Astrobiology: The Search for Life in the Universe

David Pacchioli
January 01, 2001
bottom half of Jupiter on black background

Astrobiology: The Search for Life in the Universe, by David Pacchioli

Once it was asked in whispers, or with winks. The timid among us, though undeniably curious, feared raised eyebrows. Jokes about little green men. Who could take such a question seriously, yank it from the misty realms of science fiction and drop it under the searchlight of science? Well, our national space agency, for one. What's more, NASA seems pretty confident these days about the answer: Astrobiology, as defined on an official agency Web site, is "the study of the living universe."

James Kasting is a bit more guarded. Astrobiology is the search for life in the universe, the Penn State professor of geosciences and meteorology told a keen audience at the first talk in last January's Frontiers of Science lecture series. Although the term itself may be recent, "This is not a new field," Kasting said. He got his first taste of it as an undergraduate, reading Intelligent Life in the Universe, a 1966 book by Russian astronomer I.S. Shklovskii and a young American named Carl Sagan, who later wrote, "We have every reason to believe that there are many water-rich worlds something like our own." Kasting was hooked.

In recent decades, Kasting acknowledged, the field has known a bit of a slump. It fell out of favor after the 1976 Viking mission to Mars. "Viking was very successful," he explained. "We learned a lot—but we didn't find life. The perception was that all that money was wasted."

Today, astrobiology is back. The reports, over the last five years, of some 30 planets spotted outside our Solar System —the first of these by Penn State astronomer Alexander Wolszczan —have made all those potential watery Earths that Sagan speculated about less hypothetical.

A great stir, too, has been caused by the discovery, in a melon-sized meteorite plucked from the ice of Antarctica, of a fossil-like remnant that, according to Kasting, looks a lot like Earthly bacteria—"except smaller by a factor of ten." Martian microbes? Opinions vary. The possibility was strong enough, however, to warrant a press conference at which President Clinton said, "If this discovery is confirmed, it will surely be one of the most stunning insights into our universe that science has ever uncovered."

There have been other, quieter, advances. We know now, for instance, that organic, i.e., carbon-based, molecules— crucial to any sort of life we can imagine—are virtually everywhere in the universe. And that, here on Earth, living organisms thrive in what once seemed the unlikeliest of places, from hot springs to frozen lakes—even far below the planet's crust.

In 1998, NASA announced formation of an Astrobiology Institute, a partnership formed for study of "the origin, distribution, evolution, and future of life in the universe." Penn State is one of 11 lead members. No surprise, then, that last winter's annual Frontiers of Science series, organized by the Eberly College of Science and sponsored by the pharmaceutical company Pfizer, Inc., took astrobiology as its topic. On six straight Saturday mornings, the large lecture hall in the Thomas Building at University Park was filled to overflowing with people eager to hear talks by three planetary scientists, two molecular biologists, and a geologist. Astrobiology, these listeners learned, is no loopy fringe pursuit; it is coordinated, systematic, and broadly interdisciplinary. And it involves a lot more than just outer space.

Life in the Extreme

How hardy is life on Earth? Imagine a globe cased in ice: A cap a kilometer thick over land and sea, frozen solid for ten million years. The most recent Ice Age, during which Cro-Magnon's teeth chattered and great hunks of North America and Europe were covered by glaciers, was a tropical honeymoon in comparison.

science fiction comic book cover

Astounding Stories

Now imagine life beneath all that ice. Not a lot of life, mind you— almost everything with a pulse is turned into a Popsicle. But in a few hidden niches, those hot springs under the ocean, say, the hardiest specimens—bacteria, archaea—survive. And, in the long run, life prospers. For when things eventually thaw, they do so in such a way that they accelerate the process of evolution as it has not been accelerated before or since.

Such a scenario, said Paul Hoffman, a professor of geology at Harvard University, is not at all far-fetched—nor is the idea new. Rather, he said, he wanted to share in his lecture "a variety of new evidence supporting an old theory."

In 1964, Hoffman told us, British geologist Brian Harland found glacial deposits present in the ancient rock strata of every continent, even near the Equator and at sea level—evidence, Harland claimed, of the advance of great ice sheets over much of the Earth some 600 million years ago. "Harland proposed a series of extreme Ice Ages, and suggested that the amelioration of climate following these Ice Ages might have had something to do with the great burst in biological evolution that became known as the Cambrian explosion."

Doubts were voiced. With continental drift, Harland admitted, he couldn't be sure where the land masses had been when glaciers covered them. But the real problem was that he had no good explanation for how an ice-covered Earth could have happened. How could it get so cold? "In the absence of a theory," Hoffman said, "no one believed him."

Ironically, Hoffman added, there was a contemporary theory that fit Harland's evidence. A physicist named Mikhail Budyko, at the Leningrad Geophysical Observatory, had worked through a series of calculations based on the global energy balance: the fundamental principle that the heat Earth absorbs must always equal what it gives off. "This balance includes the planetary albedo, the energy reflected back to space," the amount of which is determined largely by surface cover. Dark cover, such as trees and other vegetation, absorbs energy, while a light-colored surface—snow and ice—reflects it away.

Budyko was most interested in something called the ice-albedo feedback. (Maybe it was those long winters in Leningrad.) The ice-albedo feedback, Hoffman explained, says that for any drop in global temperature, you get an increase in surface snow and ice, which means that in turn more heat is reflected away, insuring that things will get still cooler.

