by David Toomey
Ideas of how organisms might adapt to mixes of water and ammonia or water and liquid methane are what get biologists (and especially astrobiologists, who hypothesize about extra-terrestrial life) excited about places like Saturn’s moon Titan, where the warmest midday temperature might be –179°C, water ice is hard as granite, and methane, a gas in our atmosphere, is cooled to a liquid. Exactly how low, under the limbo stick of temperature, can life go? The NRC report proffers that, given the right solvent, “it is possible that there is no low temperature limit for enzyme activity or cell growth.”21
THE CHALLENGE OF SALT
It is not quite accurate to say that all extremophiles were identified after the discoveries of Corliss and Edmond. Some, including members of a group called “halophiles” (salt lovers), were found decades earlier. In the late 1930s, a graduate student named Benjamin Volcani, then studying at Hebrew University in Jerusalem, began to look for microorganisms in the Dead Sea. It was, to many, a curious pursuit. Hydrologically speaking, the Dead Sea is a closed-basin lake. In recent years, with the diversion of water from the Jordan River, its only substantial inflow, the Dead Sea has grown saltier and more alkaline. But even in the 1930s, its waters could be five times as salty as seawater, and often reached the point of saturation.22
The threat of salt water to a cell derives from the tendency of water molecules to balance the concentration of solutes on either side of a cell membrane. Salt water outside a cell will pull water from the cytoplasm inside through the membrane, and the cytoplasm will dry up.
In the 1930s, one would have had good reason to suppose that the water in the Dead Sea was lifeless, and many did. And so it came as no small surprise when Volcani found not merely a few organisms, but a thriving microbial community.23 They had solved the salt problem, as do many archaea and bacteria in brine lakes everywhere, with an “if you can’t beat ’em, join ’em” strategy, keeping high concentrations of salt in their cytoplasm, and so balancing the concentration inside against the concentration outside. But enough salt inside a cell can cause other problems. It will, for instance, bond with the water molecules that normally coat proteins, stripping them of that protection and making them vulnerable to denaturing. It turns out that the proteins in the cells of salt-loving archaea and bacteria have defenses—like, for instance, charged amino acids on their surfaces that hold on to the watery coating.
THE TEST OF ACID
On a shelf in her tidy, book-lined office in Woods Hole, biologist Linda Amaral Zettler keeps a small glass vial that she purchased in a tourist shop. It contains a few milliliters of what, in another setting, you might suppose to be red wine—perhaps a cabernet. In fact though, it is not quaffable, at least not by mesophiles like us. The liquid in the vial is a dilute acid laced with heavy metals, and it is from the Rio Tinto, a river in southwestern Spain.
The Rio Tinto’s source is an iron ore deposit—or rather, what’s left of one. The site has been mined, literally, since Paleolithic times, and what remains is a crater filled with water more acidic than vinegar. It is this acidity that dissolves iron, and it is the iron, oxidized by bacteria and exposed to air, that gives the water its reddish color—an indication of the high concentration of metals that the river maintains for all its 600-mile length, as it winds through rust-colored hills and scrub pines to empty, finally, into the Atlantic.
For years many had assumed the river was lifeless. As Dr. Amaral Zettler will tell you, they did not look very closely.24 Even without a field microscope, anyone can see films of algae on seeping walls along the river’s edges and, attached to rocks beneath the surface, green filaments of algae and whitish filaments of fungi waving in the current. But perhaps more surprising is what is living in and among the films and filaments. There are amoebas, ciliates, euglenoids, and flagellates—a thriving microbial community—not as diverse as that in a freshwater pond, but far more diverse than anyone expected.
