First Contact (11 page)

Antonio Lazcano, Bada's longtime colleague and current collaborator in testing more of Miller's vials, added this footnote: Asked why Miller never published the robust results from the experiment that added the hydrogen sulfide, Lazcano gave a very human, if scientifically questionable, reply. “Stanley always said he hated the rotten-egg smell of the hydrogen sulfide,” he said. “I think he just didn't want to be around it.”

The legacy of Stanley Miller is now very much living again not only because of the newly found volcano results, but because he trained men like Bada and Lazcano, who in turn have trained or mentored biochemists and astrochemists such as Glavin and Dworkin. They are the golden boys of NASA's Goddard Space Flight Center, located outside Washington, D.C., who used their cachet to lobby for a new generation of high-powered instruments that can analyze chemical and biological specimens—including the rediscovered Miller-Urey material—and map their electromagnetic fingerprint (and thus their chemical makeup) with greatly increased precision. They use their new tools to study the building blocks of life in meteorites, in comets, and in the vastness of space, and they have taken the origins-of-life field in some surprising new directions. One of their focuses has been on the peculiar dynamics of chirality—the uniform and unusual “handedness” of proteins and sugars—a phenomenon as puzzling as it is potentially revelatory. It's a tricky concept to take in, but basically involves the logic of mirror images: entities with the exact same structure, but set in an opposite direction.

Scientists have worked for decades to understand chirality, which has applications in drug-making and could someday be important in determining whether a microbe found on Mars or Europa or a comet is related to life on Earth or has an entirely separate origin. But it's virtually unknown to most everyone else. Here's a quick introduction:

In the mid-nineteenth century, the renowned microbiologist and chemist
Louis Pasteur took on one of the puzzles of the day: why a solution with tartaric acid derived from living things (in this case, the discarded yeast remains of the winemaking process) behaved differently from a solution with the same tartaric acid that had been made synthetically. The difference involved how light passed through the liquid. The light entering the biologically derived solution didn't go straight through but rather was refracted off to one side, while the chemically synthesized sample allowed the light to pass through unchanged. Pasteur grew crystals of a compound including the tartaric acid and noticed that they came in two nonsymmetric forms that were nonetheless mirror images of each other. He laboriously separated the two forms of the compounds and found that solutions of one form rotated light clockwise while the other form rotated light counterclockwise. If the solution had an equal mix of the two versions of the molecule, then there was no effect on the light at all. What Pasteur correctly made of this is that the molecules of tartaric acid could exist in either a “right-handed” or “left-handed” form, rather like left- and right-handed gloves. The left-handed version was always biological, the right-handed one always synthesized.

I met Glavin and Dworkin one morning to learn about the unlikely role of chirality in astrobiology. They are astrobiologists now, but have backgrounds in physics, planetary sciences, and astrochemistry. Our destination was the Smithsonian's National Museum of Natural History and its collection of meteorites in a gallery called “Geology, Gems and Minerals.” To be precise, meteorites don't fit any of those listed categories. But among the incoming rocks on display are the Allende meteorite from Mexico, the Orgueil meteorite from France, the Murchison meteorite from Australia—all legendary in the world of meteorites, and of especially great interest to astrobiologists. You couldn't tell by looking at it, or by gauging the pedestrian display or reading the brief description, but the dark gray Murchison meteorite in particular has become something of a holy grail for astrobiology. That's why Glavin, Dworkin, and other researchers come to the museum to collect precious samples stored away in a collection room closed to the public and filled with bagged and preserved pieces of Murchison
and other rocks arrived from space. Of all the meteorite samples in the vast Smithsonian collection, about one quarter of the requests for samples are for a small piece of Murchison.

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Pieces of Murchison hardly look remarkable now, and would be easy to ignore if you passed them by on the ground—just another faded dark gray rock with some shinier white highlights inside. But when it burned through the atmosphere in 1969 and landed in the Australian countryside, it was a gift from beyond. The roughly two hundred pounds of meteorite, broken up into rocks large and small, were collected with unusual speed, and in some cases protected from contamination. (In other cases, contamination is a greater issue—including the pieces that fell in a barnyard and were tossed by the farmer into the microbe-filled manure heap.) In the years that followed, hundreds of scientific papers have been written based on the ancient geochemical treasures found inside Murchison.

