How Did Life Start?

Earth could have been just another empty chunk of rock. But something happened here, and it may have taken place on a stage of clay.

By Peter Radetsky|Sunday, November 01, 1992
On a calm, clear day in February 1977, Jack Corliss and two fellow explorers wedged themselves into the tiny, cramped cabin of the research submarine Alvin, said good-bye to the two support ships at the surface, and began a long descent into darkness. About 90 minutes later Alvin was gliding along the seafloor a mile and a half below the surface of the Pacific, and Corliss, a burly Oregon State University marine geologist, was peering out the porthole, searching for a phenomenon that had been suspected but never seen: submarine hot springs.

Searchlights blazing, Alvin cruised through black water above the Galápagos Rift, an undersea volcanic ridge along the equator 200 miles west of Ecuador. It was in just such a place, Corliss and the others surmised, that these so-called hydrothermal vents would be found--if they existed. Suddenly, just ahead, they spotted a huge cluster of clams. Tha was odd. What should so many large clams, fully a foot long, be doing so far below their sources of food?

Alvin floated nearer and Corliss pressed closer to the porthole. I saw a veil of shimmering water, he recalls. It reminded me of the way air wavers above hot pavement. Alvin extended its mechanical arm, in its grip a thermometer; 44 degrees. Not particularly warm by terrestrial standards, but at the ocean floor, where the climate is ordinarily close to freezing, this was bathtub temperature. The crew of the sub broke out in a cheer. The glistening veil was actually a sheet of water rising from the rocky floor. Corliss and his team had found their submarine hydrothermal vent.

As it turned out, Corliss’s team found four more hot springs in the Galápagos Rift, and since that initial exploration numerous other vents have been detected elsewhere on the ocean bottom. In these vents, seawater percolates through a maze of mineral-lined fractures to encounter magma deep in Earth’s crust. Heated by the magma, the water rushes upward to escape in the kind of shimmering veil Corliss noticed or in turbulent black smokers. The team also encountered strange forms of life clustered around the vents--giant worms and blind white crabs scurrying over bulging pillows of lava rock. This vast submerged world had a bizarre, elemental quality to it, as though it were a holdover from primordial Earth.

It was totally amazing, Corliss says. I began to wonder what all this might mean, and this sort of naive idea came to me. Could hydrothermal vents be the site of the origin of life?

Questions about life’s origin are as old as Genesis and as young as each new morning. For scientists, there are no definitive answers. But if no one has yet pinned down the secret, it hasn’t been for lack of trying. Those investigating the origin of life are a rambunctious, scrappy group, in which no two people see things quite the same way; and it doesn’t help that it’s awfully tough to prove or disprove any particular contention. After all, how can you really know what happened when Earth formed 4.6 billion years ago? Two things these scientists can agree on, however, are that the first kinds of life, whatever they were, must have been able to reproduce themselves and must have carried information.

Self-replication is the cornerstone of any definition of life. Birds do it, bees do it; certainly our evolutionary forebear, no matter how simple an organism, must have been able to do it. To sustain life, information about oneself must be passed from one generation to the next. It is that information, in the form of inheritable characteristics, that gives life continuity. It is the accidental altering of those characteristics over time that makes evolution possible. We do this with genes. But what is not at all clear is how our ancient ancestors did it, or what form those ancestors took. Evolutionary biologists have traced our family tree to bacteria, one-celled organisms that have been found in rock formations 3.5 billion years old. But even these primitive creatures were already quite sophisticated. They had genes of DNA and RNA and were made of protein, lipids, and other ingredients. Something simpler must have preceded them.

A hint as to what that may have been came in 1981, when Thomas Cech of the University of Colorado discovered a kind of RNA that functioned as an enzyme, partially triggering its own replication. Until then, replication had been thought possible only through a collaboration among DNA, the storehouse of genetic information, RNA, the mobile dispenser of that information, and protein, which exclusively makes up the enzymes that catalyze the process. Now Cech had shown that RNA could be an enzyme and therefore could once have taken care of the whole business by itself. The news galvanized scientists, who enthusiastically painted a picture of an ancient world inhabited by naked RNA genes, which went on their way merrily self-replicating until DNA and protein evolved to assist in the procedure. Thus ensued the development of living cells and the very bacteria we claim as our own ancestors.

But while this proposed RNA world was certainly closer to the origin of life, it clearly wasn’t the beginning. Although much simpler than bacteria, RNA is still a complicated piece of molecular machinery, containing more than 30 atoms connected in an intricate, interlocking fashion. It couldn’t have sprung wholly formed into the primordial landscape. Something preceded it. That something must have been the simple carbon-based molecules that underlie all life--organic compounds.

