Back when I was in high school, our chemistry textbook described the prevailing scientific theory of the origins of life in terms of a "primordial soup" (Figure 1). This model, generally attributed to Charles Darwin, posits that life arose from a nutrient-rich warm pond on the early Earth, and that it was a random occurrence. Before life could arise, however, there must have been complex biotic molecules (like those that make up living systems) that could somehow reproduce and therefore be subject to selection and evolve (at what point these molecules are "alive" is an interesting question, but outside the scope of this article). These molecules could only survive, let alone form, in very favorable conditions: a nice planet with liquid water, lots of nice ingredients to start with, and a source of energy to drive the chemistry that would lead to the formation of biotic molecules. What, however, those "favorable conditions" were, and how these very simple compounds could have resulted in biotic molecules, was a matter of conjecture until Miller and Urey's landmark experiments in the 1950s.
The now famous Miller-Urey experiments, devised to test primordial soup theory, showed how these kinds of biotic molecules might have arisen by cooking up a little Earth in a flask. At the time, scientists believed that Jupiter resembled the early Earth, with lots of hydrogen, methane, and ammonia gas. With a thick atmosphere of these gases and a warm ocean serving as the starting ingredients, all the recipe requires is that you heat the contents gently under a low to medium sun for a couple of hundreds of millions of years, stir with tides, and add lightning and or UV radiation to taste. When the first organisms start swimming around, it's ready to serve!
In reality, the experiment (see Figure 2) consisted of shooting sparks (lightning) into a glass sphere (the Earth) containing water (the ocean) and some simple gases such as ammonia, hydrogen, and methane (the atmosphere). The results were breathtaking. When they analyzed the stuff that was made in the experiment they found things such as amino acids (from which proteins and hence all living things are comprised) and purine bases (the steps in the spiral staircase that comprises DNA). Starting with some very simple compounds, a fairly simple apparatus, and a little energy, these biotic molecules, the fundamental components of living systems, had formed spontaneously. Although they were among the first to tackle the problem, and had only just begun, Urey and Miller had already made tremendous progress, and it seemed that very soon scientists would understand how the molecules that preceded life had come about. However, things may have been a bit more complex than was first thought.
At the time of the Miller-Urey experiments it was taken as a given that the water, air, and all the other chemical ingredients that went into the primordial soup were present on the early Earth in abundance. Although this is still presented in high school textbooks today, the notion that the oceans, atmosphere, and other components needed for life were always here has been seriously undermined by new knowledge and theories. Modern models of planet formation suggest that the Earth was a hot ball of molten rock from which the oceans and atmosphere had been swept away. Although it may be possible that air and water were sequestered deep below the surface of the forming Earth and percolated out later, it seems difficult to retain an entire terrestrial ocean and atmosphere under such conditions.
|Figure 3. This kilometer-wide crater, created by an impact releasing an explosive force greater than 10 megatons of TNT, was likely caused by a meteor merely tens of meters in diameter.|
Furthermore, we know from the number and size of craters, both on Earth and elsewhere, that the Earth has periodically been pounded by asteroids and comets, causing cataclysms equivalent to the detonation of countless atom bombs (it is now believed that our Moon formed in such an event from molten rock thrown into orbit when a Mars-sized object collided with the Earth). The largest of such impacts surely would have aborted any nascent life on the Earth at that time, and caused most of the atmosphere and oceans to boil away, depriving the Earth of any water and air that it had managed to retain from its formation (see Figure 3).
Thus, while the Miller-Urey experiments are among the greatest ever performed, there has been a realization among scientists that the Earth was probably deprived of much of its original ocean and atmosphere by its hot formation and repeated ocean-boiling impacts, such as the one which created the Moon. While this doesn't make the primordial soup impossible, it makes it much harder to cook.
|Figure 4. This IDP is about 10 microns across.|
Many scientists think that this implies that water and air must have come to the Earth after cooling and a period of intense bombardment. This new theory, which has gained strong support in recent years, proposes that the water and gases were delivered to the Earth by comets and other extraterrestrial objects; this is known as the exogenous (exo, from outside), or late veneer model. In this case we are considering not very large, Earth-shattering events, but rather the arrival of small comets and meteorites and much smaller objects such as interplanetary dust particles (IDPs). IDPs are microscopic grains of comet dust that litter the inner solar system -- they sometimes can be seen in the night sky as shooting stars (see Figure 4).
