by Ben Miller
THE ORIGIN OF THE SPECIES
There’s something wonderfully poetic about Alvin finding a whole new raft of life near the Galápagos, of course, because it was on these volcanic islands that the great Charles Darwin collected the specimens that were to inspire his theory of evolution. The story is worth retelling, not only because Darwin was the astronaut of his day, boldly going where no naturalist had gone before, but also because evolution is such a linchpin in our search for intelligent extraterrestrial life. What follows may seem like a diversion, but by taking it we will have an easier approach to the summit, so here goes . . .
Galápago is a Spanish word meaning “tortoise,” and, according to Darwin’s journal, on September 18, 1835, the crew of the Beagle brought fifteen giant tortoises on board from Chatham Island,15 ready to supply a feast. With few natural predators, the animals of the Galápagos were curiously trusting; in fact, hunting and collecting were pretty much the same thing. Darwin himself reports knocking a hawk off a branch with the tip of his rifle, and rather surreally recalls midshipman King killing a bird with a hat.
Understandably, these giant tortoises made a strong impact on our young hero, and he was intrigued by the observation of the vice-governor of the Galápagos, Nicholas Lawson, that “he could, with certainty tell from which island any one was brought.”16 In other words, each island had its own species of tortoise. Sadly, Darwin wasn’t that successful in finding specimens that proved the point. He collected three giant tortoise shells, each from a different island, but they were from young animals and there was little to tell them apart.
Back in London, Darwin presented all his specimens to the Geological Society of London. The birds he gave to John Gould of the Royal Zoological Society for examination. Among them were those from the Galápagos, which Darwin had identified as blackbirds, wrens, and finches. When Gould returned the surprise result that they were all finches, “so peculiar as to form an entirely new group, containing twelve species,” Darwin began to formulate an audacious idea. What if there had originally been no finches on the Galápagos, which were, after all, relatively new volcanic islands. Could it be that a mating pair of finches had flown there from the South American coast, and somehow their descendants on each of the various islands had metamorphosed into new species?
To prove the point, Darwin needed to be able to show that, as with the vice-governor’s giant tortoises, each island was home to a different species of finch. Unusually for the meticulous Darwin, he had failed to label his own birds accurately, but fortunately his servant Syms Covington had not been so sloppy. By combining Covington’s specimens with those of the Beagle’s captain, Robert Fitzroy, Darwin was able to reconstruct the locations where he had found his own finches. It was true: Each island had begat its own species. Gould had returned his result on January 10, 1837. That March, Darwin wrote in a notebook the words that would change the course of biology forever: “One species does change into another.”
Species could change, but how? For Darwin, the argument went something like this. The mating pair had prospered, and their offspring had populated the various islands. Since reproduction is never exact, within each island population there were a variety of traits. Some finches, for example, had thick beaks, while others had thin beaks. If the seeds on a given island were hard to crack, finches with thick beaks would have a survival advantage, and would therefore have more offspring. Eventually, given sufficient generations, the entire population of finches on that particular island would have thick beaks. Nature, in other words, did not favor all creatures the same. Some she selected, and some she did not. As Darwin put it, species evolved through a process of natural selection.
In July that same year, barely eight months after he had returned on the Beagle, Darwin picked up his notebook and wrote the words “I think,” and below them sketched the first tree of life. Starting from a single trunk—the first living creature—he drew branch after bifurcating branch, with each new outgrowth representing a new species. It was a simple drawing, but its implications were profound. Starting with a single organism, life on Earth had evolved into an ever-increasing number of species. Take any two living things, the figure said, and you could trace back their lineage to find a common ancestor. All life on Earth was one.
