Creation (7 page)

What this horizontal gene swapping means is that our techniques for tracking back through the history of life using DNA as our guide may only work convincingly from the point where descent becomes vertical. The tree of life only begins to resemble a branching thing that deserves to be named a tree
after
the emergence of complex life. From that species on, when an archaea swallowed a bacteria, the overwhelming majority of inheritance was from parent to offspring: descent with modification. UCL biochemist Nick Lane calls this the genetic event horizon: comparing genes will take us all the way back to the point where it looks like a nice, branching tree, but our vision grows blurred before that point. It just becomes far too messy to unpick the deepest past.

Therefore, while we can sensibly use DNA to infer that humans and chimps had a common ancestor around six or seven million years ago, and humans and the meager sea worm amphioxus had a common ancestor around five hundred million years ago, we cannot reliably use this to date Luca. The branches of the tree become tangled and scrambled before complex life, and tracing the changing patterns of DNA as species evolve is impossible. We are left, then, with very little genetic evidence of what Luca actually was at all.

The Creation of Luca

We can figure out some things about Luca, though. Calculations and logic predict that the last universal common ancestor had characteristics that are shared between archaea and bacteria, and that means genes, proteins, and the cellular mechanics largely similar to what we see today. That means we can use comparisons of these molecules to understand some things about Luca, even if we cannot apply an accurate date. A study by Douglas Theobald (Brandeis University, MA) in 2010 applied hard statistical analysis to the domains of bacteria, archaea, and complex life. He looked carefully at the construction of twenty-three proteins that are present in each of these domains with seemingly common descent, like words that sound and mean similar things in different languages. Based on the similarities of the sequence of amino acids that make up these proteins, Theobald calculated that the odds of their having arisen independently was 1 in 10^2,860 (i.e., a 1 with 2,860 zeroes after it).
¹³

Another clue to Luca's singular origin concerns the most fundamental cellular machinery—the ribosome. It exists in all cells as a processing plant organized in minuscule blocks of molecules. Its role is universal, and as such, again supports the single-origin idea. The ribosome reads the genetic code and translates it into protein. The intricacies of this exquisite machine are explored later in the book (and particularly in the afterword, p. 240), but in essence its job is to read the genetic code (already transcribed into an RNA version) and translate each three-letter section into an amino acid. The ribosome strings together the amino acids in accordance with the messenger RNA, and a protein feeds out like a ticker tape.

We look to the ribosome as a useful indicator of relatedness because it is fundamental—without its manufactory we have no proteins, and none of the life we are aware of can exist. We can understand the idea that mitochondria in complex cells are derived from annexed bacteria because mitochondria have their own ribosomes, separate from their host cell, and these ribosomes are much more similar to bacterial ones than animal ones. We can compare the sequences of the genes that encode parts of the ribosome in all species, and trace backward the changes that have occurred over time to predict what Luca's ribosome looked like.

This is a fruitful endeavor, but the results are contested. For example, by looking at the particular sequence of ribosome parts across many species, you can reasonably infer the temperature at which that host organism thrives. Several parts of the ribosome are built from neatly folded RNA molecules, themselves made from letters of genetic code
A, C, G,
and
U
. As in the double helix of DNA,
C
pairs with
G
and
A
pairs with
U
. But
C
and
G
form a bond that is more stable at higher temperatures. Therefore, we can infer from the relative amount of
CG
bonds in ribosomes a preference for warmer conditions. This is borne out across many species, and the ribosome in which we see the highest
CG
content are from high-temperature extremophiles—in other words, organisms that thrive in heat. Where do we find such creatures? In many places, but the most impressive habitat is in and around hydrothermal vents under the sea. There, heated by a rip in the earth's surface, plumes of noxious chemicals that can boil the sea chug out. But dozens of species thrive there: bacteria, archaea, and even large, complex creatures such as the Pompeii worm, which can endure temperatures up to eighty degrees Celsius, more than 170 degrees Fahrenheit (not least because it wears an insulating fleece of hardy bacteria).

Some models of Luca's ribosomes suggest that the amount of
C
s and
G
s was disproportionately high in their component parts. This might suggest a hot home for the base of life. Certainly, the discovery of extremely heat-loving (aka hyperthermophilic) archaea and bacteria in such environments as the hot springs in Yellowstone Park or submarine thermal vents supports this idea, as these organisms mostly occupy spaces at the base of evolutionary trees, as much as we can reconstruct them.

