Origin Story, page 10
In living organisms, energy has a new function that we don’t find in stars: it creates copies of the cell. These copies allow cells to push back against entropy by preserving their complex structures even after individual cells have died. Luca’s descendants evolved the elegant and efficient methods of reproduction that all living things still use today. Those methods are built on a key molecule, DNA, whose structure was first described in 1953 by Francis Crick and James Watson based on earlier research done by Rosalind Franklin. So much of evolution depends on understanding how DNA works that it is worth looking more carefully at this marvelous molecule.
DNA (deoxyribonucleic acid) is closely related to RNA (ribonucleic acid). Both are polymers, long chains of similar molecules. But while proteins are made from strings of amino acids, and membranes are made from phospholipids, DNA and RNA are made from long strings of nucleotides. These are sugar molecules to which are attached small groups of molecules known as bases. The bases come in four types: adenine (A), cytosine (C), guanine (G), and thymine (T). (In RNA, thymine is replaced by uracil, U.) And here’s the magic. As Crick and Watson showed, these four bases can be used like the letters of an alphabet to carry huge amounts of information. As DNA or RNA molecules link up to form huge chains, the bases stick out to the side, forming a long string of As, Cs, Gs, and Ts (or Us in RNA). Every group of three letters codes for a particular amino acid or contains an instruction, such as Stop reading now. Thus, the sequence TTA says, Add on a molecule of the amino acid leucine, while TAG is a sort of punctuation mark that says, Okay, you can stop copying now.
The information on DNA and RNA molecules can be read and copied because the bases like to link up with each other using hydrogen bonds, which can be made and broken quite easily. But they bond only in very specific ways. A always joins with T (or U in RNA), and C with G. Special enzymes expose stretches of DNA that correspond to a particular gene or code for a particular protein, and each base attracts its opposite to create a new short RNA chain of nucleotides that is complementary to the original chain. The newly created segment is then whisked off to a large molecule known as a ribosome, which is a sort of protein factory. The ribosome reads the sequence of letters in triplets and extrudes the corresponding amino acids, one by one, in just the right order to make a particular protein, which then goes off into the cell to do its work. In this way, ribosomes can manufacture all of the thousands of proteins a cell needs.
The final piece of magic is that DNA and RNA molecules can use these copying mechanisms to make copies of themselves and all the information they contain. The bases that stick out sideways from their sugar-phosphate chains reach into the cellular sludge and grab onto their complements. Thus, Cs always grab onto Gs, and As always grab Ts (or Us, in RNA). The newly attached bases attract new sugar molecules that link together, and in this way they form a new chain that is the exact complement of the first. In DNA, these two complementary chains normally stick together, which is why DNA usually exists in the form of a double chain or helix, like a pair of winding staircases. It can be wound up so tightly that it packs neatly inside each cell, and it is unwound only to be read or to make copies of itself. However, RNA normally exists as a single chain, so, like a protein, it can also fold up into particular shapes and function like an enzyme.
This small difference between RNA and DNA is hugely important because it means that, while DNA normally functions just as a store of genetic information, RNA can both store information and do chemical work. It is both hardware and software, and that is why most researchers believe that there was a time, perhaps when Luca was still around, when most genetic information was carried by RNA. Luca probably lived in such an RNA world. But RNA is a less secure information carrier than DNA because its information is constantly buffeted in the violent inner world of the cell, whereas the double strands of DNA shield their precious information from the whirlwind outside. In an RNA world, genetic information could easily get lost or distorted. Evolution really got going only after the development of a DNA world by Luca’s descendants, the true prokaryotes, which dominate the world of microorganisms today.
With membranes of their own, an independent metabolism, and more precise and stable genetic machinery, the first prokaryotes could leave the volcanic vents in which they had been born and cruise the oceans of the early Earth. They were probably already doing this 3.8 billion years ago.