What Budyko determined was what Hoffman called "an underlying instability" in the ice-albedo feedback. In short, if temperatures ever went low enough to allow that ice cover to creep to within 30 degrees of the Equator—Houston, Texas, say —"the feedback would be so strong you'd get a runaway effect. It would be unstoppable. The Earth would quickly freeze over."

Budyko didn't think a snowball Earth had ever actually happened, Hoffman said. If it had, he thought, life would have been completely wiped out. Then too, Budyko thought a snowball Earth, once in place, would be permanent: What could generate the enormous heat it would take to undo such a hammerlock? (In 1992, Penn State geoscientists Jim Kasting and Ken Caldeira estimated that such a reversal would require raising atmospheric CO2 to 350 times its present level.)

Since Budyko's day, however, "a couple of things have happened," Hoffman noted. One is the discovery of living organisms in those deep-sea vents, creatures not dependent on sunlight. "We're not certain that these organisms could have survived—ocean chemistry would change in a snowball Earth—but it raises the possibility." A parallel discovery, he added, was of frozen lakes in places like Victoria Land, East Antarctica, where despite mean annual temperatures in the range of ñ20 degrees C (ñ4 degrees F), "things never completely freeze. And the water under the ice is teeming with life.

The other thing Budyko didn't know about," Hoffman said, "was plate tectonics. Plate tectonics drives the carbon cycle, which allows Earth to be a habitable planet."

Earth's crust is made up of a dozen great plates, like ill-fitting puzzle pieces, that float atop the hot molten rock below. The bumping and grinding of these plates shapes Earth's geography, raising mountains, occasioning earthquakes, breaching and redistributing continents. Pressures that build up at the heated core beneath all this activity are released via volcanoes, which belch out CO2.

In the normal course of events, Hoffman related, "Rainwater washes this CO2 out of the atmosphere as dilute carbonic acid, which falls on silicate rocks. This weathering produces alkalinity, which is washed by rivers into oceans and winds up as carbonate sediment on the sea floor." This limestone deposit is drawn by churning and settling down to the core, where it is reheated to liquid and gas, and eventually spewed back up volcanically into the atmosphere, renewing the cycle.

A snowball Earth, however, would screw up the carbon cycle something awful. "The oceans are frozen. The air is very dry. There is no source of atmospheric moisture, no way to scrub CO2." Meanwhile, "plate tectonics is continuing. CO2 is being emitted, but there's no way of getting rid of it. CO2 builds up and up, drives temperatures higher and higher—the escape mechanism is inevitable. And boy, what an escape." After about four million years, things warm to the point that dark ponds of open water appear at the equator. This sudden switch in albedo at low latitudes then kicks off wholesale melting, and from there, "Deglaciation is extremely violent. The ice will disappear in a few hundred years—much faster than you can get rid of the excess CO2."

That thick blanket of gas means an extreme greenhouse period: "Surface temperatures at the tropics over 40 degrees C (104 degrees F), super-hurricanes, torrents of carbonic-acid rain." And—with no ice and the maximum surface area of rock exposed—powerful carbonate weathering. This combination eventually resets the atmospheric chemistry to pre-Snowball levels.

A "freeze-fry" scenario, Hoffman called the whole process. And it fits nicely, he added, with the existing rock record. "Glacial deposits world wide are capped by carbonate sediments. This has long been a puzzle—why are warm-weather rocks sitting on top of glacial rocks? But with all this alkalinity being delivered in conditions of rapid warming, massive deposition of inorganic limestone is exactly what you would predict.

It seems pretty likely, given the evidence, that a Snowball Earth did take place, somewhere between 600 and 700 million years ago. And that likelihood brings us back to the Cambrian explosion.

The extreme environmental conditions post-Snowball, Hoffman suggested, may have ramped up the rates of evolution. "The crash in population size accompanying a global glaciation," he has written, "would be followed by millions of years of comparative genetic isolation in high-stress environments," conditions "favoring the emergence of new life forms." Whether this speed-up would create new branches on the tree of life (as the molecular data would determine) as well as new body types within existing branches (as fossil evidence may show) is not clear. But changes in molecular sequence, Hoffman noted, will always show up earlier than changes visible in the fossil record. Whichever type of explosion the Cambrian was, it seems reasonable to speculate that a string of freeze-fry events could have triggered it.

And how does all this relate to astrobiology?

"We're finding there are still many things to be discovered about the history of this planet," Hoffman concluded, "which shed light on the probability of finding life elsewhere. If life's expansion here depends on an event like a Snowball Earth, that's another thing that makes the persistence and evolution of life on this planet extremely remarkable."

Life as We Know It

In 1997, Charles Fisher, professor of biology at Penn State, discovered this remarkable creature (also shown on the cover of this special report) living on mounds of methane ice under half a mile of ocean on the floor of the Gulf of Mexico. The flat, pink worms, one or two inches in length, use their appendages like oars to move around the surface of the ice as they graze for the bacteria also living there. The new worm species, Hesiocaeca methanicola, may have some influence on the formation of natural gas deposits on the sea floor and, if so, on how we go about mining gas as a source of energy. It has already helped redefine "life as we know it." The bacteria the ice worms eat, and the methane both species grow on, could provide clues about early life on this and other planets.

