Even as the debate between Miller’s advocates and the ventists was heating up, an explosion of new research dramatically changed the research community’s view of the emergence of biomolecules. When Miller first reported organic synthesis on a benchtop in 1953, the results seemed almost magical. Fifty years ago, no one could have predicted how easy it would be to make amino acids, sugars, and other key biomolecules from water and gas. But the more scientists study carbon chemistry in a wide range of plausible, energetic prebiotic environments, the more diverse and facile organic synthesis seems to be. It now appears that anywhere energy and simple carbon-rich molecules are found together, a suite of interesting organic molecules is sure to emerge. It’s all a matter of environment, and it now appears that the universe boasts an extraordinary range of productive environments.

The last place you might think to look for life-forming molecules is the black void of interstellar space, but new research reveals that organic molecules from space must have predated Earth by billions of years. Deep space, we now realize, is home to immense tenuous clouds where carbon, hydrogen, oxygen, and nitrogen combine in complex sequences of reactions.

A research team at NASA Ames Research Center at Moffett Field,

California, led by veteran astrochemist Louis Allamandola, has simulated the ultracold deep-space environments of these so-called dense molecular clouds (though these vast volumes of dust and gas are far less dense than the highest vacuum attainable on Earth). A typical interstellar cloud harbors only a measly million atoms per cubic inch, at temperatures colder than −100°C. Such high vacuums and frigid temperatures would seem to preclude any sort of chemical reaction, but in these remote regions, minute ice-covered dust particles are subjected to ultraviolet radiation from distant stars. Gradually, as molecules absorb this radiation, they become sufficiently reactive to form larger collections of atoms. Radio astronomers have long recognized the distinctive signatures of numerous organic species in these clouds. Each type of molecule absorbs or emits characteristic wavelengths of light—features that appear as sharp lines on a radio spectrum. The most abundant molecules are the diatomic and triatomic species, such as CO, H₂, CO₂, and H₂O, but more than 140 different compounds are known, including many larger molecules with a dozen atoms or more.

Theorists easily explain such molecular diversity. They calculate the efficiency with which small cold molecules condense onto tiny dust particles, forming submicroscopic ice coatings. They predict details of how icy particles occasionally absorb ultraviolet radiation, which can shuffle electrons and trigger chemical reactions. They plot reaction cascades by which small groups of atoms clump together and slowly cause new larger molecules to accumulate in the cloud. Eventually, under the pervasive inward pull of gravity, local regions of a molecular cloud can collapse into a new planetary system with a central massive star and an array of planets and moons. As each body forms, a steady rain of organic-rich comets and asteroids contributes to the life-forming inventory. So, the theorists tell us, organic molecules inevitably constitute part of any planet-forming mix.

Regardless of how convincing a theory may sound, experiments carry a lot of weight in science. Allamandola and co-workers’ experiments at NASA Ames have exploited an elegant chilled vacuum chamber, about 8 inches in diameter, crafted of shiny stainless steel, and equipped with thick glass observation ports, to produce suites of organic molecules. First, they introduce a fine spray of simple gas molecules, such as water, carbon monoxide, methane, and ammonia, into the chamber, where the gases freeze onto an aluminum disk. Then they bathe the thin ice layer with a beam of ultraviolet radiation, which

triggers the formation of larger molecules—compounds that match the distinctive molecular emissions from those distant clouds. [Plate 5]

The NASA team has used their benchtop apparatus to produce a rich variety of interesting molecules: reactive nitriles, ethers, and alcohols abound, as do ringlike hydrocarbons. One set of experiments yielded nitrogen-bearing precursor molecules to amino acids. Another set generated long chainlike molecules reminiscent of the building blocks of cell membranes.

Evidence from space amply buttresses these nifty experiments. The Murchison meteorite and many other carbon-rich meteorites are loaded with organic molecules thought to be of extraterrestrial origin. Comets, too, are known to be rich in the molecular precursors of life, as are the microscopic interplanetary dust particles that incessantly drift down to Earth’s surface. Armed with their vacuum chamber, the Ames team can reproduce the supposed deep-space synthesis processes in the lab. Theory, observations, and experiments agree: The prebiotic Earth was seeded abundantly with extraterrestrial organics.