IT’S ALL IN THE GENES
That’s such a piquant thought it’s worth taking a moment to digest it. Every single living thing on the planet is related to every other living thing. Not only are you a descendant of your great-aunt Ada, but you are a distant cousin of a flatfish, and a kinsman of an amoeba. The creatures that we find at black smokers, strange as they are, perch on the same tree of life that we do, as do the most bizarre fossils we have ever found, those of the Ediacarans.17
Completely central to Darwin’s theory of evolution by natural selection, of course, is the concept of inheritance: the passing of traits from parent to offspring. In Darwin’s day, the mechanism of reproduction was unknown; today we understand that every organism on Earth carries its own blueprint in every cell of its body, coded into the long-chain carbon molecule known as deoxyribonucleic acid, known as DNA. In short, you resemble your parents because you inherited their DNA.
Or to be more accurate, you inherited almost all of it. The system by which DNA is copied isn’t perfect, and that’s vital; it’s this less than perfect copying that gives rise to what Darwin called “variation”: the appearance of a new trait in the offspring that wasn’t inherited from its parents. Most of the time the new trait makes no difference. Sometimes it is harmful, and the offspring will be less likely to reproduce as a result, meaning the new trait dies out. In some rare cases, however, it bestows a survival advantage, and increases the chance that the offspring will, as Daft Punk might put it, “get lucky.”
Traits are coded by small sections of DNA known as “genes.”18 Take any trait—the thickness of a finch’s beak, say—and it is possible to identify the genes which control it. Indeed, as the British biologist W. D. Hamilton showed, it is at the level of genes that the struggle for survival is best understood, rather than that of an organism or species. Put simply, genes are doing it for themselves.19 All that matters to your genes is that they make as many copies of themselves as possible; we host organisms are simply a means to an end.
I’M A MAC, ZARG IS A PC
So there we have it. Evolution is the key that unlocks the mystery of life-as-we-know-it, and is able to explain two extraordinary and seemingly unrelated facts. Firstly, the older the fossils we dig up the more primitive the life-forms we find. No one has yet found a mastodon in the same layer of rock as a trilobite, nor do we ever expect them to. Speciation—the process by which natural selection creates two species where previously there was merely one—is irreversible, and the total number of species,20 both living and extinct, can only increase with time.
And, secondly, it explains the extraordinary similarity between the different branches of life-as-we-know-it. To use a computer analogy, everything we find is a Mac; nothing is a PC. Every creature on Earth is made up of one or more cells, relies on water as a solvent, stores its blueprint in DNA, and burns carbohydrate to release energy. Every molecule of living protein is made from the same twenty amino acids,21 and every molecule of living DNA is coded using the same four nucleobases.22 On an even deeper level, you might say that all life-as-we-know-it is carbon-based, because almost every single molecule that you can think of that has a biological or biochemical function is a compound of carbon.
So what does this mean for our search for intelligent alien life? Well, as the Galápagos giveth, so the Galápagos taketh away. On the one hand, its black smokers show us that there’s no limit to the kind of environments where life might thrive. On the other, its finches tell us there’s only one kind of life on Earth. So are we alone or not? What if life-as-we-know-it is a colossal fluke, a one-in-a-gazillion random event, never to be repeated? One thing is for sure: If we had just one other example of a second tree of life here on Earth, we’d
be a lot more confident that life is common in the galaxy. And yet there’s nothing. Or is there? It’s time to talk about desert varnish.
LIFE IN THE SHADOWS
The godfather of desert varnish research was the pioneering German naturalist and explorer Alexander von Humboldt. In 1799, during his groundbreaking expedition to South America, he noticed a strange metallic coating on the granite boulders in the rapids near the mouth of the Orinoco River in northeastern Venezuela that made them appear “smooth, black, and as if coated with plumbago.”23 Intrigued, he had the coating analyzed by one of the leading chemists of the day, Jöns Jacob Berzelius,24 who informed him that it was made up of manganese and iron oxides. That was puzzling, because granite contains only small amounts of manganese and iron.25 Where were these metals coming from? Presumably from the waters of the Orinoco, but what was attaching them to the rocks?
