Cold ocean water percolates through these hydrothermal vents, sinking until it reaches the vicinity of giant magma chambers that heat it beyond the boiling point. From there it rises, like heated air in the atmosphere, until it is reunited with the cold waters above. On its journey through the deep-ocean volcanoes, this heated water leaches from the crust a thick broth of minerals, gases, and other nutrients. As it cools, they precipitate like snow from humid air. Unlike snow, however, they slowly aggregate into enormous chimneys whose height can exceed sixty meters. While growing, these chimneys continue to exhale a mix of hot water and small particles, and are thus aptly named smokers—black or white, depending on the chemicals in their breath.42
The hot water rising through hydrothermal vents may seem an obvious source of energy for life, but it is not the most important one—it’s not the heat that makes a soup, but the ingredients. The vent fluids abound in energy-rich chemicals such as the hydrogen sulfide that gives rotten eggs their aroma. These volcanic chemicals would be pure poison to us, but they are fertile fuel to some microbes. Unlike plants, which photosynthesize—they extract energy from sunlight to build complex molecules from CO2—these microbes chemosynthesize.43 They build their own organic molecules from energy-rich inorganic molecules, as well as from the vent’s abundant sources of carbon and other elements.44 And theirs is not the only way to thrive around a vent. Although it is pitch-dark two thousand meters below the ocean surface—hardly any sunlight penetrates below two hundred meters—a heated vent also emits the faintest of glows, enough that some bacteria can scavenge its light for energy.45 A vent’s ways of provisioning life may be bizarre, but they are also highly effective, supporting oases with thousands of times more organisms than the surrounding seabed.46
Unlike the tepid soup of Darwin’s pond, deep-sea hydrothermal vents are primordial pressure cookers. They are prevented from boiling over only through a kilometer-high water column that pressurizes them to some 200 atmospheres—equivalent to a mass of two million tons pushing down on every single square meter. Remarkably, even these extreme conditions do not deter life, as earth’s current high-temperature champion testifies. The microbe called Methanopyrus kandleri can reproduce at temperatures above 122 degrees Celsius, which is higher than the temperature microbiologists use to sterilize their equipment.47 (M. kandleri gets too toasty to reproduce only at 130 degrees, although it still survives there.)
Since Darwin’s visit on the Beagle, the Galápagos Islands have been famous as an unusually fertile laboratory of evolution. This volcanic archipelago has already brought forth giant turtles, unique marine iguanas, and the playful Galápagos sea lion. So another seemingly unusual laboratory, the hydrothermal vents only 250 miles away, may seem a fitting if ho-hum companion. But the hydrothermal vents are not unusual. Thousands of them are fuming throughout the earth’s oceans. They occur wherever magma rises from the earth’s core and causes the seafloor to spread. And that is about everywhere along a giant chain of underwater volcanoes called the Midoceanic Ridge, a long wound reaching deep into the earth, bleeding liquid magma that constantly renews the planet’s crust. A bit like the sutures on a tennis ball, this ridge circumnavigates the globe and is four times longer than the Rockies, the Andes, and the Himalayas combined, more than twice the circumference of the planet—all of it under water. Just as impressive as its length is the volume of water passing through the hydrothermal vents that litter this chain of volcanoes: More than 200 cubic kilometers every year, which means that all of the ocean’s water circulates through one vent or another every 100,000 years.48
Hydrothermal vents have become favorite candidates for life’s origin, but hardy and primitive life forms like this are not the most important reason. More important is that sources of energy and chemical elements are everywhere in their nutritious waters. Also, these vents are old, as old as the liquid oceans themselves. They have been exhaling nutrients since long before life began. Since then, all ocean water would have passed through them more than ten thousand times, enough to seed the oceans many times over.