What were those first organic compounds? And how did they form? The questions bedevil origin-of-life researchers. Over the years they have come up with a host of imaginative and intensely debated possibilities. Perhaps the most influential first surfaced four decades ago, when in a dramatic experiment a University of Chicago graduate student named Stanley Miller simulated the creation of life in a laboratory.

Today Miller is a renowned and feisty 62-year-old professor of chemistry at the University of California at San Diego. Back in Chicago in 1953, however, he little knew what he was getting himself into. My research director, Harold Urey, gave a talk about the origin of the Earth and the solar system, he recalls. He said that if you have an atmosphere like that of early Earth you ought to be able to make organic compounds easily. I said, ‘I want to do it,’ but he tried to talk me out of it. It was a very risky experiment, and it was his responsibility to make sure that I had an acceptable thesis within a couple of years. I said that I’d give it a try for six months to a year, and if that didn’t work out, I’d do something conventional.

Urey agreed, and the two set to work. They designed a glass apparatus consisting essentially of two flasks connected within a closed circle of glass tubing. Miller pumped into the larger flask the gases thought to be present in the early atmosphere: hydrogen, methane, ammonia, and water vapor. The smaller flask he partially filled with water--it represented the primitive ocean. He then shot bolts of electric current through the gaseous mixture to simulate primordial lightning storms. For a week the electricity sparked, while Miller sat back to see what would happen.

It didn’t take long to see I had it, he says. The organics just poured out. It was very exciting.

As the scientists watched, fluids rained out of the gas chamber, turning the clear water in the ocean pink, then deep red, then yellow-brown. When Miller analyzed the brew, he found that it contained amino acids, the building blocks of protein. The lightning had reorganized the molecules in the atmosphere to produce organic compounds. It looked as though making organics was easier than anyone had suspected. Perhaps the origin of life was simplicity itself, nothing more than the routine consequence of basic conditions on early Earth.

People were stunned. Articles appeared in major newspapers across the country, prompting predictions that, like Dr. Frankenstein, researchers would soon concoct living organisms in their labs. A Gallup poll asked people whether they thought it possible to create life in a test tube. (Seventy-eight percent answered--perhaps hopefully--no.) And the simple experiment (It’s so easy to do--high school students now use it to win their science fairs, Miller says) stimulated a rush of studies, with the result that a number of other organic compounds, including adenine and guanine, two of the ingredients of RNA and DNA, were produced by similar procedures.

Thus emerged the picture that has dominated origin-of-life scenarios. Some 4 billion years ago, lightning (or another energy source, like ultraviolet light or heat) stimulated a hydrogen-rich atmosphere to produce organic compounds, which then rained down into the primitive ocean or other suitable bodies of water such as lakes, rivers, or even a warm little pond, as Charles Darwin once suggested. Once there, these simple compounds, or monomers, combined with one another to produce more complicated organics, or polymers, which gradually grew even more complex until they coalesced into the beginnings of self-replicating RNA. With that came the RNA world and ultimately the evolution into cells and the early bacterial ancestors of life.

The picture is powerful and appealing, but not all origin-of-life researchers are convinced. Even Miller throws up his hands at certain aspects of it. The first step, making the monomers, that’s easy. We understand it pretty well. But then you have to make the first self- replicating polymers. That’s very easy, he says, the sarcasm fairly dripping. Just like it’s easy to make money in the stock market--all you have to do is buy low and sell high. He laughs. Nobody knows how it’s done.

Some would say the statement applies as well to the first easy step, the creation of simple organic compounds. For example, what if the primordial atmosphere wasn’t anything like the one Miller and Urey imagined? Would it be so easy to produce organics then?

The Miller-Urey experiment was a strong foundation because it was consistent with theories at the time, says geochemist Everett Shock of Washington University in St. Louis. The problem is that subsequent research has swept away a lot of those ideas. The Miller-Urey atmosphere contained a lot of hydrogen. But now the atmosphere of the early Earth is thought to have been more oxidized.

That makes Miller’s scenario less probable, because it’s a lot harder to make organic molecules in the presence of oxygen. A hydrogen-rich atmosphere is relatively unstable. When zapped by lightning or other sources of energy, molecules in that environment readily tumble together into organic compounds. Not so in a heavily oxidized atmosphere. While an infusion of energy may cause a few simple organics to form, for the most part the results are inorganic gases like carbon monoxide and nitrogen oxide. These are the constituents of smog, says Shock. So basically what you’re getting is a lot of air pollution.

That’s worried people for the last 10 to 15 years, says Christopher Chyba, a planetary scientist based at NASA’s Ames Research Center, south of San Francisco. There seems to be a contradiction between the fact that we’re here and evidence that early Earth was not very hospitable to the formation of organics. How do you resolve the dilemma? One way is to take advantage of the fact that asteroids and especially comets are rich in organic compounds. Maybe there was a way that those organics reached early Earth intact.