Fisher came upon the worms by accident while collecting tubeworms near hydrocarbon seeps at the sea floor. Before the discovery, methane ice had been of most interest to geologists and energy companies, not biologists. The area where the ice worms live is under extremely high pressure and, at seven degrees C, very low temperatures. Adds Fisher, "The ice worm community is in itself a new ecosystem. We found an animal living in an environment that we never thought of as a habitat for animals." The ice was formed when methane gas rose up from deposits deep beneath the sea floor. Ancient bacteria that may have lived beneath the Earth's crust, feeding on this gas, migrated with it, eventually settling on the ice.

The ice worms, which are not ancient animals but are related to the common red mud worms we see after a rain, would have come along later. But the mere fact that they can survive such a harsh environment shows the long-term adaptive capabilities some animal species possess. Says Fisher, "The animals we study live in some very extreme, very strange environments and they adapt to it using special physiology, special anatomy, and special behavior."

Mars Revisited

To Bruce Jakosky, life's demonstrated ability to weather almost anything Earth can dish out makes a strong argument that life probably does exist elsewhere in the universe. One likely spot, he suggested, is an old favorite: Mars.

Given the fertility of our collective imaginings about the red planet over the years, Jakosky, professor of geology at the University of Colorado at Boulder and a member of the Laboratory for Atmospheric and Space Physics there, wisely began his talk with a few ground rules. His first slide was a cover from the tabloid Weekly World News, with a prominent photo of a shiny silver saucer hovering above a line of trees. "This," he said with deadpan aplomb, "is what I'm not going to talk about."

Mars, Jakosky went on to acknowledge, is a stone that's already been turned. Twenty-four years ago, two Viking landers touched down on the planet's surface, dug some soil samples, and headed home. Subsequent analysis turned up no trace of organic molecules, the bare-minimum evidence that would have pointed toward life. The search for extraterrestrials was dealt a stinging setback. But recent findings here on Earth, Jakosky said, warrant taking a second look. "Over the last couple of decades, our understanding of terrestrial life has evolved dramatically.

First of all, we know now that life originated quickly." Earth's early history, he explained, was exceedingly violent, with frequent catastrophic bombardments by asteroids not letting up until about four billion years ago. "Not until then could life have gained a foothold." Yet carbon-dating evidence shows that life was already firmly established by 3.8 billion years ago. "Life sprang up almost overnight once the right conditions were present," Jakosky concluded. "To me, this suggests that anywhere these same conditions exist, the odds are good that life could be—and probably is."

Second, he said, "We've found out that life on Earth is incredibly robust and capable," existing not only in surface hot springs and around thermal vents but deep within the planet's interior. "Twenty years ago we didn't know about life below the surface. Today we think that half of Earth's biomass exists there, inside rocks. We were missing half of the life on Earth!"

In short, "Life doesn't require much for its support," Jakosky said. The basic necessities are only three: a liquid medium, an energy source, and the presence of a few choice elements. Here on Earth that means water, sunlight, and an atmosphere shot through with carbon, hydrogen, nitrogen, and oxygen. "Of these elements," Jakosky said, "carbon is probably the most important," not just because of its abundance—it exists all over the universe—but also because of its versatility. "Carbon combines with oxygen to form a gas—carbon dioxide—that can be dissolved in water, so it's transportable. It can precipitate out and be stored as limestone when it's not needed. People ask, ëDoes life have to be carbon-based? What about silicon?' But carbon is so much more capable."

Does Mars meet the three basic criteria? From this distance, it's difficult to say. But "we can learn a lot," Jakosky said, "by looking at pictures." Present-day Mars is much colder than Earth, too cold to sustain liquid water on its surface. But photographs depicting what looks like erosion of crater rims and other features suggest that abundant water has been present there even very recently. Other photos show networks of branching lines that look like river tributaries; still others, broad channels up to 100 kilometers wide. "That's an hour's drive here on Earth. That much water couldn't have come from just rainfall; there must have been some catastrophic release." Yet tracked to their sources, these channels reveal nothing. "It looks like water burst forth from beneath the crust," Jakosky said. "Almost certainly there is still water down there."

What about an energy source? Granted, the sun is too far off to power Earth-style photosynthesis, but geochemical energy—from volcanoes, and even from mineral weathering—is a viable alternative, Jakosky suggested. He showed a picture of Olympus Mons, a volcanic Martian peak that is twice as tall as Earth's Everest, with a summit area 100 kilometers across. "With volcanism and liquid water," he said, "there's a possibility of hydrothermal vents, like the ones we see at Yellowstone."

As for those life-building elements—carbon, hydrogen, oxygen, and nitrogen—they are all present in the Martian atmosphere. According to the recent Pathfinder mission, magnesium, iron, aluminum, and phosphate —all potential role-players, as well—are components of Martian rocks. "So life could have originated on Mars," Jakosky said. "That doesn't mean that it did, or that it's there now. But it's reason enough to look."