Each prokaryote is an entire kingdom of staggering complexity. Billions of molecules swim through a thick chemical slurry, being nudged and pulled by other molecules thousands of times each second, rather like a tourist in a crowded market full of traders, touts, and pickpockets. If you were injected into one of these molecules, you would find this a terrifying world. Enzymes will try to glom on to you and change you, perhaps hook you up with other molecules to form a new team that can cruise the markets looking for new opportunities. Imagine millions of these interactions going on inside every cell every second and you have some idea of the frenetic activity that powers even the simplest of cells in the early biosphere.
This is a new world and a new kind of complexity. Like stars and planets formed during periods of chaotic change, cells eventually settled into a sort of stability as they began to manage and push back against tiny fluctuations in their environments. Cells would achieve a temporary balance; so, too, would entire species and lineages and groups of species. But this was never a static balance. It was always dynamic, always maintained by a constant negotiation between living organisms and changing environments, and always in danger of a sudden breakdown.
CHAPTER 5
Little Life and the Biosphere
To give Estha and Rahel a sense of Historical Perspective… Chacko… told them about the Earth Woman. He made them imagine that the earth—four thousand six hundred million years old—was a forty-six-year-old woman.… It had taken the whole of the Earth Woman’s life for the earth to become what it was. For the oceans to part. For the mountains to rise. The Earth Woman was eleven years old, Chacko said, when the first single-celled organisms appeared.
—ARUNDHATI ROY, THE GOD OF SMALL THINGS
Together, Earth and life make up the biosphere.1 The word biosphere was coined by the Austrian geologist Eduard Suess (1831–1914). Suess saw Earth as a series of overlapping and sometimes interpenetrating spheres that included the atmosphere (the sphere of air), the hydrosphere (the sphere of water), and the lithosphere (the rigid, upper levels of the Earth, including the crust and the top layers of the mantle). But it was the Russian geologist Vladimir Vernadsky (1863–1945) who first showed that the sphere of life has shaped planetary history as powerfully as the other, nonliving spheres. We can think of the biosphere as a thin wrapping of living tissue (and the remains and imprints of living tissue) that reaches from the depths of the oceans to Earth’s surface and up into the lower atmosphere. In the 1970s, James Lovelock and Lynn Margulis showed that the biosphere can be thought of as a system with many feedback mechanisms that allow it to stabilize itself in the absence of major shocks. Lovelock called this vast, self-regulating system Gaia, after the Greek goddess of the Earth.
Geology: How Planet Earth Works
Life took some time to get going, so we will begin by considering planet Earth as a purely geological system, like a stage set before the actors have arrived. That should make it easier to understand the complex dramas acted out later by living organisms.
The violent processes of accretion and differentiation, which had forged the young Earth, left a chemically rich ball of matter separated into distinct layers. There was a hot, semimolten core, made mostly of iron and nickel, that generated a protective magnetic field around Earth. Wrapped around the core was a three-thousand-kilometer-thick layer of gas, water, and semimolten rock, the mantle. The lightest rocks rose to the surface and formed Earth’s crust. Gases and water vapor bubbled up through volcanoes to create Earth’s first atmosphere and oceans. Meteors and asteroids ferried in new cargoes of rocks, minerals, water, gases, and organic molecules.
About 3.8 billion years ago, when the bombardment from space eased up, the main driver of geological change was the heat buried in Earth’s core. That heat seeped up through Earth’s mantle, to the crust, and into the atmosphere, churning up the material in each layer, transforming it chemically, and moving vast amounts of matter and gas around in huge, slow convection cycles. Like the evolution of stars, the geological evolution of our Earth was driven primarily by simple processes that fed on an initial, nonrenewable store of energy. Earth changed as it sweated heat from the core through the mantle and crust and out into space.
Heat from the core still drives a lot of geology and will continue to do so for billions of years. But not until the 1960s did geologists figure out how this huge geological machine worked. Their new understanding of geology was based on one of modern science’s most important paradigms: plate tectonics.