In The Voyage of the Beagle, Charles Darwin also describes an encounter with mysterious rock coatings. His were found below the tide line on a beach on the coast of Brazil and were a “rich brown” in color. Darwin had a bit of a thing about Humboldt, and one of his prized possessions on the Beagle was a seven-volume translation of his hero’s Personal Narrative of his South American voyage.26 He knew that Humboldt’s coatings had been much darker, and wondered if the redder color of those formed on the beach in Brazil was because they contained less manganese and more iron. Struck by how they “glitter[ed] in the sun’s rays,” he too was puzzled by what could be causing them, remarking that “the origin . . . of these coatings of metallic oxides, which seem as if cemented to the rocks, is not understood.”
Both Humboldt and Darwin found their coatings in the tropical climate of South America, but—perhaps unsurprisingly given the name—it turns out that what we now call desert varnish is found just as often in arid surroundings. The sandstone deserts of the Colorado Plateau in the southwestern United States are a classic example, where the shiny black patina on the rocks has often provided a handy surface into which Native Americans could scratch their art. Within the canyons, the swathes of varnish can be particularly striking, sometimes covering entire walls, or forming vertical stripes alternating between black and red and tan.
We’ve learned a bit more about desert varnish since the time of Humboldt, but not much. As Darwin suspected, its color does indeed vary from red to black depending on the relative quantities of iron and manganese oxide, with varnishes with equal amounts of both appearing tan in color. We know that the varnish also contains silica in the form of clay, and that it grows more readily on rocks and walls that are intermittently wet and get good sun, as if the rapid drying of water somehow helps the varnish grow. We know that the varnish accumulates slowly, growing by less than the thickness of a human hair every thousand years. And we also know—and this is where it gets controversial—that it contains microbes.
Barry DiGregorio, for example, an Honorary Research Fellow at the Buckingham Centre for Astrobiology in the UK, thinks that these microbes are photosynthesizing manganese-fixing bacteria, and that the varnish is, like the blooms that Brock found in the hot springs of Yellowstone Park, a type of microbial mat.27 On the other hand, Randall Perry, a researcher in the Earth Science Department of Imperial College London, believes that it’s the clays in the varnish that are doing all the heavy lifting. He thinks they react with moisture to form a gel, which then traps all sorts of other stuff including stray microbes, as well as catalyzing some pretty funky chemistry which concentrates the metal oxides. When this gel dries in the sun, it hardens into a varnish.28
Who’s right? Well, maybe neither, says Carol Cleland, a philosopher at the University of Colorado Boulder in the US, and an affiliate of the NASA Institute of Astrobiology. She is fascinated by the fact that we don’t know whether desert varnish is chemical or biological. As she points out, it’s a stretch to see how rocks could become coated with metal simply as a result of chemistry, but on the other hand, when we excavate desert varnish—if you can call scratching away at something a hundredth of a millimeter thick an excavation—we don’t find lots of cells, just a few fragments. What, she asks, if desert varnish is another kind of life entirely?
At first glance, that might seem slightly unhinged, but she has a point. After all, extremophiles have been around for billions of years, but it took Brock to notice them; once he had, we started to find them everywhere. In 2005, Cleland proposed the existence of a “shadow biosphere”; a microbial ecosystem living in parallel with our own, but that we have yet to identify. As she rightly points out, all of our tests assume that there is only one kind of life: our own. What if there’s something out there that doesn’t have the same DNA code, or uses different amino acids to build its proteins? What if it doesn’t have DNA or proteins at all? Or cells? Or is based on something other than carbon? How would we recognize it?
Cleland feels that we should be actively searching for life-as-we-don’t-know-it, and that desert varnish is a good place to start. Another is manganese nodules, the strange metallic boulders that populate the seabed in many of our oceans, and which always remind me of the egg hatchery in the movie Alien. These, again, are assumed to be the result of chemistry rather than biology, but how can we be sure? Once we can rid ourselves of our preconceptions of what life should look like, maybe we will start to find it everywhere, even in the hot springs of Yellowstone National Park.