But the simple truth is we don't—and possibly can't—know. Reconstructing the past using phylogenetics is a complicated art, with many confounding factors. Using the ever-changing DNA sequence, we can't see back to Luca because of the ability of bacteria and archaea to move genes sideways, not just from parent cell to daughter. That's not to say that Luca did not have specific characteristics that we can investigate and compare with those in living things. It's just that the idea of a single cell, a biological equivalent of Adam from the biblical Genesis, might be naive. If Luca was a cell, even in its most basic form, it still would have systems within it that are like their modern counterparts: DNA, RNA, proteins, ribosomes to make proteins, a cell membrane, and, crucially, a highly developed way of capturing energy—a metabolism. Otherwise, we would expect to see different and divergent mechanisms in bacteria and archaea. Not to denigrate this useful species; Luca's real use for science is as a proxy. If Luca is at the root of cellular life, it represents an all-dominant coagulation of what came before. Bill Martin, a brilliant and pugnacious origin-of-life biochemist we will meet properly later, says that the trouble with Luca is that, “like love, it means different things to different people.”

It's been almost three and a half centuries since cells were bled, ejaculated, and fished out of their natural environment and viewed under a primitive microscope. Since then, we have picked them apart to the extent that we have earned almost full command of their faculties, acquired over four billion years. We see their commonality so clearly, a beautiful neatness where everything we discover in biology serves to refine and reinforce the truth of evolution. It's a wonderful state of affairs, and reflects the maturation of a science. The essential qualities of life are known and gel into a grand vision: life shares its tools, its processes, and its language. The security of a robust unifying theory of life as we know it allows us to investigate a much more difficult puzzle: where did Luca come from? As it happens, the best way to begin to understand the emergence of life on Earth is to take a hard look at where and when it happened. Therefore, we must start at the very beginning. It's a very good place to start.

Hell on Earth

“Long is the way, and hard, that out of hell leads up to light.”

John Milton,
Paradise Lost

I
f you want to construct a picture of the earth on which life first emerged, think about how we've named it. There are four geological eons spanning the earth's 4,540-million-year existence. The most recent three names reflect our planet's propensity for living things, all referring to stages of life. The second eon is called the Archean, which rather confusingly translates as “origins.” The third eon is the Proterozoic, roughly translating from Greek as “earlier life”; the current eon, the Phanerozoic, started around 542 million years ago and means its name “visible life.”

But the first eon, the period from the formation of the earth up to 3.8 billion years ago, is called the Hadean, derived from Hades, the ancient Greek version of hell.

Life does not merely inhabit this planet; it has shaped it and is part of it. Not just in the current era of man-made climate change, but all through life's history on Earth, life has affected the rocks below our feet and the sky above us. And necessarily, the origin of life is inseparable from the fury of the formation of the earth in the first place. A picture of the Hadean earth is crucial to understanding the wild natural laboratory in which life contrived to be born. Just as the formation of our home world is an event in space, we'll come to see how the emergence of life here is essentially a cosmic event.

The study of the early geology is all rock-hard science, but the evidence is often both literally and metaphorically thin on the ground. It requires geological detective work with clues dotted around the planet, and off the world, too. Geology gives us clues to how the earth formed from the bits and pieces of matter floating in space around the sun. Yet it also begins to describe the world that will evolve into the host of the only living things that we know of. While our lives are built on the stability of the earth, we also are keenly aware that our planet is sporadically violently active. The solid surface (including the seafloor) is made up of seven or eight leviathan continental plates and a collection of smaller ones. These all float on the slowly flowing but solid rock of the earth's mantle, itself encapsulating the molten core. The plates that form the crust are in constant flux, grinding and unhurriedly jiggling together. Some, such as the Pacific and North American plates, grind against each other, forcing up new land inch by inch. The subcontinent that is now India was once an island, and crunched into mainland Asia in a process that began around seventy million years ago, inching forward and crumpling up the land into the mountains of the Himalayas. These mountains will continue to grow at a rate of a few millimeters every year as the Indo-Australian plate continues to muscle its way into mainland Asia. Others plates are pulling the earth apart at the seams. The United States' colonial expansion continues westward every day, as the coast of Hawaii grows new land at a rate of many feet per year, with molten rock gurgling up above sea level and solidifying. Earthquakes shake the land and the seabed, dislodging mountainous blocks of water that become tsunamis, such as the one that wrecked the east coast of Japan in 2011. Still, these events are currently anomalies, though they remind us that our planet is alive not just with cellular life but also with slowly flowing rock. Yet for the most part our earth is reassuringly stable.

