Origin Story, page 9
Chemical evolution can take place only in an environment that allows rich chemical experimentation. And such environments are extraordinarily rare. So what are the Goldilocks conditions for chemical experimentation? And why did the young Earth exhibit so many of them?
First, our solar system is in the right part of the Milky Way galaxy. Stars in the galaxy’s outer suburbs have thin, chemically impoverished clouds of chemicals to work with. Stars too close to the galaxy’s central business zone are battered by shock waves from the violent outbursts of black holes that lie at its core. Our solar system is in just the right place. Its orbit is about two-thirds of the way from the center of the Milky Way, in the middle of our galaxy’s “habitable zone.”
Second, chemistry works well only at lower temperatures. The early universe was too hot for atoms to combine into molecules. So is the interior of stars. Rich chemistry is possible within only a narrow range of fairly low temperatures, and you find these in habitable zones that are close to stars but not too close. Our Earth’s orbit is in about the middle of our sun’s habitable zone. Venus and Mars orbit at the inner and outer edges, respectively, of our system’s habitable zone. But we are learning that some moons farther away from the sun, such as Saturn’s moon Enceladus, may also have internal furnaces and chemistries that make them life-friendly. In 2017, scientists found that the oceans of Enceladus produce hydrogen, that gas that provided food for some of the earliest organisms on planet Earth.13
A third Goldilocks condition for rich chemistry is the presence of liquids. In gases, atoms zoom about like hyperactive kids, so it’s hard to keep them still enough to hitch up with other atoms. In solids, you have the opposite problem: atoms are locked in place. But liquids are like ballrooms, and liquid water, with its whispering hydrogen bonds, offers the best ballroom of all. Atoms can cruise, waltz, and tango, and it’s not too hard for electrons to change partners if they spot something more attractive. The presence of liquids depends on chemistry, temperature, and pressure. There is a narrow range of temperatures in which water exists in liquid form (most water in the universe is in the form of ice). But at these same temperatures, you can also find gases and solids, which makes for very interesting chemical possibilities. So, we should expect the most interesting chemistry to be on planets whose average surface temperatures lie roughly between zero and one hundred degrees Celsius, the freezing and boiling points, respectively, for water. That’s rare, but our Earth happens to be at just the right distance from the sun to have liquid water.
A fourth Goldilocks condition for rich chemistry is chemical diversity. It’s no good having the right temperature if you’ve got only hydrogen and helium to work with. And today, even in the chemically rich regions within galaxies, hydrogen and helium still make up 98 percent of all atomic matter. What chemistry needs is those rare environments in which the other elements of the periodic table are more common. In our solar system, such diversity can be found only on the rocky planets close to the sun, because the young sun boiled away much of the hydrogen and helium from the solar system’s inner orbits, leaving a concentrated distillate of all the elements in the periodic table.
As soon as the young Earth congealed, its diverse slurry of chemicals generated lumps of rock, solids consisting of many different simple molecules jumbled together. Earth’s first minerals also appeared, probably in the form of simple crystals such as graphite or diamonds.14
In such a chemically rich environment, many of the simple molecules from which life is built can form more or less spontaneously. We are talking about small molecules, with less than a hundred atoms, including the amino acids from which all proteins are made, the nucleotides from which all genetic material is made, the carbohydrates or sugars that are often used like batteries to store energy, and the fatty phospholipids from which cellular membranes are built. Today, such molecules don’t arise spontaneously because atmospheric oxygen would rip them apart. But there was hardly any free oxygen in the atmosphere of the early Earth, so these simple molecules could form when given a few jolts of activation energy.
In 1952, in an effort to demonstrate this, a young University of Chicago chemistry graduate student, Stanley Miller, created a laboratory model of the early Earth’s atmosphere by putting water, ammonia, methane, and hydrogen into a closed system of flasks and tubes. He heated the mixture and zapped it with electric charges (laboratory equivalents of volcanoes and electrical storms) to provide some activation energy. Within a few days, Miller found a pinkish sludge of amino acids. We now know that other simple organic molecules, including phospholipids, can also form in such environments. Today, Miller’s basic results still stand, even though we know that the early atmosphere was dominated not by methane and hydrogen but by water vapor, carbon dioxide, and nitrogen.
Since then, we have learned that many of these molecules can form even in the less chemistry-friendly environments of interstellar space, so lots of simple organic molecules may have arrived on Earth, ready-made, inside comets or asteroids. For example, the Murchison meteorite, which fell to Earth near Murchison, Australia, in 1969, contained amino acids and several of the chemical bases that we find in DNA. Such meteorites were much more common early in Earth’s history than they are today, which suggests that the early Earth was already seeded with many of the raw materials of life and quite capable of manufacturing more.