What makes it so important is that it is an extremely rare meteorite with lots of organic carbon (as opposed to the many others made up primarily of iron, nickel, silicon compounds, or inorganic carbons like graphite), and so is an extraterrestrial laboratory for the kind of carbon-based chemistry that ultimately produces life on Earth. To the initial amazement and delight of researchers, Murchison was found to be loaded with some of the important and complex molecules needed for life—including some of those amino acids that are used to make proteins. Just as English words are made up of the twenty-six-letter alphabet, all proteins—the workhorse molecules of life—are made up of twenty of the hundreds of amino acids known or suspected to exist. About half of those twenty amino acids essential for biology on Earth were found in Murchison. What does this mean? It does not mean what may first come to mind—that the presence of those biological amino acids says the rocks once were home to living things. Rather, it's the fact that
only
half of the twenty protein amino acids have been found. The absence of the rest has been used to bat down charges that
the meteorite was substantially contaminated by Earthly microbes. If the rocks had been widely contaminated, then all the Earth's biological amino acids would be present. In addition, Murchison has scores of other amino acids found on Earth but unassociated with life and many others with no known Earthly counterparts. All of those complex molecules, it had to be assumed, were formed from simpler elements while on asteroids, comets, or in the primordial cloud of dust and gas where the solar system was born—making Murchison a true Rosetta stone sent down from space.

Here's where it gets really interesting to Murchison researchers. On Earth, virtually all living cells containing amino acids (and therefore proteins) are entirely “left-handed.” Virtually all have sugars of a molecular structure deemed to be entirely “right-handed.” This works great for biology, because the proteins and sugars need to be of opposite structures to successfully interact. But this kind of “homochirality,” with virtually all proteins and sugars structured entirely in the same way, is unique. Everything else that isn't biological on Earth contains molecules that are mixed right-handed and left-handed in roughly the same proportion. Rocks, water, inorganic chemical solutions—they all have molecules that are mixed right- and left-handed on Earth and, as far as we know, everywhere else. Yet somehow the proteins and sugars essential to life are significantly abnormal in a way that makes biology possible. Dworkin put it like this: Biology would still work fine if amino acids were all right-handed rather than all left-handed, but wouldn't work at all if they were mixed left and right. “If you mix them, life turns into something resembling scrambled eggs—it's a mess. Since life doesn't work with a mixture of left-handed and right-handed amino acids, the mystery is how did life decide—what made life choose left-handed amino acids over right-handed ones?”

The mystery remains unsolved, but scientists have been finding clues to help explain it. And a primary repository of those clues is the Murchison meteorite. The pioneering work was done in the mid-1990s by two biochemists at Arizona State University, Sandra Pizzarello and John Cronin. The two isolated one particular amino acid called isovaline from Murchison,
concentrated it, and then did an analysis of its chirality. Since everything nonbiological was understood to have molecules equally left- and right-handed, the assumption had to be that the isovaline would also be equally mixed. But it was not. The imbalance wasn't great, but there were about 8 percent more left-handed isovaline molecules than right-handed ones in Murchison. The discovery opened the door to several remarkable possibilities: that forces in the cosmos could transform the handedness of some molecules traveling through space on meteors, asteroids, or even dust, and that the handedness could then be gradually exaggerated on Earth until—in the case of the twenty amino acids essential for life—that small excess would become complete left-handedness.

Glavin and Dworkin used their more sophisticated instruments a decade later to make similar measurements of isovaline from Murchison and other meteorites. What they found was similar but more pronounced: a left-handed excess of 18 percent. Since no other nonbiological molecules on Earth are known to have this kind of excess, the two concluded that the left-handedness of amino acids, proteins, and therefore life on Earth most likely came from beyond. That hypothesis is now broadly accepted.