Amaral Zettler is interested in many aspects of these organisms—one of which, quite naturally, is exactly how they manage to survive. Some set up defenses at the cell membrane, mostly with added proteins, that keep the inside at a more neutral pH and mostly free of metals. Others accumulate metals inside the cell, evidently without doing themselves serious harm. But research on the subject has barely begun, and it is probable—in fact, likely—that the microbial life in the Rio Tinto is protecting itself by other means as well.25
GOING WITHOUT
Readers of a certain age will recall an advertisement found in the back pages of many comic books, alongside the X-ray glasses and hovercraft plans, for “sea monkeys.” An illustration promised an underwater city bustling with miniature creatures that looked a bit like chimpanzees, if chimpanzees had spiny dorsal fins and webbed fingers and toes. It was, so we readers were led to believe, a completely self-contained alien civilization we could keep on the dresser in our bedroom. What actually arrived in the mail was less miraculous, but only slightly. It was a small foil packet containing what looked like coarse-grained paprika. If you poured it into a glass of warm salt water and held a magnifying lens to the side of the glass, you would see tiny creatures uncurl, wriggle, and swim. In fact, they were brine shrimp (Artemia salina).
The shrimp had survived without water through a trick shared by many organisms—including bacteria, yeasts, fungi, plants, and insects. It is a process called “anhydrobiosis,” by which cells shut down their whole metabolism and simply wait, as it were, for a rainy day. Some can wait a very long time. In the 1960s, archaeologists excavating Masada, the fortress in the Judean desert built around 35 BCE, found date seeds. Radiocarbon testing dated the seeds’ shell fragments to the same period, and someone thought it might be interesting to see what would happen if the seeds were planted. Of the three, one germinated and soon grew into a healthy meter-tall plant.26
These remarkable examples notwithstanding, the undisputed champions of longevity are not any particular organism, but the dormant stage in the life cycles of many bacteria, plants, algae, and fungi. They are the small, lightweight, stripped-down versions of seeds known as spores. As a group, spores are profligate (a single mushroom may release millions), but as individuals they are downright spartan. They keep within them little, if any, stored food, and evidently they don’t need much. In 1995, scientists resuscitated Bacillus spores that had been trapped in amber at least 25 million years.27 And spores are inventive, making salt (a cell’s enemy) into a shield. When salt water evaporates, it may leave deposits that have, trapped within them, tiny pockets of water called “brine inclusions,” microenvironments in which spores can survive. A Bacillus spore has been reported revived from brine inclusions thought to be 250 million years old—older, that is, than the first mammals.28
To many scientists in the late nineteenth century, spores seemed overengineered, far tougher than they needed to be; and some wondered whether they might have evolved in an environment much harsher than any on Earth. If they did, then they might explain life’s origin.
In the early nineteenth century, many natural philosophers held that organisms arise by spontaneous generation from organic matter. In 1860, French chemist and microbiologist Louis Pasteur conducted a series of experiments involving much care and many flasks and filters, and demonstrated that such could not be the case. Two possibilities for life’s origin on Earth remained: either life had arisen in the distant past, in the form of organisms far simpler than any in existence in 1860; or it had come from somewhere else. The second hypothesis, now termed panspermia, was put forth a few years after Pasteur’s work by Lord William Thomson Kelvin, who suggested that life originating on another world may have arrived on Earth via “seed-bearing meteoric stones.”29
Such a trip would not be easy. Suppose it were from Mars to Earth. An organism, actively metabolizing or dormant inside a fragment of Martian rock, would have to be well positioned—not so near a meteor strike that it would be vaporized, but near enough that it could ride the blast’s shock waves (and withstan
d tremendous g-forces and heat) up through the atmosphere and into interplanetary space. Once in space, it would have to survive vacuum, radiation, and extremes of temperature, and it would have to do so for years, decades, or perhaps centuries. Finally, it would have to withstand a fiery entry, along with more g-forces, into Earth’s atmosphere, ending its journey with an arrival violent enough to leave a crater.
Given spores’ well-known feats of endurance, many astro-biologists have wondered whether they might be up for the trip, and a few have devised experiments to simulate one. If you were a spore, you might regard astrobiologists as the sum of all fears. Astrobiologists have baked spores, frozen them, irradiated them, fired them from guns, and slammed them between quartz plates with explosives. And in case such simulations fell short of the rigors of actual space travel, they placed them aboard NASA’s orbiting Long Duration Exposure Facility and left them outside the spacecraft, unprotected except for a thin aluminum cover, for six years. At present, despite its advocacy by several respected scientists, panspermia lacks widespread support. Nonetheless, the upshot of all these experiments is that spores can withstand a violent launch and reentry, and that as long as they are shielded from ultraviolet radiation with a few centimeters of soil or rock, they are quite capable of surviving in space for decades—long enough for travel among planets within the Solar System. If life on Earth did come from elsewhere, it could have made the journey as a spore.