Not so in the past. The birth of planets is a process of summoning order from the chaos of the early solar system. Violence ensues. The sun, the star at the center of our planetary system, formed around 4.6 billion years ago as a colossal cloud of free-floating molecules collapsed under its own gravity and condensed into the huge nuclear fusion reactor that continues to heat the earth today. In the immediate aftermath, the sun sat in the center of a solar nebula, a flat disk of detritus left over by its formation but held there by its own gravity. Over the course of the next few million years, this matter, mostly dust and gas, began to stick together in clumps. At first these were the size of medium-size concert halls, but over hundreds of centuries, lumps collided and stuck together in a process called accretion. Closer to the hot sun, the temperature is higher, which makes it harder for gasses to condense and accrete. It's for this reason that the inner four planets of the solar system—Mercury, Venus, Earth, and Mars—are terrestrial, made of rock, whereas the outer four—Jupiter, Saturn, Uranus, and Neptune—are gaseous.
¹

It's impossible to describe a whole planet in simple terms, as we know very well how our home is not a unified land mass. Much of the surface of modern Earth is solid; more is ocean. Of the grounded parts, there are extremes of temperature and geography, snow, deserts, marshes, forests, plains, mountains, and so on. The rock beneath your feet is likely to be very different from the rock underneath a reader's feet on the other side of the planet. Similarly, it's virtually impossible to describe simply what Earth looked like in the Hadean. The evidence is vanishingly sparse, and as a result this period unhelpfully gets called the Cryptic era. Yet we can extract meaningful models in broad terms. We know that immediately after accretion, the planet was probably largely molten. Contrary to previous thinking, however, this period of molten Earth is now thought to have been short-lived. There are no surviving rocks from that era, so in the past we had assumed that there were no rocks. Yet the absence of evidence is not the same as evidence of absence.

Just as we look in rocks for the traces of recent life—the fossilized forms of dinosaurs, or even cells, for example—our planet's history before life is embedded in geology. The oldest dated matter on Earth comes not from rocks, but in the form of zircon crystals, an abundant mineral found all over the world but probably best known as a cheap substitute for diamonds in jewelry.

Zircons have two convenient characteristics. The first is that they can withstand metamorphosis—the brutal churning of rock over very long periods of time. The second is that the atoms of zircons are naturally arranged in a neat cubic structure. This molecular box can trap atoms of uranium inside them, as few as ten parts per million. A small proportion of uranium, like many elements, is radioactive, and over time will decompose into lead. Due to the precise nature of their crystal structure, when zircons form they can include uranium, but exclude lead atoms. Once imprisoned in this cage, radioactive uranium slowly mutates into lead over the course of millions of years, and because this decomposition happens at a fixed rate (what we call a half-life), the moment a zircon crystal forms, it sets a clock at zero. Lead found inside zircons must have begun its life as imprisoned uranium, so by quantifying it we can date the origin of that crystal with 99 percent accuracy. Out in Jack Hills in Western Australia, zircon crystals have been found that trapped their uranium 4.404 billion years ago.

Dating is not all we can tell from the constituents of cheap jewelry. We can also infer from those same crystals that their formation was part of a solidifying process, the development of a crust. This means that, although there are no rocks from that period, we can know that there was land in the Hadean. We can also determine what other ingredients were present. The Jack Hills zircons also harbor a particular type of radioactive oxygen whose presence looks like that found in modern crystals, which are formed as the earth's crust gets sucked down beneath the ocean floor. The presence of this oxygen suggests that from as little as a hundred million years after the initial molding of the earth, water was present. On Earth, there is no life without it, though this water was likely to have been extremely acidic.
²

Therefore, the early Cryptic days on Earth might not have been quite the hellish inferno of endless seas of molten lava. Within a mere hundred million years, the earth had a solid surface and oceans. Sounds pleasant enough, but let's not paint such a picturesque portrait. The earlier theories of a molten Hadean Earth were based on a solid observation: we can't find any rocks from that era. If there was a rocky surface, what in hell's name happened to it?

It turned out that we were looking in the wrong place. In fact, we were looking on the wrong heavenly body altogether. To address the question of what happened to the early earth, we sent twelve men to the moon. The moon itself was born out of the most destructive impact that Earth has yet suffered. Somewhere between fifty and one hundred million years after the formation of our solar system, the earth had its worst day. It was struck by Theia—a terrifically and inappropriately pretty name for such a harbinger of doom. Current theories suggest that Theia was a rock the size of Mars. It blasted enough matter from the embryonic earth into space that it reformed as our closest celestial neighbor: the moon. The impact was devastating, enough to rip the first atmosphere from the planet. Theia's glancing blow may be what shifted the earth's axis from vertical to its off-kilter stance of 23.5°. This lean is what causes the seasons, as the distance from the sun varies with the axial tilt of the earth.