In other words, maybe the beginnings of life came from interstellar space. The notion is not as farfetched as it may seem. If you go to the moon, says Chyba, or look at the craters on Mars or Mercury, what you see is that the whole inner solar system was being subjected to a very intense bombardment from space at that time. You can infer that the same was true for Earth. And in fact, in the early nineteenth century, organic molecules were found in a meteorite, although some people suspected that it had simply acquired earthly organics in the thousands of years since it had landed. In 1969, however, such skepticism was dispelled once and for all when a meteorite fell in Murchison, Australia. A prompt examination revealed a large number of amino acids, components of RNA and DNA, and other organic compounds.

More recently, says Chyba, in 1986, European and Soviet spacecraft flew by Halley’s comet. People had strongly suspected that comets were rich in organics, and that was absolutely borne out by the observations made by the spacecraft. And whereas the fraction of organics in meteorites is no more than one-twentieth of their mass, the flybys found Halley to be fully one-third organic compounds.

However, says Chyba, it’s likely that most organics aboard meteorites and comets never made it to Earth. At these velocities, at least 10 to 15 miles per second, the temperatures you reach on impact are so high that you end up frying just about everything. And those organics that survived would probably have been too few and too scattered to evolve into life.

But interplanetary dust particles (IDPs for short) are another matter. In contrast to their larger cousins, these particles, tiny specks no larger than .004 inch across, routinely reach Earth. They get slowed way up in the atmosphere, says Chyba. Then they remain floating around for months, even years, before they come down. NASA samples IDPs directly in the atmosphere with modified U2 spy planes fitted with adhesive collectors on the wings. What researchers have found is that IDPs also contain organic material--although only about 10 percent worth. Perhaps, then, dust seeded early Earth with the stuff of life.

Not surprisingly, not everyone thinks so. If you have to depend on such low amounts of organic material as that found in IDPs, says Miller, then from the standpoint of making life on Earth you’re bankrupt. You’re in Chapter Eleven. Because you just don’t have enough. His point rests on simple common sense: the greater the amount of organics, the greater the possibility that they would have interacted with one another. Too few organics, and odds are that they could never have gotten together to begin the process of life in the first place. Organics from outer space, Miller scoffs. That’s garbage, it really is.

There’s another possible drawback to the notion of an extraterrestrial origin of life, acknowledged by Chyba himself. The surface of early Earth would have been a very hostile place, he says. The biggest impacts would have generated enough heat to evaporate the entire ocean, probably several times. And leaving the biggest impacts aside, the upper tens of meters of the oceans would routinely have been evaporated and the surface of Earth sterilized by these giant impacts.

Where, then, in such a nightmarish environment, could emerging life have been sufficiently protected? The only safe place--safe, at least, after the last total evaporations were over and done with--would have been in the deep ocean. And that, says Jack Corliss, is where hydrothermal vents come into the picture.

Since his discovery of the Galápagos hot springs, Corliss, who now works at NASA’s Goddard Space Flight Center, in Greenbelt, Maryland, and a growing number of his colleagues have been promoting the notion that hydrothermal vents were the birthplace of life. The thing about the hot springs, Corliss says, is that they provide a nice, safe, continuous process by which you can go from very simple molecules all the way to living cells and primitive bacteria.

The crux is the word continuous. For besides providing safe harbor for the development of life, vents offer a natural temperature gradient. The vents have it all, from the cracking front in the interior, where temperatures reach 1300 degrees and cool water filtering down from above cracks the superheated rock, to the 40-degree seafloor. Whatever temperature you want, says Corliss, you have your choice. And any chemist will tell you that where you find a temperature gradient is where you’ll find chemical reactions--maybe even the ones that began life.

The reactions Corliss envisions began at the cracking front, half a mile deep in the planet’s crust, where seawater encountered hot magma. There, in this seething caldron, elements like carbon, oxygen, hydrogen, nitrogen, and sulfur interacted to form new, organic compounds. Just as in the Miller-Urey experiments, says Corliss, if you heat simple molecules to high temperature, you can make organic compounds.

But heat is a double-edged sword. It facilitates chemical reactions, but it can also destroy the products of those reactions. If exposed to high heat for too long, organic compounds decompose. It’s a very simple argument: if you keep a roast too long in an oven that’s too hot, it’s going to get charred, says Miller, who has little use for this scenario either. The vent hypothesis is a real loser. I don’t understand why we even have to discuss it, he says, his voice rising to an exasperated falsetto.