NOTHING LIKE THE SUN
Microbes collectively called “intraterrestrials” have been found several kilometers beneath Earth’s surface, making for a kind of subterranean biosphere.30 Bacteria have been found in rock samples taken several hundred meters below the seafloor, even in places where the seafloor itself is several kilometers below sea level. No one knows how many organisms are living in this environment, but the number may be large. One recent study found between a million and a billion bacteria per gram of rock. It may be that a large proportion of all bacteria on Earth live below the floor of the sea, where their metabolisms are driven by energy from various sources (like natural radioactivity) that are utterly independent of the Sun. But even extremophiles on Earth’s surface have been discovered exploiting unusual energy sources. One fungus was found in the water core of the Chernobyl nuclear reactor, ingeniously and fearlessly converting nuclear radiation into usable energy and managing radiation damage by keeping copies of the same chromosome in every cell.31
THE PRESENT
A list of extremophilic world record holders that elicits a “wow” also risks a dismissal—an assumption that they are freaks in a biological sideshow, having little to do with biologists’ larger interests. In fact, though, there are real, baseline reasons to count extremophiles as important players in the epic of life on Earth. Until the late twentieth century, many biologists supposed that all life on Earth began in the “warm little pond” that Darwin’s contemporaries favored or its more sophisticated successor, the “prebiotic soup” that Stanley Miller and Harold Urey tried to replicate in their famous experiments in the 1950s.* These suppositions and many others, along with a century or so of thought, were challenged when, three years after Corliss and Edmond discovered hydrothermal vent communities, Corliss and a group of colleagues published a paper arguing that life might have begun in or near a hydrothermal vent.32 Recent evidence suggests that thermophiles much like those now living near the vents may have been the ancestors of all life on Earth.33
These discoveries come at a time when many mesophiles are being discovered and catalogued for the first time. Especially in a moment when species are being made extinct at a terrifying rate—exceeding that of the five great extinctions in the last half-billion years, and at least a hundred times faster than the normal background rate—it may come as a surprise to learn that since 2005, about 400 species of mammals have been newly identified. But this is not necessarily good news. Many were discovered precisely because, with their habitats destroyed by logging, human settlement, climate change, pesticides, invasive species, and so on, they were disturbed, made suddenly visible—and vulnerable. We are, as it were, burning down the forest and watching to see who runs out.
It is here that extremophiles bring cause for a kind of big-picture optimism. If individual organisms and whole species are fragile, then life in general is resilient, tenacious, and, in its willingness to exploit any and all environments, downright aggressive. It is also inventive. When a suitable environment does not exist, life may create one. The most extreme extremophiles are of the domain called Archaea—the domain whose members were the first life on Earth, and a billion or so years from now, when our ever-warming Sun will have baked the ground and boiled away oceans, are likely to be the last. Even now, if the worst happened and a nearby star exploded, roasting Earth with gamma rays and exterminating all life on the surface and in the upper layers of the oceans, those assemblages of bacteria and fungi living a kilometer deep would go on as if nothing had happened. In time they would colonize the surface, probably learn the trick of synthesizing sunlight, and start things all over again.
