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Anybody Out There? By: Sacks, Oliver, Natural History, 00280712, Nov2002, Vol. 111, Issue 9
Environment Complete
Anybody Out There?
Section: REFLECTIONS Or is life, instead, "a glorious accident"?
One of the first books I read as a boy was H. G. Wells's 1901 fable, The First Men in the Moon. The two men, Cavor and Bedford, land in a crater, apparently barren and lifeless, just before the lunar dawn; then, as the Sun rises, they realize there is an atmosphere. They spot small pools and eddies of water, and then little round objects scattered on the ground. One of them, as it is warmed by the Sun, bursts open and reveals a sliver of green. ("'A seed,' said Cavor . . . . And then . . .very softly, 'Life!'") They light a piece of paper and throw it onto the surface of the Moon. It glows and sends up a thread of smoke, indicating that the atmosphere, though thin, is rich in oxygen and will support life as they know it.
Here, then, was how Wells conceived the prerequisites of life: water, sunlight (a source of energy), and oxygen. "A Lunar Morning" the eighth chapter in his book, was my first introduction to astrobiology.
It was apparent, even in Wells's day, that most of the planets in our solar system were not possible homes for life. The only reasonable surrogate for the Earth was Mars, which was known to be a solid planet of reasonable size, in stable orbit, not too distant from the sun, and so, it was thought, having a range of surface temperatures compatible with the presence of liquid water.
But free oxygen gas--how could that occur in a planet's atmosphere? What would keep it from being mopped up by ferrous iron and other oxygen-hungry chemicals on the surface, unless, somehow, it was continuously pumped out in huge quantities, enough to oxidize all the surface minerals and keep the atmosphere charged as well?
It was the blue-green algae, or cyanobacteria, that infused the Earth's atmosphere with oxygen, a process that took between a billion and two billion years. The fossil record shows that cyanobacteria go back three and a half billion years. Yet, amazingly, some of them still thrive today, in odd corners of the world, forming strange, cushion-shaped colonies called stromatolites [see photograph on opposite page]. It is an extraordinary experience to go to Shark Bay in western Australia, where stromatolites flourish in the hyper-saline waters, to watch them slowly bubbling oxygen, and to reflect that, three billion years ago, this was how the Earth was transformed. The cyanobacteria invented photosynthesis: by capturing the energy of the sun, they were able to combine carbon dioxide (massively present in the Earth's early atmosphere) with water to create complex molecules--sugars, carbohydrates--which the bacteria could then store and tap for energy as needed. This process generated free oxygen as a by- product--a waste product that was to determine the future course of evolution.
Listen American Accent
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Although free oxygen in a planet's atmosphere would be an infallible marker of life, and one that, if present, should be readily detected in the spectra of extrasolar planets, it is not a prerequisite for life. Planets, after all, get started without free oxygen, and may remain without it all their lives. Anaerobic organisms swarmed before oxygen was available, perfectly at home in the atmosphere of the early Earth, converting nitrogen to ammonia, sulfur to hydrogen sulfide, carbon dioxide to formaldehyde, and so forth. (From formaldehyde and ammonia the bacteria could make every organic compound they needed.)
There may be planets in our solar system and elsewhere that lack an atmosphere of oxygen but are nonetheless teeming with anaerobes. And such anaerobes need not live on the surface of the planet; they could occur well below the surface, in boiling vents and sulfurous hot pots, as they do on Earth today, to say nothing of subterranean oceans and lakes. (There is thought to be such a subsurface ocean on Jupiter's moon Europa, locked beneath a shell of ice several miles thick, and its exploration is one of the astrobiological priorities of this century. Curiously, Wells, in The First Men in the Moon, imagines life originating in a central sea in the middle of the Moon and then spreading outward to its inhospitable periphery.)
It is not clear whether life has to "advance"--whether evolution must take place--if there is a satisfactory status quo. Brachiopods-lampshells--for instance, have remained virtually unchanged since they first appeared in the Cambrian Period, more than 500 million years ago. But there does seem to be a drive for organisms to become more highly organized and more efficient in retaining energy, at least when environmental conditions are changing rapidly, as they were before the Cambrian. The evidence indicates that the first primitive anaerobes on Earth were prokaryotes: small, simple cells--just cytoplasm, usually bounded by a cell wall, but with little if any internal structure.
By degrees, however--and the process took place with glacial slowness--prokaryotes became more complex, acquiring internal structure, nuclei, mitochondria, and so on. The microbiologist Lynn Margulis of the University of Massachusetts, Amherst, has convincingly suggested that these complex so-called eukaryotes arose when prokaryotes began incorporating other prokaryotes within their own cells. The incorporated organisms at first became symbiotic and later came to function as essential organelles of their hosts, enabling the resultant organisms to utilize what was originally a noxious poison: oxygen.
Primitive as they are, prokaryotes are still highly sophisticated organisms, with formidable genetic and metabolic machinery. Even the simplest ones manufacture more than 500 proteins, and their DNA includes at least half a million base pairs. Hence it is certain that still more primitive life forms must have preceded the prokaryotes.
Perhaps, as the physicist Freeman Dyson of the Institute for Advanced Study in Princeton has suggested, there were "pro- genotes" capable of metabolizing, growing, and dividing but lacking any genetic mechanism for precise replication. And before them there must have been millions of years of purely chemical, prebiotic evolution--the synthesis, over eons, of formaldehyde and cyanide, of amino acids and peptides, of proteins and self-replicating molecules. Perhaps that chemistry took place in the minute vesicles, or globules, that develop when fluids at very different temperatures meet, as may well have happened around the boiling midocean vents of the Archaean sea.
