Origins: Fourteen Billion Years of Cosmic Evolution (21 page)

We do not know whether life already existed 4 billion years ago, having survived the early impact storm, or whether life arose on Earth only after relative tranquility began. These two alternatives include the possibility that incoming objects seeded our planet with life, either during the era of bombardment or soon afterward. If life began and died out repeatedly while chaos rained down from the skies, the processes by which life originated seem robust, so that we might reasonably expect them to have occurred again and again on other worlds similar to our own. If, on the other hand, life arose on Earth only once, either as homegrown life or as the result of cosmic seeding, its origin may have occurred here by luck.

In either case, the crucial question of how life actually began on Earth, either once or many times over, has no good answer, though speculation on the subject has acquired a long and intriguing history. Great rewards lie in store for those who can resolve this mystery. From Adam’s rib to Dr. Frankenstein’s monster, humans have answered the question by invoking a mysterious
élan vital
that imbues otherwise inanimate matter with life.

Scientists seek to probe more deeply, with laboratory experiments and examinations of the fossil record that attempt to establish the height of the barrier between inanimate and animate matter, and to find how nature breached this dike. Early scientific discussions about the origin of life imagined the interaction of simple molecules, concentrated in pools or tide ponds, to create more complex ones. In 1871, a dozen years after the publication of Charles Darwin’s marvelous book
The Origin of Species
, in which he speculated that “probably all of the organic beings which have ever lived on this Earth have descended from some one primordial form,” Darwin wrote to his friend Joseph Hooker that

It is often said that all the conditions for the first production of a living organism are now present, which could ever have been present. But if (and oh! what a big if!) we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, &c., present, that a proteine [
sic
] compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly absorbed, which would not have been the case before living creatures were found.

In other words, when Earth was ripe for life, the basic compounds necessary for metabolism might have existed in surplus, with nothing in existence to eat them (and, as we have discussed, no oxygen to combine with them and spoil their chances to serve as food).

From a scientific perspective, nothing succeeds like experiments that can be compared with reality. In 1953, seeking to test Darwin’s conception of the origin of life in ponds or tide pools, Stanley Miller, who was then a U.S. graduate student working at the University of Chicago with the Nobel laureate Harold Urey, performed a famous experiment that duplicated the conditions within a highly simplified and hypothetical pool of water on the early Earth. Miller and Urey partly filled a laboratory flask with water and topped the water with a gaseous mixture of water vapor, hydrogen, ammonia, and methane. They heated the flask from below, vaporizing some of the contents and driving them along a glass tube into another flask, where an electrical discharge simulated the effect of lightning. From there the mixture returned to the original flask, completing a cycle that would be repeated over and over during a few days, rather than a few thousand years. After this entirely modest time interval, Miller and Urey found the water in the lower flask to be rich in “organic gunk,” a compound of numerous complex molecules, including different types of sugar, as well as two of the simplest amino acids, alanine and guanine.

Since protein molecules consist of twenty types of amino acids arranged into different structural forms, the Miller-Urey experiment takes us, in a remarkably brief time, a significant part of the way from the simplest molecules to the amino-acid molecules that form the building blocks of living organisms. The Miller-Urey experiment also made some of the modestly complex molecules called nucleotides, which provide the key structural element for DNA, the giant molecule that carries instructions for forming new copies of an organism. Even so, a long path remains before life emerges from experimental laboratories. An enormously significant gap, so far unbridged by human experiment or invention, separates the formation of amino acids—even if our experiments produced all twenty of them, which they do not—and the creation of life. Amino-acid molecules have also been found in some of the oldest and least altered meteorites, believed to have remained unchanged for nearly the entire 4.6-billion-year history of the solar system. This supports the general conclusion that natural processes can make amino acids in many different situations. A balanced view of the experimental results finds nothing totally surprising: The simpler molecules found in living organisms form quickly in many situations, but life does not. The key question still remains: How does a collection of molecules, even one primed for life to appear, ever generate life itself?