Nevertheless, the Miller crowd is unpersuaded by these studies, too. Says Miller, “Organics from outer space, that’s garbage, it really is.” Jeff Bada echoes, “Even if cosmic debris struck the prebiotic Earth at 10,000 times the present levels, the resultant prebiotic soup would still have been much too weak to engender life.”

Meteorites and comets carry a rich inventory of organic molecules, but can these molecules survive the catastrophic insults of collisions with Earth? Deep-space synthesis, no matter how fecund, would be irrelevant to life’s origin if the intense temperatures and pressures of impact disintegrated molecules.

It’s hard to imagine an environment more destructive to life and its molecules than the shattering surface impact of an asteroid or comet. Nevertheless, carbon-rich meteorites like the Murchison contain a significant store of amino acids and other potential biomolecules; evidently impacts don’t destroy all organic molecules. In fact, recent experiments suggest quite the opposite. Jennifer Blank and her colleagues at the Lawrence Berkeley National Laboratory, in Berkeley, California, use a giant experimental gas gun that hurls hyperfast chunks

of metal at innocent rocks. Their goal is to trace the fates of organic molecules during these violent collisions.

Blank’s experiments begin with a flattened cylindrical stainless-steel capsule about 1 inch in diameter that is filled with a solution of five different amino acids in water. She carefully positions the sealed sample in a metal well—the target at the end of a 40-foot-long gun barrel.

“Clear the room!” she demands, as they close the gun chamber.

A technician powers up her weapon. “Three, two, one, fire!” and blam!, a tremendous shock wave shakes the building as a massive metal projectile hurls down the barrel at more than 4,000 miles per hour and squashes her neatly prepared sample like a bug. For a few microseconds, the amino acid solution experiences pressures in excess of 200,000 atmospheres at temperatures approaching 900°C.

Then the fun begins. Blank pries out her deformed steel capsule and mills down the metal to extract a few drops of liquid. The original clear solution has turned a dark brown color—something interesting has happened to the amino acids. The organic chemists’ standard analytical techniques, chromatography and mass spectrometry, tell the story. To be sure, most of the original amino acids are lost in every run. But, remarkably, some of the delicate molecules react with each other to form pairs of amino acids. The formation of these peptide bonds between amino acids is a crucial step in the assembly of proteins.

Jennifer Blank’s highly publicized conclusion: Impacts on the early Earth may have reduced the quantity of organic molecules, but at the same time they increased the diversity of complex prebiotic chemical species.

Of all scenarios for the prebiotic production of organic molecules, none is more original (and correspondingly controversial) than the idea of Friedemann Freund, a longtime researcher at the NASA Ames Research Center. He claims that igneous rocks were, and still are, a principal source of Earth’s organic molecules. “Maybe,” he remarked to Wes Huntress, the Geophysical Lab’s director, “the next chapter in the origin of life is written in the solid state—in the dense, hard, seemingly hostile matrix of crystals.”

Freund, who is as persistent and unflappable as anyone you’re ever

likely to meet, smiles and quietly presents his case. Tall, lean, with a shock of graying hair, he speaks gently, with a slight German accent and lots of eye contact. He’s always ready to talk about what he’s doing and seldom expresses the slightest doubt that he’s onto something important.

Here’s how he claims it happens. At high temperatures, every melt contains some dissolved impurities. Molten rocks are no exception; they always incorporate a little bit of water, carbon dioxide, and nitrogen. As the melt cools, minerals begin to crystallize one after another. The first mineral might be rich in magnesium, silicon, and oxygen, but inevitably it will also incorporate a small amount of carbon and nitrogen—elements that don’t easily enter the crystal lattice. These residual elements concentrate along crystal defects—zipperlike elongated spaces where the foreign atoms can react and, according to Freund, ultimately form chainlike molecules with a carbon backbone. Freund suspects that every igneous rock has the potential to manufacture such organic molecules. When the rocks weather away, so the story goes, they release vast amounts of organic carbon into the environment.

Many scientists would say that’s a wacky idea. “I am a hundred percent sure that the Freund paper is utter nonsense,” asserts Washington University mineralogist Anne Hofmeister. “Most igneous rocks form from an incandescent melt at temperatures greater than 1,000°C—temperatures at which even the hardiest organic molecule is fragmented into carbon dioxide and water. By contrast, organic contamination is everywhere in our environment.” What causes Freund’s observed organics? “It’s surface residues,” Hofmeister says, “probably sorbed out of the air.”