Oh, and there's one more reason: whatever it is that's embedded in the small set of Martian meteorites that have been recovered over the last 20 years. From a pocket Jakosky produced a sliver of dark mineral cased in clear plastic, and held it aloft. "This is part of one of about 15 rocks that have been picked up on the Antarctic ice sheets," he said, "where if you find a rock, the only place it can have come from is out of the sky. These rocks are young, volcanic, which means they came from a planet with recent geologic activity: Earth, Venus, or Mars." Gases trapped within the samples show that they're unearthly: there's not enough oxygen present for them to have been trucked down from New Zealand, say. More positively, the levels of argon, xenon, and krypton are identical to what is present in the Martian atmosphere—"and nowhere else," Jakosky said. "If these rocks didn't come from Mars, we don't know anything about the solar system."

In 1996 NASA created a splash by reporting that one of the Martian meteorites, known as ALH84001 (for its discovery in the Allan Hills region of Victoria Land, in 1984), contains some rather interesting tidbits. Lodged within limestone deposits formed in cracks in the rock were tiny tube-shaped structures that just might be fossilized life-forms. Make that extremely tiny: The largest of them is less than 1/100th the width of a human hair. "Nano-fossil-like structures," NASA has called them. "They look like terrestrial bacteria, except they're a thousand times smaller" in volume, Jakosky said. Apparently they formed, whatever they are, the same way fossils occur in limestone on Earth. But could they really be remnants of life?

"We don't have enough data to tell," Jakosky said. Researchers at Johnson Space Center, he noted, have also identified organic molecules in ALH84001 and some of the other fragments: polycyclic aromatic hydrocarbons, to be precise. "These could be precursors of life, but they are also typical of decay products from the earthly combustion of fossil fuels." They could be simple contamination, in other words. Again, "We will only find out by getting more samples from the Martian surface and bringing them back to study."

The chief difference between now and the Viking mission days, Jakosky said, is that, "We know better what to look for now. Twenty years ago, we didn't know to look for hydrothermal vents." He and his colleagues at NASA also have a better idea of where to look: "In river channels and canyons, places where there has been liquid water." Or at crater rims, some of which appear from photographs to be rimed with ice.

"It's possible that we won't find any evidence of life," Jakosky said. "But that would also be an important result. It would lead us to question again what we have learned about life's origin here on Earth."

An Ocean in Space

For a long time," said Chris Chyba, the last Frontiers of Science speaker, "the difficulty with looking for life on other planets was finding water." The concept of the "habitable zone," developed by Stephen Dole of the Rand Corporation and Michael Hart of NASA's Goddard Space Center and further elaborated by Penn State's Kasting, along with Ray Reynolds of NASA Ames and Dan Whitmore of the University of Southwest Louisiana, put this dilemma in black and white. Of the planets in our Solar System, Earth, Kasting and his colleagues calculated, is the only one close enough to the Sun to be warm enough for liquid water, yet not so close that the water boils away. Actually, Mars is in the ballpark too, except that present-day Mars has too little atmosphere to retain the necessary heat—at the surface. But what about down below?

Recent research has heightened interest in "worlds that may be rich in liquid water below the surface," said Chyba, associate professor of geological and environmental sciences at Stanford University and director of the Center for the Study of Life in the Universe at the SETI Institute. Mars is one such world. Another, in some ways even more tantalizing, is Europa.

Fourth largest of the 16 known satellites of Jupiter, Europa is a chunk of rock and metal about as big as Earth's moon, sheathed in ice. Voyager photographs taken 20 years ago show a smooth surface scored heavily with cracks, like a favorite skating pond in late winter. The absence of craters, Chyba said, shows that unlike our moon, Europa is geologically active. "Its surface is being reworked every 10, or 20, or 30 million years," by new material churned up from below.

The reason for this activity, he said, is the strong tidal pull exerted by Europa's giant parent, which causes bulging and shrinking of the satellite's crust as Europa moves through its orbit. All that movement creates friction—and heat. "And we can calculate how much," Chyba said. Doing so, he added, "enabled one of the few important successful predictions in the history of planetary science": that Io, Jupiter's closest satellite, was so heated by friction it would be "the most volcanically active world in the Solar System. And it in fact is—Voyager has taken pictures of its volcanoes erupting."

The pull on Europa, farther out, is less than that on Io. But there's still enough friction to heat Europa's core substantially —enough to melt away most of its icy layer from the inside, Chyba said. So, although the surface, which has no atmosphere, remains a rock-solid ñ170 degrees C (ñ274 degrees F), beneath Europa's ice in all probability lies a vast body of water.

The evidence of resurfacing seems to corroborate this, Chyba said, with smooth areas suggesting water flowing out from the interior only to be quickly re-frozen, like the contents of a bucket spilled across a frigid sidewalk. The wealth of cracks, he added, "seem to be related to stretching ice as it rides up on top of an ocean deforming underneath." But in a way the most compelling argument for an ice-bound sea is the magnetometer data.

Jupiter has the strongest magnetic field of any planet in the Solar System. That field sweeps past Europa every ten hours, as the giant planet spins on its axis. "If there were a conductor on Europa—salty water, for example—the changing magnetic field would set up a current in that conductor," Chyba explained, and that current would create Europa's own magnetic field. Such a field has now been measured—its strength consistent with an ocean 100 kilometers deep with a salt content about equal to that of the ocean on Earth.