Since the early Earth had not weeks but many million years in which to bring forth life, the Miller-Urey experimental results seemed to support the tide-pool model for life’s beginnings. Today, however, most scientists who seek to explain life’s origin consider the experiment to have been significantly limited by its techniques. Their shift in attitude arose not from doubting the test’s results, but rather from recognizing a potential flaw in the hypotheses underlying the experiment. To understand this flaw, we must consider what modern biology has demonstrated about the oldest forms of life.

Evolutionary biology now
relies on careful study of the similarities and differences between living creatures in their molecules of DNA and RNA, which carry the information that tells an organism how to function and how to reproduce. Careful comparison of these relatively enormous and complex molecules has allowed biologists, among whom the great pioneer has been Carl Woese, to create an evolutionary tree of life that records the “evolutionary distances” between various life forms, as determined by the degrees to which these life forms have nonidentical DNA and RNA.

The tree of life consists of three great branches, Archaea, Bacteria, and Eucarya, that replace the biological “kingdoms” formerly believed to be fundamental. The Eucarya includes every organism whose individual cells have a well-defined center or nucleus that contains the genetic material governing the cells’ reproduction. This characteristic makes Eucarya more complex than the other two types, and indeed every form of life familiar to the non-expert belongs to this branch. We may reasonably conclude that Eucarya arose later than Archaea or Bacteria. And because Bacteria lie farther from the origin of the tree of life than the Archaea do—for the simple reason that their DNA and RNA has changed more—the Archaea, as their name implies, almost certainly represent the oldest forms of life. Now comes a shocker: Unlike the Bacteria and Eucarya, the Archaea consist mainly of “extremophiles,” organisms that love to live, and live to love, in what we now call extreme conditions: temperatures near or above the boiling point of water, high acidity, or other situations that would kill other forms of life. (Of course, if the extremophiles had their own biologists, they would classify themselves as normal and any life that thrives at room temperature as an extremophile.) Modern research into the tree of life tends to suggest that life began with the extremophiles, and only later evolved into forms of life that benefit from what we call normal conditions.

In that case, Darwin’s “warm little pond,” as well as the tide pools duplicated in the Miller-Urey experiment, would evaporate into the mist of rejected hypotheses. Gone would be the relatively mild cycles of drying and wetting. Instead, those who seek to find the places where life may have begun would have to look to locales where extremely hot water, possibly laden with acids, surges from Earth.

The past few decades have allowed oceanographers to discover just such places, along with the strange forms of life they support. In 1977, two oceanographers piloting a deep sea submersible vehicle discovered the first deep sea vents, a mile and a half beneath the calm surface of the Pacific Ocean near the Galápagos Islands. At these vents, Earth’s crust behaves locally like a household cooker, generating high pressure inside a heavy-duty pot with a lockable lid and heating water beyond its ordinary boiling temperature without letting it reach an actual boil. As the lid partially lifts, the pressurized, superheated water spews out from below Earth’s crust into the cold ocean basins.

The superheated seawater that emerges from these vents carries dissolved minerals that quickly collect and solidify to surround the vents with giant, porous rock chimneys, hottest in their cores and coolest at the edges that make direct contact with seawater. Across this temperature gradient live countless life forms that have never seen the Sun and care nothing for solar heating, though they do require the oxygen dissolved in seawater, which in turn comes from the existence of solar-driven life near the surface. These hardy bugs live on geothermal energy, which combines heat left over from Earth’s formation with heat continuously produced by the radioactive decay of unstable isotopes such as aluminum-26, which lasts for millions of years, and potassium-40, which lasts for billions.

Near these vents, far below the depths to which any sunlight can penetrate, the oceanographers found tube worms as long as a man, thriving amidst large colonies of bacteria and other small creatures. Instead of drawing their energy from sunlight, as plants do with photosynthesis, life near deep sea vents relies on “chemosynthesis,” the production of energy by chemical reactions, which in turn depend on geothermal heating.

