Sky & Telescope June 1992 [antikvár]

I M ' l.i-JI,FOCAL POINTWhence Came Life?Scientists now generally accept that life arose on Earth early in its history The initial step was to produce organic building blocks, which in turn combined to form ever more complex structures that ultimately became self-replicating. But how did the early Earth's "primordial soup" get thick enough to allow life to gain a foothold here? Two investigators of this problem come to differing conclusions.SIMPLE organic compounds would have been synthesized very easily on the primitive Earth if the chemical environment was suitably reducing, that is, if the atmosphere contained reasonable amounts of free hydrogen (Hj) or methane (CH4) from which to create organic compounds. Even a trace of methane not much more than the current levels of water vapor or sulfur dioxide would have been enough for the needed reactions. Some mixtures of carbon dioxide and hydrogen would have worked as well.Given such a reducing atmosphere, the production of organic matter on the early Earth would have far outstripped any possible input from meteorites, asteroids, comets, or cosmic dust. Reactions would be driven by energy from electrical discharges and ultraviolet light. These synthesizing processes are so efficient that they could have maintained a relatively rich broth of amino acids in the primitive ocean estimated at 0.003 percent (3 x 10-" molar). However, in the absence of a reducing atmosphere, the production rates of amino acids and other organic compounds would plummet to rates and concentrations perhaps only 'Ao.m as high.Thus, knowing the makeup of Earth's primitive atmosphere is fundamental to this whole discussion. Unfortunately, that composition is simply not known. Most atmospheric chemists and geochemists now favor neutral atmospheres of CO. and N,. (The present atmosphere of COj, N2, and O2 is termed oxidizing.) However, their supposition is based not on hard evidencebut on calculations and extrapolations backward in time from the geological record of 3.5 and 3.8 billion years ago. Prebiotic chemists, on the other hand, generally favor reducing atmospheres. After all, this is what we find on Jupiter, Saturn, and Titan along with plenty of organic molecules!The input of amino acids from mete-orites and asteroids, assuming they survived impact, would have been negligible. Organic-laced interplanetary dust particles would have contributed little compared to the production rates in a suitably reducing atmosphere or been at most comparable to the rates in a neutral atmosphere. Thus extraterrestrial sources become important only by assuming the best case for them and the worst case for Earth-based reactions.Even interplanetary dust particles have their problems. Their carbon is the wrong type an unknown, intractable polymer that has been "cooked" by interstellar cosmic rays and ultraviolet light. It is difficult to see what role they could have played in the processes that created life.By contrast, the important organic compounds in living organisms amino acids, purines, pyrimidines, sugars, and so on are among those produced in the highest yields in experiments designed to simulate prebiotic synthesis. Thus, the early Earth's own chemical factories would have been excellent sources of the organic com-pounds important in biology today and, presumably, in the first life forms.Comets do contain large amounts of hydrogen cyanide (HCN) and formaldehyde (H2CO), both of which are central to the prebiotic reactions that produce amino acids, purines, and sugars. But, again, comets would be a minor player if Earth initially had a reducing atmosphere. Further, it's not even clear that the comets' organic material could have survived the high-velocity impacts that brought it here.Another key question is whether either extraterrestrial sources or Earth-based synthesis in a neutral atmosphere provided a thick enough "broth" for life to arise. Prebiotic polymer synthesis could not take place if the ocean's concentration of organic matter was too low. The means to concentrate these building blocks did exist, but they would have been ineffective if there were too little of the organic matter to begin with.Taking all this together. it seems the primitive Earth's own synthesis of organic compounds would have been the dominant source unless it had, as now. a neutral atmosphere. In the latter case, extraterrestrial inputs would have provided a significant fraction of the Earths organic inventory, but perhaps not in quantities sufficient for life to arise.STANLEY L. MILLERA professor of chemistry at the University of California, San Diego. Miller ond the late Harold Urey won internaiional acclaim in 1953 for their pioneering synthesis of prebiotic compounds from simple gases.The LAST several years have seen a resurgence of interest in the role that extraterrestrial objects may have played in delivering life's precursor molecules to the early Earth. The idea is not new. Ever since the Swedish chemist Jons Berzelius first analyzed the Alais meteorite for organic molecules in 1834, the possibility has remained alluring though often deceptive. But only recently have we learned enough about extraterrestrial sources to604 Sky & Telescope, June, 1992