"It's hard to avoid the conclusion that there's a salty conducting ocean on Europa," Chyba concluded. "But we're not completely certain. And we would like to be, because if there is a second ocean in the Solar System, we're going to go back and have a program of exploration on Europa that rivals the Mars program. I would go so far as to say that if there is an ocean on Europa, it is the most exobiologically interesting place in the Solar System. That is to say, there might be life there."

What do we mean by life? That's the first thing that needs to be agreed on. "There have been many attempted definitions—thermodynamic, metabolic, biochemical—but all of them seem to either leave something out that we know is life, or let something in that we know isn't," Chyba said. "So we have to fall back on a simpler idea, that of life ëas we know it,'" made of liquid water, organic molecules, and an energy source. On Europa, "there is almost certainly liquid water present. There are hints that there are organic molecules present." What about an energy source?

"It's hard to say anything at all about this," Chyba admitted. "You can't have photosynthesis. Light couldn't penetrate that surface ice." Might there be hydrothermal vents at the bottom of that ocean? "We have no idea."

A look at life on Earth, he continued, shows that higher life forms—eukaryotes—require something beyond the three basics: they need oxygen, too, to help metabolize energy. "Even tubeworms and clams at hydrothermal vents need oxygen; it's produced at the surface and finds its way down. If not for photosynthesis these organisms would die." Oxygen, whether in Europa's atmosphere or in its ice-covered ocean, is likely to be scarce, Chyba said. So, "as much as I would like to see giant squid swimming in Europa's ocean, we probably have to content ourselves with one-celled organisms analogous to bacteria or archaea."

On the other hand, he noted, there are some creatures on Earth that get along fine with no oxygen at all. Methanogens, for example, are a class of bacteria that digest hydrogen and carbon dioxide to produce methane. "And they probably get that hydrogen from rocks. If Earth froze over tomorrow and became a world that looked like Europa, we would probably continue to have an ecosystem living underground for billions of years."

Conceivable, too, are energy sources on Europa that we simply don't know about: Chyba offered a suggestion based in photochemistry. Jupiter's strong magnetic field, he said, acts like a particle accelerator, shooting charged particles—radiation— into Europa's ice. "We know from Galileo's observations that there are carbon dioxide molecules mixed in with that ice. Once you irradiate carbon-dioxide-bearing ice, you make simple organic molecules, like formaldehyde. And you can make oxidants from the ice itself. These molecules are frozen together, and at melt-through events they could get mixed into the ocean." Using Earth analogies, Chyba said, "We can estimate that Europa's ocean, in this way, could support a bacterial ecosystem." Not a very robust one—"only about 1/10,000 as dense as that in Earth's ocean"—but, hey, it's a start.

The only way to know whether such an ecosystem is out there," he said, "is to go look." That's the rationale behind NASA's Europa Orbiter, planned for launch in 2006. The Orbiter's primary objectives, which Chyba helped to draft, are to verify the presence of an ocean, measure how thick the ice is, and spot evidence of organics. "After the Orbiter, there is planned a Lander. And after that, maybe a series of missions, to get beneath the ice."

All of which is a bit more involved than missions to Mars. "It takes three years just to get there," Chyba said, "and another to get into orbit. And once you're there, you have that punishing radiation," conditions so harsh that the Orbiter is expected to survive for only a month. Then, too, there's the possibility of contamination, "both forward and back, but it's the forward contamination I'm worried about. We need to be extremely careful that we don't introduce organisms that would interfere with Europa's possible ecosystem. And if the ocean is sterile, we don't want to introduce any false positives."

Securing answers from far-off Europa will be an extraordinarily complicated endeavor, as difficult, perhaps, as humans have ever attempted. To Chyba, however, the effort required will be well worth it.

"My suspicion is that if we find an ecosystem on Mars, it's quite possible that it will share a common ancestor with life on Earth," he explained. "Whichever world evolved life first will have inoculated the other," through asteroids or other space-borne debris. "But I think that if we find life on Europa, it's probably an altogether different form of life." Something beyond even our current power to imagine.

Reflections From A Warm Little Pond

Back in 1953, Jim Kasting said, scientists thought they had the origin of life figured out. Chemists Stanley Miller and Harold Urey at the University of Chicago had simulated that crucial instant around 3.9 billion years ago when a batch of simple inorganic molecules, zapped by a bolt of lightning (or maybe just the sun's warmth during a break in the clouds), fell together to form the prototypes for the complex organic compounds that life is made from.

Now that was a moment. Remember it on Star Trek? The muddy puddle of ooze on the edge of Nowheresville? The awful humidity? The onset of bubbling? Before, everything was dead as Play-doh. After came a chain of eye-popping events that just keeps unfolding, across the eons, into alligators and astronauts, puppies and banana figs, mosquitos and lichens and particles of ebola virus . . .

In their lab, Miller and Urey shot flashes of lightning, in the form of cascades of sparks, through a flask containing an "ocean" of liquid water and an "atmosphere" of strongly reduced (that is, hydrogen-rich) gases—methane, ammonia, hydrogen sulfide, and water vapor. After a couple of days, they tested what was left. "They had formed all sorts of compounds," Kasting said, "including large quantities of amino acids," the molecules that join to form proteins. This simple experiment seemed to corroborate a vision Darwin (and not Gene Roddenberry) had described a hundred years earlier, of life emerging "in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc., present."