But it was what came after the formation of the moon that concerns us. It's the characteristic pock-marked lunar visage that gave us clues to the state of the Hadean earth. Between 1969 and 1972 the Apollo program of the U.S. National Aeronautics and Space Administration (NASA) landed six missions and twelve explorers on the moon, beginning with Neil Armstrong's famous first small step. Over those missions, astronauts collected around half a ton of rock and brought it back home for analysis. The last man to walk on the moon, Commander Gene Cernan in Apollo 17, is quoted as saying that “we went to explore the Moon, and in fact discovered the Earth.” There is a great truth in that quotation, as it was in the subsequent analysis of lunar rocks that we were to discover the nature of the earth's formative years. Unlike the earth, the moon has no atmosphere or winds, and no shifting geology, so the craters formed by meteorite impacts are left undisturbed alongside the footprints of the Apollo pioneers. What that means is that we have a record of meteorite activity in the local solar system, footprints unscathed by the winds, seas, and tectonic grind of the earth. Geologists dated lunar rocks bearing the hallmarks of meteor strikes. These are called impact melt rocks; they all occurred in a precise window of time, between 4.1 and 3.8 billion years ago. We can deduce that this was a period of intense local meteoric activity, and by inference, that the earth also suffered this hellish pummeling from above. The young solar system was crowded with debris and leftovers from its birth, and for a period of three hundred million years, until the end of the Hadean, we got the full brunt of it. This period is called the Late Heavy Bombardment—so named because it was mercifully the last time Earth would suffer such a battering.

How heavy is heavy? Meteors fall from the skies all the time. Almost all, thank goodness, are tiny, burn out, then fade away in the atmosphere as shooting stars. Even big ones, such as the dramatic fireball that lit up the sky above Chelyabinsk, Russia, in February 2013, mostly disintegrate in the atmosphere. Occasionally, a big one hits, whereupon they become meteorites, such as one that fell on the small Australian town of Murchison in September 1969, just a few weeks after Apollo 11 returned Armstrong, Buzz Aldrin, and command module pilot Michael Collins to Earth. That one weighed more than two hundred pounds, and carried a payload of interest to this story, as we will find out later.

If you're prone to making a wish when you see a shooting star, why not hope that we don't get to witness anything near the size of the best-known meteorite. Sixty-five million years ago a five- or six-mile-wide rock smashed into an area of what we now call Chicxulub in Mexico. The crater is now hidden, mostly beneath the sea, but its 110-mile-wide shadow remains and was spotted by oil prospectors in the 1970s. In the ground and seabed, there is a broken but detectable ectopic circle of tiny glass beads that were forged from molten rock during the heat of impact. And from space we can see the same circle in minuscule gravity distortions only measurable from precision equipment in satellite orbit. There has not been an impact anywhere near that magnitude since then, and be thankful for that. The Chicxulub meteorite was the trigger that extinguished the reign of the dinosaurs and paved the way for small mammals to evolve, eventually into us. An impact of that order means that it's very likely that the meteorite instantaneously wiped out many millions of creatures, with an expanding circumference of mile-high megatsunamis racing away from the impact site, leveling the land like a wave crashing on a beach. With that there also would have been a fireball hot enough to melt sand and rock into those telltale glass beads. But the full impact of the meteorite would have taken thousands of years, a dust cloud thrown up that blotted out the sun. The Chicxulub impact irreversibly changed the earth's system, wiping out once-dominant life-forms. And yet compared to what was happening on the infant planet, the place on which life began, Chicxulub was a drop in the ocean.

Scientists have estimated that during the Late Heavy Bombardment something like fifteen astronomical rocks over one hundred miles wide, twenty times the size of Chicxulub, bruised our world. Of these maybe four of them were two hundred miles wide. For three hundred million years, giant rocks rained down from the sky, some as big as decent-size islands. The power of any one of the tens of thousands of impacts during this time would make the most destructive nuclear bomb seem like a firecracker. Global environmental destruction would have occurred at least every few centuries. Any potential surface habitat for living organisms would have been destroyed over and over and over again. The relentless pounding the planet suffered during the Late Heavy Bombardment was enough to boil the oceans and vaporize the land.

And then it significantly calmed down. The meteoric blitz of the Hadean ended around 3.8 billion years ago, leaving a frazzled earth, still tempestuous and rough, but at least not barraged from the skies. The sun was dimmer than today, probably less than three-quarters of its current strength. Because of that, the earth cooled quickly, and water from volcanoes and comets condensed into oceans that covered the planet.