Corliss, however, thinks he has an ace in the hole: a vent’s temperature gradient. He thinks it likely that the circulating seawater cooled the newly formed compounds almost immediately. If you quenched them very rapidly, you could preserve them, he says. Then they rose and mixed and worked their way up in the hot springs, through this huge complex of fractures, cooling as they went.

Finally the organic compounds were deposited onto the clay minerals lining the mouth of a vent. And there they stayed. Rather than simply emerging and dissipating into the vast ocean where they might never encounter another organic molecule, the compounds accumulated on the clay surface. There, in a concentrated colony, they were able to interact with one another and with the endless supply of new compounds rising in the hot springs, until over time the first stirrings of primitive life emerged.

It’s the perfect environment, Corliss says. You couldn’t design it better. With the clay minerals lining the fractures in the upper part of the hot springs, the organic material has something to stick to. It’s an ideal way to concentrate the organic material made at the cracking front. Now it can build up and evolve.

The prospect is bolstered by the likelihood that in the turbulent early Earth there were many more hydrothermal vents than today. Presumably it was hotter within primitive Earth, so there was even more hydrothermal circulation to cool things down, says Everett Shock. And, therefore, more safe havens in which life might have evolved.

Furthermore, the clay lining the vents could have been far more than just a convenient medium on which organic compounds could evolve. Chemist A. Graham Cairns-Smith of the University of Glasgow sees clay as a solution to the mystery of how simple organics made the leap all the way to self-replicating genetic material. In fact, Cairns-Smith sees clay itself as the first genetic substance, what he calls a crystal gene.

Clay minerals, he explains, are crystals formed from the weathering of rocks by water. And clay, like any crystal, grows by itself-- think of crystals of frost expanding on a windowpane. Crystals, in other words, self-replicate. So if self-replication is the key, life did not start with organic molecules. Life started with crystals. That is, it started with clay.

It’s not a new idea--the Bible proposed it long ago, in a slightly different form. But in Cairns-Smith’s hands the notion takes on an evocative modern flavor. With clay, I’m advocating an earlier genetic material that is fundamentally different from DNA and RNA, he says. You needed a previous stage of evolution in which the present means of evolution was itself evolving.

Again, picture a hydrothermal vent, with organic compounds settling on clay crystals lining the fissures. But this clay was no inert surface upon which organic reactions happened to take place--it was living, growing, even assisting those reactions. As the crystals grew, they developed nooks and crannies that were a perfect fit for the organic molecules rising in the swell of water. As snugly as pegs settling into holes in a pegboard, these molecules made themselves at home in this surface. Once there, they reacted with other molecules comfortably ensconced in niches next door. Because the positioning was so precise, similar reactions could occur over and over again. The crystals, then, actually catalyzed the formation of new organic compounds.

In time the organics evolved into RNA, which, with its strong interlocking structure, returned the favor, helping out the growing clay crystals. I don’t think RNA’s genetic function came first, Cairns-Smith says. My guess is that at first it had a structural function. It helped stick the crystals together. Finally, as it became a self-replicating molecule, RNA jettisoned its clay scaffolding. And early life struck out on its own.

This scenario, attractive as it may seem, is--like so many others--too farfetched for Miller. It’s not that I don’t want to entertain new ideas--that’s fine, he says. The question is, does this chemistry work? Actually work in the lab? Either it does or it doesn’t. His point is well taken. Whatever else may be said about Miller’s ideas, his experiments worked. Talk, even informed talk, is cheap. If they’re to have an impact comparable to Miller’s, these champions of crystals and vents and interstellar particles must demonstrate their scenarios.

But how? You can’t try to make early life at existing hot springs--they’re already replete with bacteria and other life-forms, so the environment just can’t be the same as it was on the primordial planet. And re-creating an ancient hydrothermal vent in the lab is a mind-boggling prospect. Still, vent researchers are busily conducting experiments designed to do just that. Elsewhere, Chyba is collaborating with Carl Sagan and others in an attempt to nail down the possible link between extraterrestrial objects and the origin of life. And Cairns-Smith is investigating the chemical relationships between minerals and organic compounds.

But while he recognizes the importance of experimental proof, Cairns-Smith cheerfully acknowledges that he may never come up with any. I’m hoping that people with new techniques or people who make the appropriate discoveries will phone me up and say, ‘By the way . . .’ The origin of life depended on all sorts of accidental circumstances. Proving how it happened will take another piece of luck.
Comment on this article
ADVERTISEMENT

Discover's Newsletter

Sign up to get the latest science news delivered weekly right to your inbox!

ADVERTISEMENT
ADVERTISEMENT
Collapse bottom bar
DSCJulyAugCover
+

Log in to your account

X
Email address:
Password:
Remember me
Forgot your password?
No problem. Click here to have it emailed to you.

Not registered yet?

Register now for FREE. It takes only a few seconds to complete. Register now »