Certainly the resulting scenery wouldn’t satisfy the aesthetic of, say, nineteenth-century American landscape painters. But then again, the assemblages of bacteria and fungi probably wouldn’t much care for the aesthetic of nineteenth-century American landscape painters—or ours, for that matter. And yet they and we are distant relatives. In fact, all life we know shares certain basic features. If you could take a cell from any organism—an alga, a giant sequoia, a condor, or your second cousin—and dive through its cell membrane and into its cytoplasm, you would find precisely the same nucleic acids and proteins doing precisely the same things in precisely the same ways.34
In fact, these shared features are what lead evolutionary biologists to suspect that everything that lives and has ever lived is descended from a single common ancestor, a microbe that metabolized some 3.5–3.8 billion years ago and (luckily for us) reproduced.35 You might expect that, given its role as the very origin of life on Earth, this microbe would have been granted a name evocative of grandeur and myth. But, perhaps doing an end run around cultural politics, or realizing that any name would have to be borrowed from one of the microbe’s descendants, biologists call it, somewhat prosaically, the last universal common ancestor, or LUCA.36
What interests a great many biologists is that many of the features shared by all known life seem to have no “selective advantage.” In other words, it didn’t have to be this way. There were, and are, alternatives. Chemists can imagine billions of organic compounds, but life uses only about 1,500. Those working in the new field of synthetic biology can imagine other amino acids, other proteins, and other metabolisms (or at least parts of metabolisms) that use other processes and would work just as well, perhaps better.
Quite naturally, a question arises. Was LUCA truly universal? Might there have been, in the 4.6 billion years of Earth’s history, a second genesis—a moment when complex molecules gave rise to another living organism, independent of and unrelated to LUCA? Might this organism have then reproduced? Might it even have established a line of descent that has endured as microscopic, single-celled Sasquatches into our own time? And if such organisms exist, since they arose from a chemistry different from that which produced LUCA, might they survive and even flourish beyond the limits of the most extreme of extremophiles?
These are profound and haunting questions, and they much occupy the thoughts of the several scientists (and one philosopher) we will meet in the next chapter.
* * *
* A few years after Corliss and Edmond discovered the community near the Galápagos, scientists returning to the site and conducting a more thorough reconnaissance discovered rows of slender white tubes with red filaments emerging from their tips—organisms now called tube worms, or Riftia pachyptila. In 2002 another expedition visited the site—informally termed the “Rose Garden”—and found that it had been covered with hardened lava. The midocean ridge giveth, and the midocean ridge
taketh away. And, it seems, giveth again. The same expedition found tiny tube worms and mussels the size of a fingernail—in a place they called, naturally enough, “Rosebud.” (Nevalla, “On the Seafloor”)
* Or, thermophiles and hyperthermophiles, psychrophiles or cryophiles, barophiles or piezophiles, acidophiles, alkaliphiles, halophiles, and radiophiles.
* It’s worth noting that organic does not mean “living or once living”; it means “denoting or relating to chemical compounds containing carbon.”
* Experiments enshrined in textbooks but whose presuppositions about the chemistry of the early Earth’s atmosphere are now largely discredited.
CHAPTER TWO
A Shadow Biosphere
Darwin was excruciatingly careful to distinguish what he knew from what he did not know, and to distinguish both from what, given the limits of biology in his day, he could not know. In an 1871 letter to botanist Joseph Hooker, Darwin refers to the then fashionable idea of an origin for life in “some warm little pond.” But contra many who have taken the phrase out of context, Darwin did not claim the idea as his own, and he observed later in the same letter, “It is mere rubbish thinking at present of the origin of life; one might as well think of the origin of matter.”1
If anything, the origin of life has proved the more difficult problem. Since the mid-1920s, biologists have agreed that life is the product of complex chemistry, but other aspects of the subject have been, and continue to be, vigorously debated. Many have conjectured as to its place of origin: that “warm little pond” and variations like the ocean and drying lagoons, surfaces of clays, deep-ocean hydrothermal vents, mineral surfaces of ice veins in glaciers, the pores of rocks deep within the Earth, even clouds. As mentioned in the previous chapter, some have suggested that life began elsewhere in the Solar System and was delivered to Earth via meteorite. There have been at least as many ideas as to its first form: enzymes, viruses, genes, and cells, to name a few. All these ideas have played out against the more fundamental question of life’s sheer probability, with the pendulum of informed opinion swinging on a wide arc between “improbable in the extreme” and “almost inevitable.” About the only point on which there has been general agreement is that if we could trace the ancestry of all living organisms back far enough, we would find them converging, some 3.5–3.8 billion years ago, at a single genesis. Life on Earth, so most scientists believe, began at one place and at one time.