Freund rests his case on two sets of samples he has been studying for almost a quarter century. Two-inch-long synthetic magnesium oxide (MgO) crystals, produced by cooling a white-hot MgO melt from 2,860°C, serve as a simple model system. Pure MgO should be clear and colorless, but Freund’s crystals have a cloudy, turbid interior, suggestive of pervasive impurities. Infrared spectra reveal the sharp absorption features of carbon-hydrogen and oxygen-hydrogen bonding, both characteristic of organic molecules. Studies of the crystals’ unusually high electrical conductivity and other anomalous properties have further convinced him that the supposed MgO crystals are loaded with excess carbon and hydrogen. The clincher: Subsequent analyses of molecules extracted from crushed MgO crystals reveal the presence

of carboxylic acids, which just happen to be essential molecules in the metabolism of all cells.

Studies of natural gem-quality olivine, an attractive green mineral that is among the commonest constituents of igneous rocks, complement Freund’s work on synthetic MgO. Once again, his spectroscopic studies revealed C–H and O–H bonds; once again, he extracted organic molecules from crushed powders. Olivine crystals hold an astonishing 100 parts per million carbon, he claims. Furthermore, much of that carbon occurs in biologically interesting, chainlike organic molecules.

Others remain unconvinced. Caltech mineralogist George Rossman duplicated some of Freund’s olivine results with dirty crystals. “I ran a sample of ours that had been standing around for a while,” he told Anne Hofmeister in 2002. “It had the organic bands. I washed it off with organic solvent and re-ran it. No organic bands.” Organic contamination is everywhere, so any surface—especially any powder—no matter how well cleaned, will quickly become loaded with adsorbed organic molecules. Freund counters that the types of molecules he extracts, carboxylic acids, are not typical of any ordinary environmental contamination. They must have come from inside the mineral.

Freund had won relatively few converts by the summer of 2003, when he came to George Cody’s lab to duplicate his extraction of molecules from olivine. For several weeks, a white-coated Freund was an amiable fixture at Cody’s lab bench. He meticulously washed and powdered the semiprecious stones, extracted carbon compounds with strong solvents, and analyzed the samples with Cody’s battery of high-tech instruments. Sure enough, every crystal seemed to release a small hoard of carbon-rich molecules. There wasn’t much, certainly, but the volume of igneous rock that has formed and eroded over the course of geological history is immense. So, by Freund’s estimates, solid rocks have provided one of Earth’s largest and most continuous sources for the emergence of biomolecules.

Scientific progress involves a long process of hypothesis and testing, bold claims, and critical counterarguments. Not surprisingly, Freund’s hypothesis has received a lot of scrutiny and not a little disdain. But those unexplained carboxylic acids can’t be ignored. And so, for the time being, the jury is still out.

Where did life’s crucial molecules form? In spite of the polarizing advocacy of one favored environment or another by this group and that, experiments increasingly point to the possibility that there was no single dominant source.

It’s not a matter of Millerites versus ventists, or deep space versus Earth’s surface. Many ancient environments boasted carbon atoms and sufficient energy to initiate their chemical transformations. Many environments must have contributed to the prebiotic inventory. Lightning-sparked gases were a major source, to be sure, as were UV-triggered reactions high in the atmosphere. Deep in the ocean, in environments ranging from lukewarm to boiling hot, molecules must have been made in abundance, as they certainly were within some reactive rocks of the crust (and, if Tom Gold is correct, perhaps in the much deeper mantle). A wealth of organic products also rained down from space, formed in remote dense molecular clouds and concentrated in the carbon-rich meteorites and asteroids that coalesced to make our planet.

The bottom line is that the prebiotic Earth had an embarrassment of organic riches derived from many likely sources. Carbon-rich molecules emerge from every conceivable environment. Amino acids, sugars, hydrocarbons, bases—all the key molecular species are there.

So the real challenge turns out to be not so much the making of molecules, but the selection of just the right ones and their assembly into the useful structures we call macromolecules. That process required a higher level of emergence.

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