Meteorites bombarded the Earth 4 billion years ago and they delivered an impressive amount of carbon-based material to Earth, something on the order of hundreds of thousands of tons per year. The infall is much smaller now, on the order of 5,500 tons per year and most in the form of micrometeorites or even smaller particles of interplanetary dust. So how did those space-faring amino acids develop their slight excesses of left-handedness? Long-term exposure to the cosmic radiation of neutron stars is the most likely explanation, since chirality has been shown to change slightly when exposed to certain kinds of highly energized light. But no firm consensus has yet produced an answer.

All of a sudden, the undistinguished chunk of Murchison on display at the museum looked not so homely to me anymore. A gray-black, alternately smooth and craggy rock the size of a baked potato and mounted on a simple blue rod, it has an improbable and important story to tell. Murchison,
it is commonly believed, is somewhere between 4 billion and 4.5 billion years old—meaning that it was formed around the time that Earth was. It was initially a part of the asteroid belt between Mars and Jupiter, and no doubt was spun in the direction of Earth after another asteroid crashed into it. At some point it was exposed to water. Ironically, it can tell us more about the early history of the universe than anything on Earth because the planet has been so altered by oceans, volcanoes, the cycling of elements like carbon, and, most important, by life.

Glavin and Dworkin walked me down to the museum's meteorite collection, a treasure trove of gifts from outer space. Curator Linda Welzenbach got us gloved and began to take out more and more samples of Murchison, some in glass vials, some in plastic bags. In all, the museum has about sixty-five pounds of the meteorite, purchased by NASA from private collectors in 1971, two years after it fell. They sit in covered shelves inside a heavy-duty storage cabinet, hopefully not gathering dust. Welzenbach didn't hesitate when asked if we could take a look at some of her collection.

Holding pieces of Murchison was an unexpectedly moving experience. To be in physical contact with something so profoundly and undeniably old was to be, for one transporting moment, connected to that near eternity, to be drawn back to the Big Bang, the great mystery that science can now describe but not really explain. No wonder meteoriticists, as they're called, are a famously intense crew. They're in daily touch with the concrete reality of deep space and deep time, and their job is to understand what the rocks are telling them about both.

“There is certainly a constant debate in the origins-of-life community about what fraction of the organic material on the early Earth was made in situ, locally, by reactions on Earth versus extraterrestrial input,” said Glavin. “A lot of people argue the amount coming in from [extraterrestrial] forces—meteorites, interplanetary dust—wouldn't be enough… to get good chemistry to take place. You could argue it either way.”

“But the problem is the evidence is lost,” Dworkin interrupted. “You can infer things based on modern observations of meteorites and based on
laboratory experiments of what the chemistry might have been like. But the record is gone. And the best you can do is make intelligent guesses and see what's out there.” Glavin finds it paradoxical that no clear record exists of ancient, prebiotic chemistry on Earth, but that clues into the nature of those precursors to life arrive regularly from outer space. And so while trained and still immersed in the Miller-Urey tradition, Glavin and Dworkin nonetheless focus on meteorites and comets rather than the origins of the Earthly ingredients that might once have made up the primordial soup. A recent discovery of theirs, for instance, involved an Earth-bound meteorite that was tracked in space, landed in northern Sudan in 2008, and was collected in more than 600 pieces within weeks. The rock was initially formed when two asteroids collided and caused a heat shock of up to 2,000 degrees Fahrenheit, which is considered to be well beyond the point where all complex organic molecules (including amino acids) should have been destroyed. But Glavin and his team found the amino acids there anyway. Having determined that Earthly contamination was not involved, Glavin concluded the meteorite is telling us the very important news that there appear to be numerous ways to form the seemingly abundant amino acids in space, a dynamic that “increases the chance for finding life elsewhere in the universe.” Just as those amino acids fell on Earth and may well have played a role in the origin of life here, so too would they be falling on Mars, Europa, and planets and moons throughout the cosmos.