Of course, all of this begs a question: If we do find a second genesis of life here on Earth, what do we call it? Seeing as it’s from Earth, the word “alien” doesn’t seem right. The cosmologist Paul Davies has suggested ditching the word “alien” altogether, and using the term “weird life” to describe anything that doesn’t share a common origin with life-as-we-know-it; he has even proposed a “mission to Earth” to seek it out. And what do we call the kinds of life that are emerging from the ever-growing field of synthetic biology? Is that weird too, or just weird-ish?
IN OUR BACKYARD
However you slice it, one thing is certain: The discovery of extremophiles has opened our minds to the idea that our own solar system might be teeming with life after all. There may not be herds of wildebeest sweeping majestically across the plains of Jupiter’s moon Europa, but there may well be blooms of microorganisms in the giant oceans we now know to be hidden beneath its icy crust. Likewise Ganymede, the largest of Jupiter’s Galilean moons, is now known to be hiding a saltwater ocean sandwiched between layers of ice. Knowing what we now know about life’s appetite for weird and wonderful environments, what might be lurking in its depths?
Remember Enceladus, the tiny moon of Saturn which appeared to Voyager to be a solid lump of ice? Well, in July 2005, NASA’s Cassini spacecraft arrived to finish the job. To everyone’s surprise, Enceladus wasn’t a cold, dead world after all, but violently active. At its south pole was a giant volcanic hot spot, from which plumes of ice particles and water vapor were erupting hundreds of miles into space; in fact, it’s this fire hose of material that supplies Saturn’s vast outer ring. By 2014, further measurements by Cassini had confirmed what many suspected: Beneath the pack ice at Enceladus’ south pole was a giant superheated ocean.29
Although Mars appeared lifeless to the Viking lander in the late seventies, circumstantial evidence has grown that it too was home to microbial life in the past, and may still be today. Thanks to recent missions like the Mars Reconnaissance Orbiter (MRO) we know that the Red Planet had copious amounts of water until as recently as two billion years ago. That means conditions on Mars were right for life-as-we-know-it round about the same time as they were on Earth; some have even suggested that our kind of life seeded on Mars first, and then came to Earth on a meteorite. As recently as 2015, NASA’s Curiosity rover found nitrates, a compound essential to many forms of life, and the MRO even found evidence of running water.
Last, but most definitely not least, is the glorious Titan. Again, it may not have been love at first sight, but he’s growing on us. Voyager 1, as you will remember, saw n
othing but an orange methane smog, and Voyager 2 decided it had better things to do than follow up. Yet many wouldn’t let it go. Just as the temperature of the Earth sits near the triple point of water—that is, the temperature at which water at atmospheric pressure can exist as a solid, a liquid, and a gas—Voyager 1 had confirmed what we had suspected since the middle of the twentieth century: Titan was at the triple point of methane, with a methane-rich atmosphere.30 When NASA’s Cassini spacecraft made its flight to Saturn, hitching a ride was a probe named Huygens, built and paid for by the European Space Agency.31
If it’s weird life we’re looking for, the images beamed back by Huygens showed us that Titan is just the place to find it. Martian meteorites reach us so frequently, and Mars’s early climate was so similar to ours, that any life we find there is quite likely to share a common genesis, or at least a similar biochemistry. And that’s leaving aside the issue that the Viking lander wasn’t sterilized properly, and may have contaminated Martian soil with earthly microbes. But Titan is a whole other deal. This is a really big moon with a proper atmosphere, with methane clouds, methane lakes, methane ice, and maybe even methane snow. What kind of abominable methane snowman might be lurking there?
Abominable microbial methane snowman, I should say. While our solar system no longer appears to be the graveyard we once thought, no one is expecting to find anything within it other than simple, single-celled life. That’s extraordinarily exciting just in itself—imagine what we could learn from just one weird microbe—but its implications would be even more exciting still. It would mean that biology is as universal as chemistry. And where there’s biology, there’s evolution. And where there’s evolution, there’s complex, intelligent life.