Even better, hydrothermal vents solve several problems that plague warm little ponds. They provide exactly the needed test tubes, and in vast quantities. For the chimneys that rise when minerals precipitate from the hot rising water have a shape that is neither smooth nor simple. As the chimneys accrete from the precipitating vent fluid, they become suffused with numerous pores and channels, each one a minuscule test tube where microscopically small volumes of molecules can mingle and recombine without getting washed into the open ocean. Think of these chimneys as ever-growing laboratories stuffed with millions of tiny reaction chambers.49
As if that were not enough, these laboratories also come equipped with catalysts, not enzymes but minerals such as iron sulfide and zinc sulfide, some of them floating as particles in the vent fluids, others coating the surface of the reaction chambers.50 And yet another benefit comes from the mixing of hot vent fluids and cold water. Both the reactions that build life’s complex molecules and those that destroy them proceed faster when it’s hot. The searing heat in a vent’s core would render life’s molecules unstable, whereas in the coldest areas around it life’s reactions might proceed too slowly. But because waters of all temperature mix around a vent, a suitable temperature niche exists for any one of proto-life’s chemical transformations.
Hydrothermal vents may well have been the laboratories that created the first metabolism. But even if we were sure of that—origin-of-life researchers don’t agree on much—this knowledge by itself wouldn’t specify which chemical reactions comprised the first innovation of life’s history. The best candidates are the reactions found in the oldest parts of our own metabolism, those we share not only with other animals but also with plants and microbes, including the hardy ones around hydrothermal vents. Out of those possibilities, one candidate sticks out: a short cycle of chemical reactions called the citric acid cycle.
The citric acid cycle uses ten chemical reactions to transform one molecule of citric acid, the substance that gives lemons their sour taste, through several intermediates with uncommon names—pyruvate, oxaloacetate, acetate, and others—until it has completed one turn and manufactured another molecule of citric acid.51
A chemical cycle that creates two molecules from one sounds fishy, like the long-discredited perpetual motion machines of the nineteenth century. But this cycle does not violate any laws of physics. It cleaves the starting citrate molecule into two smaller molecules, from which its reactions build new molecules step by step, using as materials the carbon from carbon dioxide, and feeding on energy-rich nutrients.
Portions of the citric acid cycle appear in the planet’s oldest known life forms, but its ancient heritage is not the only reason it is a prime candidate for the earliest metabolism.52 The molecules that it creates are also ingredients for many other building blocks of life. Oxaloacetate provides atoms to build multiple amino acids and DNA nucleotides, pyruvate does the same for some sugars, acetate contributes to lipids—all-important components of cell membranes—and so on.53 If you sought one metabolic core from which you could build what life needs, the citric acid cycle would be it.
What is more, the citric acid cycle is extremely versatile, for it can run in two directions.54 In the first direction, described above, it operates a bit like an engine that performs the work of building new molecules, powered by a chemical battery of inorganic molecules. The kinds of bacteria that live in hydrothermal vents, bacteria that chemosynthesize for a living, use it in this way.55 Run in the opposite direction, the cycle charges the chemical batteries that power life. Our bodies run it in this way to create chemical energy from the food we eat.
Even though the citric acid cycle’s ancient heritage, source of building blocks, and versatility all advocate for its primacy, we are still waiting for an experiment like Miller’s that would jump-start the cycle. Don’t hold your breath. Such an experiment would be much harder than Miller’s, because hydrothermal vent
s create such extreme conditions. Moreover, a chimney’s reaction chambers have a complex shape and chemical coating that might have been the essential habitat for early life. You can’t exactly order test tubes like this in the mail. But although we do not know yet how the entire cycle can emerge spontaneously, some experiments already point the way: With catalysts like iron sulfide and zinc sulfide, pyruvate, a key cycle molecule, has already been created spontaneously at high temperatures and pressures, and some of the cycle’s reactions advance on their own in the laboratory.56
The citric acid cycle is attractive for one more reason: It makes more of itself. With each turn, it transforms a starting molecule into two, each of which spawns a new cycle and all its molecules, eventually creating four molecules, and so on. Chemists call this property autocatalysis, a fancy word for a defining feature of modern cells and primitive RNA replicators alike: They all make more of themselves.