But the Miller-Urey experiment, important as it was, had a flaw. Urey had based his primitive-Earth atmosphere on astronomical data just then coming in, the first spectra from the giant planets in our Solar System: Jupiter, Saturn, Uranus, and Neptune. These characteristic bands of color showed that the giants were swathed in atmospheres rich in methane and ammonia, thought to be left over from the planets' formation.

At the time, people thought all of the planets had once shared a "primordial" atmosphere, the result of their common birth. Because of their stronger gravity, the giants were believed to have retained this early atmosphere, while the atmospheres of Earth and the other, smaller planets had lost some of their lighter gases, hydrogen among them, to space. Thus, Urey reasoned, an early Earth atmosphere, before its hydrogen had escaped and the life-driven process of photosynthesis had boosted its oxygen, would have been a lot like a present-day giant's.

Shortly after the Miller-Urey experiment was published, however, geologists came up with new findings on Earth's volcanic emissions—and threw the old reasoning for a loop. "What comes out of volcanoes is not methane and ammonia," Kasting said, "but about 80 percent water vapor, 15 to 20 percent carbon dioxide, and traces of carbon monoxide and molecular hydrogen." James C. G. Walker, one of Kasting's graduate advisers at the University of Michigan during the 1970s, took these emissions data and balanced them against the rate at which hydrogen would be expected to escape from a planet with Earth's gravity. ("He did all this stuff on the back of an envelope," Kasting said.) What Walker came up with was a much different picture of Earth's early atmosphere: an oxygen-rich mix of carbon dioxide, nitrogen, and water vapor.

The catch is that oxygen, although an absolute necessity for multicellular, advanced life, is poison to pre-biotic synthesis. Do a Miller-Urey experiment in an oxygen-rich atmosphere, Kasting said, and "you don't form things like amino acids. There are too many oxygen atoms in there." So, over the years, "enthusiasm for the warm little pond theory has waned."

Two competing theories have emerged instead. The discovery of microbes and other small organisms living in and around hydrothermal vents—underwater hot springs boiling from the ocean floor—has led to the idea that life may have started at the bottom of the sea. Sharp differences in temperature and oxygen concentration at the boundaries around these vents make good catalysts for chemical reactions, Kasting said. "The problem with this theory is that the complex organic compounds likely to form life cannot remain stable for long at such high temperatures." Amino acids, instead of joining up, would tend to break down.

The other scenario has life first coalescing in the frigid climes of outer space—specifically, within the cold dark hearts of interstellar dust clouds. "Long, complex organic molecules can be made when ionizing radiation leads to ion-molecule reactions," Kasting explained. "The intense cold prevents them from breaking down." In this so-called "seeding from space" model, these complex molecules are brought to Earth by incoming meteorites and comets. The weak link here is that most of a meteor is vaporized on impact with our atmosphere. "The survival potential for organisms is low. They get pyrolized: Burned to a crisp."

Kasting, for his part, is not ready to give up on the warm little pond. Using computer models of light-triggered atmospheric processes, he is working to reconcile Darwin's vision with the constraints imposed by a relatively oxygen-rich atmosphere.

"My idea," Kasting said, "is that this atmosphere did contain some methane: just enough to allow for the formation of hydrogen-cyanide molecules, one of the key starting materials for making both amino and nucleic acids. Ten to 100 parts per million would be enough."

Present-day life, he explained, requires three types of molecules: DNA, to store the genetic information that allows cells to replicate; RNA, which transfers that genetic information from the nucleus to the rest of the cell; and the proteins that catalyze these reactions. "It's a very complicated system." Yet in 1989, molecular biologists Thomas Cech of the University of Colorado and Sidney Altman of Yale shared a Nobel prize for showing that under some circumstances RNA can replicate on its own. Not only that, but it can store genetic information.

RNA, in other words, can do it all. "Early life is now believed to have passed through a stage in which only RNA was present," Kasting said: the so-called "RNA world." All you need for life, besides those crucial amino acids, are the ingredients for RNA: ribose, a sugar; phosphate, a salt; and the four bases —adenine, cytosine, guanine, and uracil (the last replaces the thymine in DNA). The question is, can you get these molecules in an atmosphere where significant oxygen is present? The answer, Kasting said, is yes—assuming there's a little bit of methane around.

Ribose, Kasting explained, "is simply five molecules of formaldehyde strung together," and formaldehyde is easy to make where there is carbon dioxide and light. Phosphate occurs routinely with the weathering of rocks. And all four bases, A, C, G, and U, can be synthesized from hydrogen cyanide, for which you need that sprinkling of methane.

"So the key to making Darwin's little pond," Kasting said, "is to figure out if there was a good source for methane in the early atmosphere." That source, he suggests, is under the sea, in the volcanic activity that fires up those super-hot hydrothermal vents. Currently, the carbon released from the vents run about 99 percent carbon dioxide, he said, and about one percent methane, a slightly different mix than what comes from volcanoes on land. "And there are good geochemical reasons to believe that the Earth's mantle 3.9 billion years ago was much more strongly reduced than it is today, which means the methane component of these emissions would have been that much higher." Plenty high enough to allow for the formation of organic molecules.

That's not to say this is the way life sparked into being, Kasting quickly added. But it's a plausible scenario. And if it did happen that way here, what's to stop the same process from repeating itself, around the universe, wherever conditions happen to be the same?