Humans have been able to visualize Earth’s surface only in the past five hundred years, when, for the first time, they were able to sail all around it. But most people continued to assume that at large scales, the world’s geography was more or less fixed. Volcanoes might erupt and rivers change course, but surely the layout of continents and oceans, of mountains, rivers, and deserts, of ice caps and canyons, was unchanging. Some, though, began to have doubts. And, just as Darwin showed that life had changed profoundly over the eons, evidence began to accumulate that Earth, too, had a history of profound change.
In 1885, Eduard Suess suggested that about two hundred million years ago, all the continents had been joined together in one supercontinent. We now know he was dead right. Three decades later, Alfred Wegener, a German meteorologist who had done research in Greenland, assembled a lot of evidence that supported Suess’s idea. Wegener published that evidence in 1915, the middle of World War I, in a book entitled (perhaps with a nod to Darwin’s Origin of Species) The Origin of Continents and Oceans. Just as Darwin proposed that living organisms had evolved, Wegener proposed that continents and oceans had evolved, by a mechanism he called continental drift. Once joined in the supercontinent of Pangaea, or Pan-Gaia (a Greek word meaning “all Earth”), they had gradually diverged and moved to their present positions.
Wegener offered plenty of evidence. On a world map, many parts look as if they once fit together, something people had noted since the creation of the first world maps in the sixteenth century. Just before 1600, a Dutch mapmaker, Abraham Ortelius, commented that the Americas seemed to have been “torn away” from Europe by some catastrophe.2 If you look at a modern world map, you’ll see that the shoulder of Brazil fits nicely into the armpit of western and central Africa, while West Africa looks as if it would fit snugly into the huge arc of the Caribbean. In the 1960s, geologists realized that the fit is even better if you focus on the edges of the continental shelves.
Wegener showed that there were almost identical fossils of ancient reptiles in South America and central and South Africa. The early nineteenth-century German scientist Alexander von Humboldt, one of the first scholars to write a modern, science-based origin story, had also noticed similarities between the coastal plants of South America and Africa.3 Then there were rock strata that seemed to start in West Africa and continue in eastern Brazil without missing a beat. As a meteorologist, Wegener was particularly interested in climatic evidence. In tropical Africa, you could find the telltale scratches and gouges of moving glaciers. Could tropical Africa once have hovered over the South Pole? In Greenland, Wegener had found fossils of tropical plants. Something had certainly moved over long distances in the deep past.
But it takes more than some suggestive evidence to make a good scientific hypothesis. Publishing in the middle of World War I didn’t help Wegener’s case, and the fact that he was German and not a geologist ensured that few geologists in the English-speaking world took his ideas seriously. Was it really possible that whole continents could plow through the oceans? Wegener had no idea what force could have pushed them around, and in the eyes of most professional geologists, the absence of an explanation was enough to kill off his hypothesis. In November 1926, Wegener’s theory of continental drift was decisively rejected by the influential American Association of Petroleum Geologists. And that seemed to be that.
Except that a few geologists were intrigued. A British geologist, Arthur Holmes, argued in 1928 that the interior of Earth might be hot enough to act like a slowly moving liquid, like lava. If so, perhaps the motion of material inside Earth could float entire continents around the globe. But not until the 1950s would new evidence show that Wegener, Holmes, and other supporters of the idea of continental drift had been following the right geological scent.
That’s where sonar (the word comes from “sound navigation ranging”) enters the story. Sonar technology can detect and locate objects underwater by bouncing signals off them and analyzing the returning echoes. Many animals use sonar, including dolphins and bats. Human sonar technology, like radiometric dating, was a product of wartime science, in this case attempts to detect enemy submarines. Harry Hess, a geology professor at Princeton, was a naval commander during World War II, and he had used sonar to track German submarines. After the war, he used sonar to map the seafloor, which was still unknown territory to marine geologists. Most expected the seafloor to consist of a flat ooze washed off the continents. Instead, Hess found chains of volcanic mountains running through the Pacific Ocean. No geologist had expected that. After discovering a similar chain running through the middle of the Atlantic Ocean in the early 1950s, he began to develop a theory to explain these mid-oceanic ridges. His task was helped by paleomagnetism, or studies of the magnetism of the seafloor. It was already known that at intervals of up to a few hundred thousand years, Earth’s north and south magnetic poles had swapped places many times. These flips left their traces in lava that seeped up through the ocean floor and took on the prevailing magnetic orientation as they solidified. Measurements of the magnetic orientation of rocks on either side of the volcanic ridges showed a series of north/south flips as you moved away from the ridges. This puzzled Hess.