This is not wild conjecture; such cometary and asteroidal dust has been collected in the upper atmosphere using NASA's ER2 airplanes (a slightly larger version of the old U2) flying at 70,000 feet, and it has been estimated that literally tens of tons of such dust falls to Earth every day. Presumably, there was even more debris floating around earlier in the history of the solar system, so the flux of dust was greater when life arose, perhaps 4 billion years ago, than it is now.
To what extent can the dearth of water, for example, on the early Earth, have been compensated for by the arrival of such compounds from space? The ocean does not appear to be straight melted comet ice, since the abundance of deuterium (heavy hydrogen) in cometary water is three times greater than that in ocean water. But most scientists agree that rubble from space must have contributed a large volume of water (or an equivalent) to the Earth that was then diluted, either by another source (say asteroids), or by terrestrial water.
While it may be hard to conceive that even a fraction of the Earth's water could have come to us from space, one must recall that our oceans are only a thin layer on the planet's surface, a very small volume relative to the whole. Furthermore, comets are comprised mostly of water ice, so over the course of many, many years a planet could accumulate very large quantities of water as small pieces of comets were swept up. If the Earth did receive enough water from comets to make even a part of our oceans, then other planets must have, too. Thus, reports suggesting that there were once oceans of liquid water on Mars and Venus, and that there is still polar ice on our Moon, make sense.
Why, then, don't all planets and moons have oceans? The problem is not getting the water, but retaining it. Small planets and moons do not have sufficient gravity to hold onto oceans or an atmosphere, so the molecules tend to evaporate and escape into space. While small comets, IDPs, and meteorites were delivering water and gas throughout the inner solar system, the conditions on the other planets were not favorable for forming and maintaining life. This is what I like to call the Goldilocks theory: some were too small, some were too hot, only the Earth was juuust right. That is, just the right size, and in just the right place, to retain its water and air so it could harbor life for billions of years.
And, if water was brought to Earth, then so too were other compounds as well. Scientists are still assessing and revising what is known about the formation, modification, and delivery of molecules from space, and the implications of this work for the origins of life, but we now know that the molecules that were made in the Miller-Urey primordial soup experiment literally fell to Earth from the sky, free of charge. The list of such compounds reads like a chemical catalog of amino acids, purine and pyrimidine bases, ketones, aromatic and aliphatic hydrocarbons, and so on. Basically, everything that you could possibly want to make a habitable planet.
My own research attempts to understand where and how these kinds of compounds could arise in space, and by what processes they come to Earth. Do they form in our solar system as a result of fairly unusual circumstances, or through a more general galactic or even universal process? This bears directly on the question of how likely it is that we will find habitable, or inhabited, planets elsewhere in the galaxy. More and more evidence is mounting that indicates that large organic compounds, of the kind we see in meteorites, are seen not only throughout our galaxy, but in others as well!
Most chemists are used to thinking in terms of reactions that create molecules in liquids, such as water, because of their normal human experience. So, when scientists first began to consider how to do chemistry in the context of the creation of life, it was assumed that it would happen in liquid water, just as one sees in the kitchen. That's why it's a primordial soup. While many such reactions certainly did happen on the surface of the Earth, and they may have been critical to the whole process, it doesn't help us understand how to make large molecules out in the depths of space between the stars, where the temperature is so low that air would be frozen solid. Our group at NASA's Ames Research Center (and the SETI Institute) is engaged in simulating the conditions in space, and we find that it's surprisingly easy to make interesting organic compounds under common interstellar conditions.
What we do, in effect, is a Miller-Urey type experiment in space. Instead of cooking up a little Earth in a flask, as they did, we prepare a little comet, or a bit of interstellar space, in a metal box. We're serving up a frozen space daiquiri, rather than a primordial soup, but the ultimate goal is the same: to provide sustenance for organic molecules. The apparatus in which we make a little bit of space here on Earth is shown in Figure 5.