But most molecules inside cells, such as proteins or nucleic acids, are much more complex than these simple molecules. They consist of polymers, long, delicate chains of molecules, and forming polymers is not so easy. You need just the right amount of activation energy, and environments that can nudge molecules together in just the right way. One environment on the early Earth that might have provided the right conditions for stringing polymers together can be found at suboceanic vents, where hot material from Earth’s innards oozes through the ocean floor. These environments were protected from solar radiation and from the violent bombardments on the surface. They also contained diverse chemical elements, lots of water, and gradients of heat and acidity, as hot, chemically rich magmas seeped into cold oceanic waters. Alkaline vents, which were discovered only recently, in 2000, provide particularly promising environments, and the porous rocks that form at these vents offer tiny protected refuges for chemical experimentation, like Miller’s flasks and tubes. You can even find claylike surfaces with regular molecular structures that can create physical or electrical templates on which atoms can be wrangled into regular patterns and held still until they form polymerlike chains.
From Rich Chemistry to Life: Luca, the Last Universal Common Ancestor
Life appeared early in the history of planet Earth, and that suggests that creating simple forms of life may not be too hard where the right Goldilocks conditions exist. But identifying exactly when life appeared is tricky because the first organisms lived more than three billion years ago, because they were microscopic, and because the rocks they were buried in have eroded away. At present, the best direct evidence for the earliest life on Earth consists of microscopic fossils found in Western Australia’s remote Pilbara region in 2012. They seem to be of bacteria that lived about 3.4 billion years ago.15 In September 2016, an article in Nature described 3.7-billion-year-old fossils of what looked like coral-like stromatolites that were found in Greenland.16 If these are what many think they are, life must have begun evolving millions of years earlier than previously believed and must have appeared soon after the end of the Late Heavy Bombardment, about 3.8 billion years ago. And early in 2017, on the basis of fossil formations discovered in northern Quebec, scientists claimed that life might have appeared as early as 4.2 billion years ago; we will have to wait to see if these claims stand up.17
Biologists don’t yet have a complete explanation for how the first living organisms evolved. But they understand many steps in the process.
Though they don’t know exactly what it looked like, biologists refer to the first living organism as Luca (or LUCA, from “last universal common ancestor”). Luca certainly lived earlier than the earliest life-forms we have discovered so far, and it shared many features with the modern organisms known as prokaryotes: single-celled organisms whose genetic material is not protected within a nucleus. Today, prokaryotes are found in two of the three large domains of organisms, Eubacteria and Archaea. (The third domain, of which our species is a member, is the Eukarya.)
We’ll never find fossils of Luca because Luca is really a hypothetical creature, a sort of composite picture of the first living organism, a bit like a police sketch of a criminal on the run. Still, such a portrait might help us understand how life began.
Luca might have been sort of alive but not fully, somewhere in the zombie zone between life and nonlife. This is not as evasive an idea as it might seem. Viruses are not fully alive because they don’t tick all the boxes in our definition of life. They have no metabolism, and they have extremely fragile membranes, so it’s not even clear that we can describe them as cells. They are little more than packets of genetic material that glom on to more complex organisms. They enter another cell, hijack the cell’s metabolic mechanisms, and use it to make copies of themselves. When you have the flu, viruses are siphoning energy from your metabolic pipelines. But when they can’t find cells to hijack, viruses shut down and lurk in a sort of suspended animation. Some cells live deep inside rocks and have extremely slow metabolisms; they survive on tiny scraps of water and nutrition. They may be able to shut down entirely for long periods, like the rock guitarist Hotblack Desiato, in Douglas Adams’s novel The Restaurant at the End of the Universe, who spends a year dead, for tax purposes. The tax these organisms avoid, of course, is entropy’s complexity tax. Luca might have lived in a similar twilight zone.
Composite sketches of Luca have been built up by identifying several hundred genes that are present in most modern prokaryotes and are probably extremely ancient. They suggest the type of environment Luca evolved in, because they tell us what sort of proteins Luca was manufacturing in order to survive.18
The composite Luca (or family of Lucas, because we’re really talking about billions of them) could adjust to changes in its environment. It had a genome, so it could reproduce. And it evolved. Luca may have lacked both its own membrane and its own metabolism. Its cell walls were probably made of porous volcanic rock, and its metabolism depended on geochemical flows of energy over which it had little control. The proteins Luca made suggest that it lived at the edge of alkaline oceanic vents, probably inside tiny pores in lavalike rocks, and it got its energy from nearby gradients of heat, acidity, and flows of protons and electrons. Luca’s chemical innards probably sloshed around in warm liquids from inside the Earth that were alkaline, which meant they had an excess of electrons. Just outside the volcanic pores Luca called home were cooler ocean waters that were more acidic, which meant they had an excess of protons. Like a charged battery, the tiny electrical gradient between Luca’s insides and the outer world provided the free energy needed to drive its metabolism, draw in nutrients from outside, and expel waste materials.