Eventually, Hess figured out that the undersea mountain chains were being created by magma squeezed up through cracks in the oceanic crust. This made sense, because oceanic crust is thinner than continental crust, so it can be punctured easily by hot magma. As magma climbed through cracks in the seafloor crust, it elbowed the crust apart, creating new seafloor that was imprinted with the magnetic orientation of the period when it formed. The alternating magnetism of mid-oceanic rocks provided a way of dating the formation of these underwater mountain ranges.
Lurking in these discoveries lay the driver of continental drift that Wegener had looked for in vain. Mountain chains, continents, and the seafloor were created and pushed around by huge amounts of hot magma that rose from Earth’s mantle and squeezed through cracks in the seafloor crust. The magma was heated by radioactive elements and by heat from Earth’s core, which retained much of the energy stored during the violent processes of accretion and Earth building. And there in the planet’s core lay the missing driver. Like fusion at the center of a star, heat leaking from the center of the Earth drives most important geological processes on the surface.
We now have abundant evidence that Earth’s crust, both oceanic and continental, is broken into distinct plates that jostle for position as they are dragged back and forth by the semimolten magma on which they float. Hot magmas rising from deep within the Earth circulate under the crust, like water boiling in a saucepan. It is these convection currents of semiliquid rock and lava that move the tectonic plates floating above them. Detailed studies of paleomagnetic bands have allowed earth scientists to trace the movements of plates over hundreds of millions of years, giving us an increasingly precise idea of Earth’s changing geography over the last billion years or so. We now know that these movements have created supercontinents like Pangaea and then broken them up several times in a cyclic process that probably began early in the Proterozoic eon, about two and a half billion years ago. Before that, there were probably no large continents. But some geologists argue that the machinery of plate tectonics may have powered up much earlier. Evidence from the Hadean eon suggests that some form of plate tectonics was already at work 4.4 billion years ago, as soon as Earth differentiated into distinct layers.4
Like big bang cosmology, plate tectonics was a powerful unifying idea. It explained and showed the links between many different processes, from earthquakes to mountain building and the movement of continents. It explains why so many violent geological events take place where tectonic plates meet and grind their way past, over, and under each other. Plate tectonics also explains why Earth’s surface is so dynamic, as it is continually renewed by the arrival of new materials from the mantle, while surface material, in turn, is subducted deep into the Earth.
To understand how plate tectonics works in detail, it helps to focus on the borders between tectonic plates. At divergent margins, like those described by Harry Hess, material rises from the mantle and pushes plates apart. Elsewhere, though, at convergent margins, plates are pushed together. If the two plates have about the same density—if, say, they both consist of granitic continental plates—then, like two bull walruses competing for mates, they will rear up. This is how the Himalayas formed; within the past fifty million years, the fast-moving Indian plate traveled north from Antarctica and smashed into the Asian plate. But if two converging plates have different densities—if, say, one consists of heavy, basaltic oceanic crusts and the other of lighter granitic continental crust—the story is different. The heavier oceanic plate will dive under the lighter plate at a subduction zone. It will travel downward, like a runaway elevator crashing through a concrete floor, carrying crustal material back into the mantle, where it dissolves. As the descending plate drills into the mantle, it will generate so much friction and heat that it can melt the crust above it, splitting it and punching up new volcanic mountain chains. This is how the Andes formed, as the Pacific plate burrowed beneath the plate carrying the west coast of South America.