This apparatus is essentially an evacuated metal chamber that can be cooled to a few degrees Kelvin (-440 F), at the center of which a substrate (either a single large salt crystal, or a square of metal foil, depending upon on what is being measured) is suspended. It's difficult to get the temperature this low unless the chamber is under vacuum, so ours contains around one hundred billionth of an atmosphere. We then grow ices on the surface of this substrate by leaking gases into the chamber; at such low temperatures, almost everything (except hydrogen and helium) sticks and freezes solid. Once we've grown an ice, we measure its light absorption (including frequencies not visible to the human eye), in order to compare our results with astronomers' observations of interstellar matter; a taste test of our daiquiri, if you will. In addition, we can make other measurements, watching as the simple frozen gases are converted into larger molecules under the action of high-energy photons, identical to those in space.
For instance, from simple molecules of one carbon atom commonly observed in the interstellar medium, such as methyl alcohol, we make much larger, more complex molecules under conditions that are representative of dense interstellar clouds. These molecules are very similar to those that are seen in meteorites and IDPs, both in terms of structure and other properties they display. For example, we have shown that interstellar ice photochemistry provides an explanation for the presence of the chemical structures called quinones in meteorites. These structures are interesting because they are ubiquitous in nature playing key biochemical functions in all living things (i.e. mediating electron transport).
In addition to demonstrating structural similarities, it is also edifying to note other (especially functional) qualities our simulated products share with authentic extraterrestrial molecules. My colleague, Dr. Jason Dworkin, has shown that the molecules he makes in his simulations are able to self-assemble into vesicles (hollow bilayer membrane-like structures), which are believed by many to have been absolutely essential for the development of life. These structures had already been observed by our friend Dr. David Deamer (UC Santa Cruz) in his studies of carbon-rich meteorites. Thus, the first homes in which life resided may well have been constructed from materials made in the interstellar medium.
Another interesting attribute of molecules from space is that because they form or are modified near absolute zero, they are often rich in deuterium, as mentioned above. Indeed, this is so common and well known among space scientists that the presence of deuterium is often used as a method of testing if a compound is actually from space. While this use of deuterium as a demonstration of the interstellar pedigree of meteoritic molecules is accepted, it is still not fully understood how the deuterium gets there. In our experiments we sometimes deliberately add deuterium to one of the starting materials to see where it goes to try to understand the distribution of deuterium observed in meteorites and IDPs.
Finally, the feature that may constitute the most provocative evidence of a link between extraterrestrial amino acids and life on earth is chirality, or handedness. Certain organic molecules are constructed such that mirror-image forms of the molecule are identical in every way, except that they are not physically superimposable. This characteristic is called handedness, as it is analogous to our left and right hands, which are mirror images but not superimposable. This trait of certain organic molecules, including many amino acids, may provide a crucial clue to the link between extraterrestrial organic molecules and the origin of life.
It has long been known that life on Earth favors left-handed amino acids. Drs. John Cronin and Sandra Pizzarello of Arizona State University have recently shown that there is a slight excess of a few left-handed amino acids in carbon-rich meteorites. The fact that a number of different amino acids from more than one meteorite share this property with life on Earth suggests that the left-handedness of the amino acids in our bodies may have been determined by extraterrestrial input.
But why left-handed? It is well known that certain kinds of energy can have handedness, just as molecules do -- for example, photons and high energy particles (but not heat). There are a number of theories that involve the action of some kind of handed force or radiation selecting for left-handed amino acids. For example, it has been suggested that left-handed amino acids should be very slightly more stable because the (subatomic) weak force does not act equally on left- and right-handed molecules. However, this effect seems far too small to account for what Cronin and Pizzarello have observed.
It has been shown that one can destroy molecules of one particular handedness slightly more quickly than the other using circularly polarized radiation (cpr). So, it was suggested that perhaps cpr in the interstellar medium created the bias for left-handedness by destroying the right-handed ones preferentially. The problem here is that this process seems to be very inefficient, so one would need to have started with an absurdly large quantity of amino acids floating in space in order to end up with what we see today.