Life as we know it is not imaginable without proteins, and proteins are built from peptides, and ultimately from amino acids. It is easy to imagine that amino acids were abundant in the early Earth, either formed as a result of lightning discharges or brought to the planet by comets and meteors.
The real problem is to get from amino acids and other simple compounds to peptides, nucleotides, proteins, and so on. It is unlikely that such delicate chemical syntheses would occur in "some warm little pond," as Darwin imagined, or on the surface of a primordial sea. Instead, they would probably require unusual conditions of heat and concentration, as well as the presence of special catalysts and energy-rich compounds to make them proceed. The biochemist Christian de Duve of Rockefeller University suggests that complex organic sulfur compounds played a crucial role in providing chemical energy, and that these compounds may have formed spontaneously early in Earth's history, perhaps in the hot, acidic, sulfurous depths of the seafloor vents (where, it is increasingly believed, life probably originated). De Duve imagines this purely chemical world as the precursor
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of an "RNA world," believed by many to represent the first form of self-replicating life. He thinks that the movement from one to the other was both inevitable and fast.
The two preeminent evolutionary changes in the early history of life on Earth--from prokaryote to eukaryote, from anaerobe to aerobe--took the better part of two billion years. And there then had to pass another 1,200 or 1,300 million years before life rose above the microscopic forms, and the first "higher" multicellular organisms appeared. So if the Earth's history is anything to go by, one should not expect to find any higher life on a planet that is still young. Even if extraterrestrial life has appeared, and all goes well, it could take billions of years for evolutionary processes to move it along to the multicellular stage.
Moreover, all those "stages" of evolution, including the evolution of intelligent, conscious beings from the first multicellular forms--may have happened against daunting odds. Stephen Jay Gould spoke of life as "a glorious accident"; Richard Dawkins of Oxford University likens evolution to "climbing Mount Improbable." And life, once started, is subject to vicissitudes of all kinds: from meteors and volcanic eruptions to global overheating and cooling; from dead ends in evolution to mysterious mass extinctions; and finally (if things get that far) from the fateful proclivities of a species like ourselves.
We know there are microfossils in some of the Earth's most ancient rocks, rocks more than three and a half billion years old. So life must have appeared within one or two hundred million years after the Earth had cooled off sufficiently for water to become liquid. That astonishingly rapid transformation makes one think that life may develop readily, perhaps inevitably, as soon as the right physical and chemical conditions appear.
But can one argue from a single example? Can one speak confidently of "earthlike" planets, or is the Earth physically, chemically, and geologically unique? And even if there are other "habitable" planets, what are the chances that life, with its thousands of physical and chemical coincidences and contingencies, will emerge? Life may be a one-off event.
Opinion here varies as widely as it can. The French biochemist Jacques Monod regarded life as a fantastically improbable accident, unlikely to have arisen anywhere else in the universe. In his book Chance and Necessity, he writes, "The universe was not pregnant with life." De Duve takes issue with this, and sees the origin of life as determined by a large number of steps, most of which must have had a "high likelihood of taking place under the prevailing conditions." Indeed, de Duve believes that there is not merely unicellular life throughout the universe, but complex, intelligent life, too, on trillions of planets. How are we to align ourselves between these utterly opposite, but theoretically defensible positions?
What we need, what we must have, is hard evidence of life on another planet or heavenly body. Mars is the obvious candidate: it was wet and warm there once, with lakes and hydrothermal vents and perhaps deposits of clay and iron ore. It is especially in such places that we should look, suggests Malcolm Walter, an expert on fossil bacteria that date from the Earth's earliest epochs. If the evidence shows that life once existed on Mars, we will then need to know, crucially, whether it originated there, or was transported (as would have been readily possible) from the young, teeming, volcanic Earth. If we can determine that life originated independently on Mars (if Mars, for instance, once harbored DNA nucleotides different from our own), we will have made an incredible discovery--one that will alter our view of the universe, and enable us to perceive it, in the words of the physicist Paul Davies, as a "bio-friendly" one. It would help us to gauge the probability of finding life elsewhere instead of bombinating in a vacuum of data, caught between the poles of inevitability and uniqueness.
In just the past twenty years life has been discovered in previously unexpected places on our own planet, such as the life-rich black smokers of the ocean depths, where organisms thrive in conditions biologists would once have dismissed as utterly deadly. Life is much tougher, much more resilient, than we once thought. It now seems to me quite possible that microorganisms or their remains will be found on Mars, and perhaps on some of the satellites of Jupiter and Saturn.
It seems far less likely, many orders of magnitude less likely, that we will find any evidence of higher-order, intelligent life forms, at least in our own solar system. But who knows? Given the vastness and age of the universe at large, the innumerable stars and planets it must contain, and our radical uncertainties about life's origin and evolution, the possibility cannot be ruled out.
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And though the rate of evolutionary and geochemical processes is incredibly slow, that of technological progress is incredibly fast. Who is to say (if humanity survives) what we may not be capable of, or discover, in the next thousand years?
For myself, since I cannot wait, I turn to science fiction on occasion--and, not least, back to my favorite Wells. Although it was written a hundred years ago, "A Lunar Morning" has the freshness of a new dawn, and it remains for me, as when I first read it, the most poetic evocation of how it may be when, finally, we encounter alien life.
Douglas Prince, Signs of Life: Oak Leaves over Mars, Schiaparelli Hemisphere, 1997
PHOTO (COLOR): Stromatolites are colonies of cyanobacteria. They began charging the Earth's atmosphere with oxygen some 3.5 billion years ago.
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By Oliver Sacks
Oliver Sacks is a neurologist and the author of nine books, including, most recently, Uncle Tungsten: Memories of a Chemical Boyhood and Oaxaca Journal. He lives in New York City.
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