The autocatalysis of the citric acid cycle differs from that of an elusive RNA replicase. Citric acid does not copy itself directly, nor do the cycle’s other molecules. Instead they get copied indirectly through the entire network of reactions in the cycle. The hypothetical RNA replicase would be a self-replicating molecule, while the citric acid cycle is an autocatalytic network of chemical reactions. This isn’t a shortcoming of the citric acid cycle, but another hint that a defining feature of life may not require RNA replicators and their genetic information: Life can exist before genes.57
We do not know—yet—whether the citric acid cycle is the grandfather of all metabolic activity. Nor do we know whether a metabolism of any sort came before RNA replicators. We do know, however, that the very first thing in the planet’s history that deserves to be called alive needed an autocatalytic metabolism to still its hunger. Such a metabolism is more than a mere supply chain of parts, because each of its suppliers creates more suppliers, which can produce parts in ever-increasing numbers. And once both the factory and its supply chain are in place Darwinian evolution can kick in. It can preserve better factories, which demand improvements in the supplier, which permit better factories, and so on, in the unending cycle of evolution that lifts all boats.
It is perhaps more than a coincidence that hydrothermal vents can help close this cycle too. For they contain another curious catalyst called montmorillonite, named after the French town Montmorillon, where farmers use this clay mineral to retain water in drought-prone soils. Late in the twentieth century, the chemist Jim Ferris and others revealed another useful quality of montmorillonite, when they discovered that it can rally small RNA building blocks to assemble spontaneously into RNA strings more than fifty nucleotides long.58
Once metabolism and replication were in place, life was almost ready to crawl out of its cradle, but it still needed a travel bag. All of today’s life uses the same kind of material to pack up its molecules, lipid molecules that are amphiphilic, from the Greek words for “both” and “love.” An amphiphilic molecule “loves” both water and fat, because one of its ends likes to mingle with water, whereas the other avoids water—like oil that spreads in a thin film on a puddle. Observe lipid molecules in a solution and you are in for a surprise: They can form vesicles, minute hollow droplets enclosed by a tiny spherical membrane, in which the lipid molecules are arranged, as shown in figure 2.59 How they could arrange themselves into complex highly ordered membranes without a guiding hand may seem mysterious but is not that hard to understand: This arrangement satisfies both parts of each molecule. The water-loving parts (solid circles in the figure) are close to water, whereas the water-avoiding parts (sticks in the figure) are away from it and close to each other. What is more, these membranes can grow spontaneously, incorporating new lipid molecules as you add them to the solution. And they grow autocatalytically: The larger they are, the faster they can grow.
FIGURE 2. Biological membranes
To see where the building blocks of these membranes came from, we do not have to look far. The citric acid cycle produces one of their precursors, and they arise even in extraterrestrial rocks like the Murchison meteorite. Heat up powdered meteorite with water, and you will find molecules that self-assemble into vesicles.60 What is more, montmorillonite, the same vent mineral that can string together RNA, accelerates membrane assembly. And hydrothermal vents can help in further ways, by concentrating membrane ingredients. This is what a team around Jack Szostak from Harvard University found when they re-created tiny reaction chambers like those of hydrothermal vents in their laboratory. They heated small amounts of lipids in tiny capillaries and saw that the lipids become concentrated at one end, until they start to form vesicles.61 All by themselves.
It smacks of Van Helmont’s spontaneous generation, all this complexity emerging from nothing but the right ingredients. But there is a crucial difference. Spontaneous generation—of mice, maggots, or microbes—requires the mysterious and perhaps supernatural vital force that Buchner’s discovery of enzymes began to expose as an old wives’ tale.62 In contrast, the spontaneous creation of membranes and molecules—self-organization in modern language—requires only mundane physics and chemistry. Assembling membranes requires nothing but the attraction of similar molecules. Like the self-aggregation of volcanic particles into towering underwater buildings, or the spinning of RNA strings by clay minerals, the self-organization of both membranes and molecules is explained by well-known laws of nature.