In a variation on this idea, recently some of us have been considering whether the amino acids could have been created by some kind of cpr such that the bias for left would be there from the start. Still, this process would have to be more efficient than destruction at selecting one particular handedness, and it has not yet been shown that one can selectively form molecules of a particular handedness using cpr, let alone if this process is efficent enough to explain what was reported by Drs. Cronin and Pizzarello. My colleague, Dr. Scott Sandford, and I hope to test this hypothesis in the next few years.
But even if we are right and cpr is the cause of handedness, how does one get cpr in space? Cpr can result from a rotating source, such as a spinning neutron star, which produces radiation of one particular handedness below, and of the opposite handedness above. If such a star were properly oriented relative to the dense molecular cloud from which our solar system formed, then left-handed amino acids would have been favored here; had the cloud been on the other side of the star, then right-handed amino acids would have been favored. An interesting consequence of this scenario is that left-handed amino acids would be favored not only in our solar system, but also in any other system forming from this cloud. However, planetary systems that formed elsewhere, from other clouds, might favor right-handed amino acids. On such a planet the food might well look edible, and chemical analysis would indicate that it was made of digestible amino acids, but it might be poison to an earthling.
In the absence of circularly polarized radiation, the initial molecules might show no propensity for either handedness. It is interesting to speculate about how life might arise in such a planetary system. Perhaps the first reproducing molecules would contain amino acids of one type or the other, and all life would follow from that one case. But what if it arose in a way that allowed for amino acids of both types to be useful? Presumably one handedness wins out over the other in any given lineage because the cost of maintaining two different sets of chemical machinery to deal with both kinds would be prohibitive. But one can imagine cases in which different organisms that used amino acids of different handedness would enjoy mutual advantages by symbiosis. If plants or colony organisms grew in layers, one composed of left-handed amino acids and the other of right-handed, then any predators that came to eat them would only get through one layer before having to break off since the other handedness, in our experience at least, can have ill effects.
This speculation about mirror image biota is wild conjecture, of course, but it is interesting to contemplate the consequences of compounds from space. The trajectory of early evolution, if not the origin of life, may have been dependent on the formation of molecules in ice grains in deep space billions of years ago. Given that the processes that make and deliver these compounds are universal, this may increase the chances that, if there is life elsewhere in the universe, it looks like us -- on a molecular level.
Dr. Max Bernstein is a space scientist working at the NASA Ames Research Center through a cooperative agreement with the SETI Institute. He studies the photochemistry of ices in interstellar clouds, comets, and other bodies in the outer solar system; you can learn more about his research by visiting his lab's website. Dr. Bernstein purchased the soul of this article's editor for 50 cents when they were in high school together in the late 1970s; keep an eye out for it on eBay.
This article has been translated into Portuguese.
Bakes, E.L.O. The Astrochemical Evolution of the Interstellar Medium. A nice slim basic text that covers everything from the first seconds of the universe to the origins of life.
M. P. Bernstein, S. A. Sandford, L. J. Allamandola, J. S. Gillette, S. J. Clemett, and R. N. Zare. "UV Irradiation of Polycyclic Aromatic Hydrocarbons in Ices: Production of Alcohols, Quinones, and Ethers." Science, Vol. 283, February 19, 1999.
M.P. Bernstein, S. A. Sandford, and L. J. Allamandola. "Life's Far-Flung Raw Materials." Scientific American, July 1999.
Bernstein, M. P., J. P. Dworkin, S. A. Sandford, and L. J. Allamandola. "Ultraviolet Irradiation of Naphthalene in H2O Ice: Implications for Meteorites and Biogenesis." Meteoritics and Planetary Science, Vol. 36, 2001.
Pendleton, Y. J., and J. D. Farmer. "Life: A Cosmic Imperative?" Sky and Telescope, Vol. 94, No.1, July 1997.
"Stuff of Life" -- an online article from New Scientist on the related work of Dr. Jason Dworkin.
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