Can You Relate?

"We have an amazing world," said Janet Siefert, Keck fellow in molecular biology at Rice University. "Full of extravagant beauty and diversity." The projection screen above her head flashed a series of images: of tigers, and swordfish, and the bluebonnets of her native Texas, whose balmy winter temperatures Siefert had left to give the second Frontiers of Science lecture last January.

"But there's another world that underpins everything that goes on," she said, and the focus suddenly shifted. Single-celled microorganisms now filled the screen. Diatoms, Euglena, paramecia. Giardia. "These are eukaryotes," she said. "Very closely related to humans."

Eukaryotes, she explained, are distinguished from other microbes by their complexity: the internal membranes, the machine-like organelles, and, most important, a core nucleus. "It's this structure that allows for differentiated cells, and lets multicellar organisms arise."

All the world's animals are eukaryotes, she noted, and all the insects and the plants too, not to mention fungi and algae. Among animals alone, by far the smallest subset, there are over a million species. "But eukaryotes are only a small fraction of the biological diversity on Earth."

A kind of family tree called a phylogeny helps to make the point. On screen, this tree of life consists of three main stems emerging from a sturdy trunk. The first, labeled "eukaryotes," looks stunted, dwarfed. The other two branches, much larger and fuller, are the prokaryotes: bacteria and archaeabacteria.

Amore humble class of organisms, these. No impressive innards: no mitochondria, no nuclei. No internal membranes enforcing structure. "They look very simple," Siefert admitted, "but they have remarkable biological diversity." Not only do they have us badly outnumbered; it seems that we need them more than they need us. "If you took away the eukaryotes," Siefert said, "you'd still have a living planet. If you take away the bacteria and archaea, everything crashes."

She showed us some common bacteria: helicobacter (the cause of most ulcers), E. coli, salmonella. And some that are not so common. Thiomargarita, the "scuba-tank" bacteria (so-called because of its ability to store nitrate for respiration), is "one-fifth the size of a bumblebee," a true giant among its peers, one billion of whom (on average) can fit in the eye of a needle. Size ranges aside, she said, "They all look pretty similar. They all have a similar morphology."

So also with the archaea. "These are very interesting organisms, with amazing biochemistry," Siefert said. "They grow in strange environs: at the bottoms of rice paddies, where there's no oxygen; in highly acidic hotsprings; in hydrothermal vents at the bottom of the sea. They can live almost anywhere. But they are boring to look at." The point is not merely aesthetic. Their similarity, Siefert said, makes these organisms hard to tell apart—and telling them apart is the first step to creating a more complete, and more accurate, family tree. For Siefert, an accurate tree, or phylogeny, is the key to reconstructing the early evolution of life on Earth.

Establishing relationships demands comparisons. And making comparisons requires a yardstick, something common to every living organism. But what to choose? Prokaryotes don't have noses, or feathers, or feet, to lend them character. They do, however, have DNA—and RNA, too. More specifically, they, like every organism on Earth, have ribosomes.

A ribosome is a maker of proteins: A sub-unit of RNA that reads the string of bases that makes up a genetic code and translates it into whatever the cell needs. "An incredible machine," Siefert called it. This machine itself has two sub-units. And, as it happens, the gene that codes for the smaller of the ribosome's sub-units, called 16S in prokaryotes and 18S in eukaryotes, makes a great universal point of comparison. It is easy to get. And, Siefert emphasized, "is found in every single living organism."

The process, then, is straightforward: Take the 16S genes from any two organisms; compare the sequence of bases in each. The more differences in the sequence, the farther apart on the family tree the two organisms belong.

"If I gave you a truck, a Humvee, and a Cadillac," she said, "and asked you to find out what a Model T must have looked like, what would you do? You'd take away everything those three vehicles didn't have in common, and you'd look at what's left. This is exactly what we do. If you can compare the entire genetic blueprint of an organism with that of another one, take away everything that's not common, the idea is that what's left must be what was in a common ancestor."

In 1996 evolutionary geneticists used this approach to conclude that the "minimal" genome for an ancestor that could have given rise to all of life would have to include at least 256 genes. (Yeast, a fungus, has 5,000 genes; humans have roughly 100,000.) The current debate, Siefert said, is over that murky early period before the three present-day domains emerged. How exactly did the bacteria, archaea, and eukaryotes take shape? And how did eukaryotes evolve their complexity?

Siefert showed us a timeline: the origin of life marked at 3.9 billion years ago, the earliest known fossil cells at 3.8. "Already at this point," she said, pointing to the latter, "you've got a very sophisticated organism, with ribosomes, protein-making machinery, structural molecules. How did it become so miraculously complex in so short a time?"

Unlocking this mystery won't be easy. "Genomics and phylogeny," Siefert said, "can tell us a lot about the evolution of life from 3.8 billion years ago to the present. Getting back beyond that is trickier." As for discovering life's origin, "We don't even know how to define it." Did life begin, as some suggest, with that first biochemical reaction, the synthesis of amino acids? With the pre-cellular molecules—capable of copying themselves and passing on their genetic information—that would have populated an RNA world?

"As far as I'm concerned," Siefert said, "to call it life you also need that compartmentalization. You need a cell."