Self-organization permeates the universe so completely that most of us don’t even notice it. Much older than life and natural selection, self-organization is how stars and solar systems form, how the earth accreted, how it acquired a moon, oceans, and an atmosphere, and how the continents started to shift. Self-organization creates the microscopic symmetry of a snowflake and the raging clouds of a hurricane, the shifting shapes of sand dunes and the timeless beauty of a crystal. We shouldn’t be surprised to find self-organization in life’s precursors, because it is everywhere else too.
Life’s self-organizing membranes can solve another one of early life’s puzzles: the mechanism by which the first cells divided. Modern cells use very sophisticated machinery—dozens of proteins—to constrict and divide cells, and to make sure that each daughter cell receives a copy of the mother’s DNA. But they could do the trick in simpler ways, as Szostak’s team found in 2009. The researchers observed how rapidly growing membrane droplets change their shape when they divide, and transform into threadlike hollow tubes. Unstable tubes, I should say. Agitate them a bit and they fragment spontaneously into smaller droplets. Even better, when the researchers placed RNA molecules inside these tubes, they were partitioned among the droplets. Lifeless membrane droplets can divide like living cells—an innovation without an innovator, emerging from a simple property of the system’s chemistry. All by itself.
Although we have come a long way from the first musings about a primordial soup, there are some problems that still defy solution. One of them is the last obstacle on the path from a self-dividing membrane droplet to a primitive cell. If the RNA inside this cell replicated faster than the cell grew, it would divide until the vesicle was ready to burst. But if the cell outpaced the RNA in growing, the RNA inside would become increasingly dilute, and many droplets would spawn empty-shelled offspring. To succeed, life needed to balance, to regulate replication and growth with precision, such that RNA replicated no faster than its container grew. How it learned to do that remains a mystery that twentieth-century science has left for another generation.
Fast-forward from the first wheel to the Ferrari. Although some of life’s features did not change in the thirty million centuries since it began—molecules, regulation, and metabolism are still wellsprings of innovation, as we shall see in later chapters—evolution transformed just about everything else about it. Early RNA replicators have been replaced by complex protein machines. Life has learned to regulate not just RNA and lipids but thousands of molecules. And innumerable innovations have turned the metabolism of a modern cell—the Ferrari’s engin
e—into a miracle of chemical technology.
Imagine driving home in this Ferrari from an evening picnic and running out of gas on a lonely stretch of highway in the middle of the night. No gas station is in sight, nor is anybody you could hitch a ride with. But no matter. You open your trunk, where a cooler contains leftover food and drinks. You pour a bottle of orange juice into the tank, and after it a quart of milk, and then a glass of wine. That will be enough to tide you over to the next gas station. And on you drive.
Modern metabolic engines are just like that. They can run on many different fuels. And more than that, they can also use each fuel as raw material to manufacture the smallest molecular parts of their body, parts the body needs to grow, to reproduce, and to heal. It’s as if a car could use the stuff in its gas tank not only to operate the engine but also to patch a leaky tire or mend a broken windshield.
The molecular parts in question comprise a few core molecules, some sixty biomass building blocks from which our bodies are constructed and repaired.63 The most important are the four DNA building blocks of our genome, the nucleotides composed of a sugar, a phosphate group, and one of the four nitrogen-containing bases adenine (A), cytosine (C), guanine (G), or thymine (T). Next are the four building blocks of the RNA into which this DNA is transcribed, and that still controls much of life. They—A, C, G, and U, for uracil—differ only in a single oxygen atom from DNA building blocks, but this single atom makes a huge chemical difference. It makes RNA the better catalyst, and DNA the better—because more stable—information repository. Then there are the twenty amino acid building blocks of the amino acid strings translated from RNA, some of them familiar, like the tryptophan blamed for post–turkey dinner drowsiness, or the glutamic acid of the flavor enhancer monosodium glutamate (MSG). Together with the lipids in membranous bags, some energy storage molecules for hard times, and molecules that help enzymes do their job, these comprise the sixty different kinds of bricks from which cells build themselves.
Arrival of the Fittest: Solving Evolution's Greatest Puzzle Page 6