How does this chemosynthesis occur? The hot water gushing from the deep sea vents emerges laden with hydrogen-sulfur and hydrogen-iron compounds. Bacteria near the vents combine these molecules with the hydrogen and oxygen atoms in water molecules, and with the carbon and oxygen atoms of the carbon dioxide molecules dissolved in sea water. These reactions form larger molecules—carbohydrates—from carbon, oxygen, and hydrogen atoms. Thus the bacteria near deep sea vents mimic the activities of their cousins far above, which likewise make carbohydrates from carbon, oxygen, and hydrogen. One set of microorganisms draws the energy to make carbohydrates from sunlight, and the other from chemical reactions at the ocean floors. Close by the deep sea vents, other organisms consume the carbohydrate-making bacteria, profiting from their energy in the same way that animals eat plants, or eat plant-eating animals.

In the chemical reactions near deep sea vents, however, more goes on than the production of carbohydrate molecules. The iron and sulfur atoms, which are not included in the carbohydrate molecule, combine to make compounds of their own, most notably crystals of iron pyrite, familiarly called “fool’s gold,” known to the ancient Greeks as “fire stone” because a good blow from another rock will strike sparks from it. Iron pyrite, the most abundant of all the sulfur-bearing minerals found on Earth, might have played a crucial role in the origin of life by encouraging the formation of carbohydratelike molecules. This hypothesis sprang from the mind of a German patent attorney and amateur biologist, Günter Wächtershäuser, whose profession hardly excludes him from biological speculation, any more than Einstein’s work as a patent attorney barred him from insights into physics. (To be sure, Einstein had an advanced degree in physics, while Wächtershäuser’s biology and chemistry are mainly self-taught.)

In 1994, Wächtershäuser proposed that the surfaces of iron pyrite crystals, formed naturally by combining iron and sulfur that surged from deep sea vents early in Earth’s history, would have offered natural sites where carbon-rich molecules could accumulate, acquiring new carbon atoms from the material ejected by the nearby vents. Like those who hypothesize that life began in ponds or tide pools, Wächtershäuser has no clear way to pass from the building blocks to living creatures. Nevertheless, with his emphasis on the high-temperature origin of life, he may prove to be on the right track, as he firmly believes. Referring to the highly ordered structure of iron pyrite crystals, on whose surfaces the first complex molecules for life might have formed, Wächtershäuser has confronted his critics at scientific conferences with the striking statement that “Some say that the origin of life brings order out of chaos—but I say, ‘order out of order out of order!
’
” Delivered with German brio, this claim acquires a certain resonance, though only time can tell how accurate it may be.

So which basic model for life’s origin is more likely to prove correct—tide pools at the ocean’s edge, or superheated vents on the ocean floors? For now, the betting is about even. Experts on the origin of life have challenged the assertion that life’s oldest forms lived at high temperatures, because current methods for placing organisms at different points along the branches of the tree of life remain the subject of debate. In addition, computer programs that trace out how many compounds of different types existed in ancient RNA molecules, the close cousins of DNA that apparently preceded DNA in life’s history, suggest that the compounds favored by high temperatures appeared only after life had undergone some relatively low-temperature history.

Thus the outcome of our finest research, as so often occurs in science, proves unsettling to those who seek certainty. Although we can state approximately when life began on Earth, we don’t know where or how this marvelous event occurred. Paleobiologists have recently given the elusive ancestor of all Earthlife the name LUCA, for the last universal common ancestor. (See how firmly these scientists’ minds have remained fixed to our planet: they should call life’s progenitor LECA, for the last Earthly common ancestor.) For now, naming this ancestor—a set of primitive organisms that all shared the same genes—mainly underscores the distance that we still must travel before we can pierce the veil that separates life’s origin from our understanding.