A Question of Timing

"Aphylogenic tree," Blair Hedges began his Frontiers of Science lecture, "shows the relationships between organisms. It shows that we're closer to chimpanzees than we are to gorillas—and that birds, which are warm-blooded, are actually closer to lizards than they are to mammals."

But a phylogenetic tree, added Hedges, an associate professor of biology at Penn State, cannot show us how things got that way. To follow evolution's path, you need to put a stopwatch on the stages. You need to know when.

asteroid on black background

Portrait of the asteroid Eros. A time tree of evolution, compared against a documented Earth event like the period of heavy asteroid bombardment can help predict where else life may show up. Credit NASA and Johns Hopkins Applied Laboratory

The chemist Linus Pauling, then at Cal Tech, first proposed a molecular clock back in 1962. James Watson, Francis Crick, and Maurice Wilkins at Cambridge University had recently revealed the secrets of DNA's structure—the celebrated double helix—and geneticists were busily decoding the sequences of nucleotides, or bases, that add up to genes. "As soon as enough sequences were generated that you could make comparisons between species," Hedges said, "people started recognizing that molecular data"—the order of bases in a given strand of DNA—"are different from morphological data"—that is, an organism's gross characteristics, like noses, feathers, and feet. While morphology tends to evolve in spurts, dependent on the forces of natural selection, molecular change happens at a fairly constant rate, at least when measured over hundreds of millions of years.

Within a given gene, Hedges said, base pairs are constantly being damaged or otherwise altered, and only sometimes being repaired. "Quite a few of these changes will be deleterious. They will negatively affect important functions of the gene, turn it off, cause the organism to die." Other mutations will confer some evolutionary advantage. The majority, however, will have no effect whatsoever. These "neutral" changes are made possible by a redundancy built into the system: With four "flavors" of bases —A, T, G, and C—and a string of three base pairs required to make an amino acid, there are 64 possible trios of base pairs, to form only 20 amino acids.

Neutral changes are shifts in position, not ingredients; a C-G pair replaces a G-C, say, but in essence the amino acid is the same. "If it doesn't cause a change in the amino acid," Hedges said, "natural selection can't ësee' it, so it doesn't have an effect in terms of evolution." These changes, in other words, are completely random. And while that means they aren't completely regular—they tend to happen in clusters, Hedges said—over the long haul a given gene evolves at a constant rate. If you know that rate, and you know that the gene is present in a pair of organisms, counting the number of changes that have occurred in each will yield the length of time since the two diverged from a common ancestor.

To make sure a DNA clock is accurate, Hedges said, you have to calibrate it. The best calibration so far—"the closest thing we have to proof that molecular clocks actually work"—comes from studies of influenza. "Thirty or 40 years ago, people started freezing flu virus for later study. When some of these viral particles were thawed out in the 1980s and sequenced, researchers compared their DNA sequences to those of today's flu strains, and found a significant number of nucleotide changes. They knew exactly the number of years since those viruses had been frozen, so they could do a precise comparison."

DNA clocks have been used to clarify some of evolution's biggest questions. To trace out the early history of the vertebrates, for instance, Hedges and his collaborators have looked at 7,000 different genes, and some 300 species, using for a calibration point the separation of birds and mammals around 310 million years ago. ("This is a really good split," Hedges said. "There's an excellent fossil record, based on bone characteristics.") The results are encouraging. "We've come up with divergence times for early splits in vertebrates that match up well: amphibians from reptiles and mammals at 360 million years ago; trout and salmon from other fishes, 450 million. . . . For the split between humans and chimps we got 5.5 million, which is close to the time assumed by most anthropologists."

Other findings are more controversial. Take the Cambrian "explosion," sometimes known as Evolution's Big Bang. The fossil record is rich with specimens from the dawn of the Cambrian period, 540 million years ago, Hedges said. Beyond that boundary, animals, and many plants, are virtually absent. "What it suggests is a tremendous proliferation of these higher species all at once." In a few short millions of years, according to the bones, Earth's biological diversity zoomed from next to nothing to virtually all its modern variety.

But molecular data collected in labs around the world over the last 20 years, Hedges said, tell a different story. According to the DNA, "Animals diverged one billion years ago, not 540 million." What could account for a 500-million-year gap? "Maybe animals were smaller, microscopic even," Hedges suggested. "Maybe they were soft-bodied, and therefore rapidly decaying. Right around the Cambrian border animal tracks are very small. Then they get much larger. Maybe there was an increase in size right at that boundary.

"Most paleontologists don't accept these dates," he acknowledged. "Only time and the weight of accumulating evidence will show who's right." It is already clear, however, that molecular clocks can be a powerful tool for understanding the effects of the environment on biological evolution. "Once you have a time tree of evolution," Hedges said, you can compare it against documented events in Earth's history, like the period of heavy asteroid bombardment between 4.4 and 4.0 billion years ago, or the steady rise in atmospheric oxygen to its present 18 percent.

The latter information will come in handy when, in a few years, we are able to detect the atmospheres of planets outside the Solar System, he told us. "We will be able to find out whether there is oxygen in those atmospheres, and how much. And if there is a relationship between the level of oxygen present and the rise of life, then we can use that information to better predict the possibility of life elsewhere."

Last Updated January 01, 2001