so-subtle differences in their metabolism, might live beyond those boundaries. But we’ve been hugging the shoreline and reconnoitering a few nearby islands. Much of the remainder of this book will describe ideas of life that is much, much weirder. We’ll venture into waters that are little charted, and are sometimes out of sight of the shoreline altogether. Before we do, though, we would be prudent to establish exactly where that shoreline is, and to take a good look back at it.
* * *
* The equivocation in that word “almost” derives from the fact that scientists actually know of twenty-two naturally occurring amino acids on Earth. The genetic code of certain organisms can include selenocysteine and pyrrolysine, although the latter has been found so far in only one organism—an archaean called Methanosarcina barkeri.
* The idea of cooperation between species was (of course) not lost on Darwin, who noted, “A flower and a bee might slowly become, either simultaneously or one after the other, modified and adapted in the most perfect manner to each other, by the continued preservation of individuals presenting mutual and slightly favourable deviations of structure.” (Origin of Species, 85)
* The same uncertainty surrounds the detection of trace amounts of methane in Mars’s atmosphere, which may indicate life but may also be produced by a geochemical process. (Tenenbaum, “Making Sense of Mars Methane”)
* There are also trace elements, like iron and zinc, for which many organisms will make substitutions. Some mollusks, for instance, carry oxygen in their blood not with iron (the standard choice), but with copper.
CHAPTER THREE
Defining Life
More than once in recent years, planetary scientists have been surprised to find water where they didn’t expect to find it. On Mars, for example. The planet’s atmosphere is so thin—so near a vacuum, in fact—that any liquid water on its surface would evaporate immediately, and even ice would be likely to sublime directly into water vapor. But since the mid-1970s, when NASA’s twin Viking spacecraft imaged dry riverbeds and meandering tributary channels, it became apparent that once upon a time, perhaps 4 billion years ago, the surface of Mars had seen cataclysmic flooding. In the first decade of the twenty-first century, a small armada of spacecraft found more evidence of a watery Martian past—an outcropping that may be the shore of an ancient sea, and pack ice covered in volcanic ash. Most surprising were dry gullies and streambeds formed only a few thousand years ago, and evidence of a flash flood that happened even as the reconnaissance was under way. Since the 1970s, scientists had believed that when the planet lost most of its atmosphere, much of its water went with it. Now they know that some held on as permafrost, and still more remains in liquid form beneath the surface.
Astrobiologists have long noted that water on the surface of a planet too close to a star will boil; water on the surface of a planet too far away will freeze. Naturally, there is a slender range of distances from a star at which water could exist on the surface of a planet or moon—just the right distances at which water is liquid, the state necessary to life as we know it. It’s called the habitable zone or Goldilocks zone—the second because, like the porridge favored by the girl in the fairy tale, it is neither too hot nor too cold, but “just right.” It turns out to be a shell-shaped section of space whose inner surface is just inside Earth’s orbit and whose outer surface is just beyond Mars’s.* A lot of space, we might say. But when we see it pictured within the much larger volume of the whole Solar System, we realize how small a region it is. Although there is a lot of porridge in the universe, very little of it is just right.1
Or so it once seemed.
WATER, WATER EVERYWHERE
In 1995, NASA’s Galileo spacecraft detected magnetic fields near three of Jupiter’s large Galilean moons—evidence that miles beneath their frozen surfaces are briny oceans, warmed by natural radioactive heating from their cores and, in the case of Europa, the tidal heating caused by the push and pull of gravity. Some suspect that Europa’s ocean, beneath a crust of ice tens or hundreds of meters thick, may hold twice as much water as that in all of Earth’s oceans combined.2 Planetary scientists have reason to believe that great reservoirs of water-ammonia mixes are also held beneath the surfaces of Callisto and Ganymede.
Scientists have evidence for water inside planets and moons in still-colder regions, kept in liquid state by pressure, tidal heating, radioactive decay, or some combination of these. Adam Showman, a planetary scientist at the University of Arizona who has modeled interiors of moons of the Solar System’s outer planets, estimates that at least twelve hold some liquid water.3 In a 2006 paper, German physicist Hauke Hussmann and his colleagues modeled interior structures for medium-sized icy worlds in the outer Solar System, assuming heating in their cores from natural radioactivity, and concluded that subsurface oceans are feasible on Rhea, a smaller moon of Saturn; Titania and Oberon, moons of Uranus; Neptune’s moon Triton; and the dwarf planet Pluto.4 As to worlds beyond the Solar System—in the last several years NASA’s Kepler space observatory has made provisional identifications of more than 2,000 candidates for planets, almost fifty of which are in their stars’ habitable zones.
It is not difficult to imagine Earth’s extremophiles doing well in at least some of these places—the intraterrestrials beneath the surface of Mars, hydrothermal vent communities in Europa’s vast dark ocean, and Antarctic single-celled algae in the icy fissures of Saturn’s moon Enceladus. In fact, some have suggested t
hat if and when humans decide to “terraform” Mars—that is, initiate a centuries-long process through which Mars might be provided a breathable atmosphere and balmy temperatures—they might begin by seeding it with a few especially hardy extremophiles.5
All this water has been very good news for NASA, because the agency’s strategy in its search for extraterrestrial life, informally termed “follow the water,” has long assumed that any habitable extraterrestrial environments must have liquid water. It’s a reasonable assumption, especially given current knowledge of biochemistry and the limited resources of a government agency whose funding depends, after all, on taxpayers. And as we now know, there are certainly plenty of places to look.*** Nonetheless, the authors of the NRC report suspected that following the water might limit any discovery to organisms like those we know. And they began their work contending that if NASA scientists expected to look for life in all the places it might be possible, and if they expected to recognize it when they (or their instruments) saw it, they needed a proper definition of life—that is, a definition open to all possibilities, yet rigorous nonetheless.
If you thought that biologists might have an all-purpose definition at the ready, you’d be wrong. There are at least nine specialties within biology, and biologists tend to define life according to their own.* A physiologist might call life “a system capable of eating and metabolizing”; a molecular biologist might call it “a system that contains reproducible hereditary information coded in nucleic acid molecules.” Seeking a broader definition, we might look to a field of study with a wider view—say, philosophy. In fact, philosophers have been formulating definitions of life through much of recorded history. Most have characterized life not according to what it is made of (until the late nineteenth century no one had any idea), but according to what it does. And most have come up short. The problem is that any reasonably complete list of an organism’s functions is bound to include some that are performed by things that are nonliving, and—just as problematic—some nonliving things perform functions that some living organisms cannot perform.
