Arrival of the Fittest: Solving Evolution's Greatest Puzzle (8 page)

But 3.8 billion years ago, protein catalysts had yet to be invented. Darwin’s “warm little ponds” are poor sources of catalysts, which is one reason why many scientists became disenchanted with them.
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Another is that two molecules have to meet before they can react. Because molecules are jostled erratically through water by heat’s atomic vibrations, molecular encounters are chance events, and the chances are directly proportional to the number of molecules in a given volume of water: too few molecules, too few reactions. In other words, a metabolism can get going only if its molecules are concentrated. Dilute them in a bowl too large, and primordial life would end before it began. This is why chemists perform experiments in small test tubes and not in swimming pools. Washed out into the primordial ocean, newly created molecules would never be seen again.

Some think that tidal pools, a variant of Darwin’s warm ponds, might solve this last problem. At low tide, water in such a pool evaporates through heat, thus concentrating chemicals. At high tide, new water flooding in can stir up the broth. But earth’s violent youth casts doubt on this scenario as benign as a beach vacation. The moon orbited only a third as far away as it does today and tugged ferociously at the oceans, creating gigantic tides at least thirty times higher than today’s. What is more, the moon positively whirled around the earth (which itself rotated twice as fast), circling it at least every five days, and would have created these extreme tides every few hours, leaving little time to concentrate life’s ingredients.
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Evolutionary biology had been aching for better and smaller test tubes for decades, when an answer to its prayers arrived out of the blue—the deep blue. In 1977, the research submarine
Alvin
discovered an exotic menagerie on the Pacific seafloor near the Galápagos Islands, more than two thousand meters below the surface.
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Red-plumed mouthless tube worms more than two meters long, snails with feet and shells armored with iron minerals, and eyeless shrimp thrived down there, cushioned by lawns of never-before-seen microbes that do double duty as food. But even more bizarre than this community is how it survives: Its raw materials come straight from Mother Earth herself, through searingly hot fissures in the earth’s crust that overflow with nutrients, chemical energy, and the very catalysts that warm little ponds lack.

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.
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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 CO
₂
—these microbes
chemosynthesize.
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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.
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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.
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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.
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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.
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(
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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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 vents 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.
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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.
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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.
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