Here is how one of the pioneers of early life studies, Nick Lane, describes Luca:
She [Luca] was not a free-living cell but a rocky labyrinth of mineral cells, lined with catalytic walls composed of iron, sulphur and nickel, and energised by natural proton gradients. The first life was a porous rock that generated complex molecules and energy, right up to the formation of proteins and DNA itself.19
Though simple by comparison with modern organisms, Luca already contained a lot of neat biochemical gadgets, including many of the recipes for the metabolic and reproductive machinery of modern cells. It probably had a genome based on RNA so it could reproduce much more accurately and precisely than mere chemicals, and that suggests it may have been evolving fast. It was also using the energy flows it tapped to make ATP (adenosine triphosphate), the same molecule that transports energy inside modern cells.
From Luca to Prokaryotes
Luca and its relatives had already done a lot of the heavy lifting needed to evolve the first true living organisms. But Luca lacked a membrane that it could carry wherever it went, and a metabolism that was not tethered to energy flows near volcanic vents. Luca also seems to have lacked the more sophisticated reproductive mechanism that is present in most modern organisms and is based on RNA’s close relative, the double helix of DNA. At present, we know what had to evolve, but we do not understand the precise pathways by which these things evolved.
Explaining the evolution of personal protective membranes is not too difficult. Cell membranes are made from long chains of phospholipids, and it is not hard to persuade phospholipids to link up in layers that form semipermeable bubblelike structures under the right conditions. Perhaps, as Terrence Deacon has argued, autocatalytic reactions evolved and generated phospholipid layers, molecule by molecule. If so, it may not be too fanciful to imagine some version of Luca knitting itself a personal membrane.20
Explaining how cells evolved better ways of getting energy and reproducing is trickier, but the mechanisms involved are so fundamental and so elegant that it is worth trying to understand how they work.
Evolving new ways of tapping energy flows so that cells could move away from volcanic vents meant creating the cellular equivalent of an electricity grid that molecules could plug into as they went about their work. Enzymes played a crucial role here. These are specialist molecules that can act as catalysts, speeding up cellular reactions and reducing the activation energy needed to get them going. Today, enzymes play a fundamental role in all cells. Most enzymes are proteins, made from long chains of amino acids. The exact sequence of amino acids matters, because that determines how the protein will fold up into the precise shape it needs to do its particular job. Enzymes cruise through the molecular sludge, looking for target molecules that they fit on to, the way a wrench fits a particular nut or bolt. Then the enzyme uses tiny shots of energy to tap, bend, crack, or split the molecule, or bind it to other molecules. Most reactions in your body could not happen without enzymes or would require activation energies so high they would damage the cell.
Once the enzyme has knocked its target molecule into shape, it breaks away and goes hunting for other molecules that it can bend to its will. Enzymes can also be switched on or off by other molecules that bind to them and slightly alter their shape, and this is how, like billions of transistors in a computer, enzymes govern the fantastically complex reactions that go on inside cells.
Enzymes get the energy they need to do their work from the cellular equivalent of the electrical grid. This is a system that must have evolved very early in the history of life. Energy is carried to enzymes and other parts of the cell by molecules of ATP, or adenosine triphosphate, and ATP was probably hard at work already inside Luca. Enzymes and other molecules tap ATP’s energy by breaking off a small group of atoms, releasing the energy that binds that group to the molecule. The depleted molecule (now called ADP, for adenosine diphosphate) then heads off to special generator molecules that recharge it by replacing the lost atoms. The generator molecules are powered by a remarkable process called chemiosmosis, which was discovered only in the 1960s but seems to have been at work since the time of Luca. Inside each cell, food molecules are broken down to capture the energy they contain, and some of that energy is used to pump individual protons from inside the cell (where there is a low concentration of protons) to outside the cell (where there is a high concentration of protons). This is like charging a battery. It creates an electrical gradient between the outside and inside of the cell, with a voltage similar to what Luca may have used at alkaline vents. Special generator molecules (ATP synthase, for the technically minded) that are embedded in cell membranes use the electrical voltage created by protons returning from outside the membrane to drive nano-rotors. Like rotary assembly lines, the rotors charge up ADP molecules by replacing the group of molecules they have lost, then the charged-up ATP molecules go back into the cell and wait for other molecules to plug into them and get the energy they need to keep working.
This elegant cellular electrical grid is present in all cells today. It untethered cells from the energy flows around volcanic vents, allowing the earliest prokaryotes to roam Earth’s oceans, scrounging energy from food molecules and using them to create ATP molecules that could supply the energy needed to power the cell’s innards.
These delicate flows of energy maintained the complex inner structures of cells just as fusion maintains the structures of stars. Like fusion, they allowed the first living cells to pay the complexity taxes demanded by entropy, because in cells, as in stars, a lot of energy goes into keeping complex structures functioning. But also as in stars, a lot of energy is wasted because no reactions are 100 percent efficient, and of course, entropy loves wasted energy. In both cells and stars, concentrated flows of energy are needed to pay entropy’s taxes and overcome the universal tendency of all things to degrade.
