The Miller-Urey Experiment

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Icons of Evolution? Why Much of What Jonathan Wells Writes about Evolution is Wrong Alan D. Gishlick, National Center for Science Education THE MILLER-UREY how the early atmosphere was probably differ- EXPERIMENT ent from the atmosphere hypothesized in the original experiment. Wells then claims that the THE EXPERIMENT ITSELF actual atmosphere of the early earth makes the he understanding of the origin of life Miller–Urey type of chemical synthesis was largely speculative until the 1920s, impossible, and asserts that the experiment Twhen Oparin and Haldane, working does not work when an updated atmosphere is independently, proposed a theoretical model used. Therefore, textbooks should either dis- for “chemical evolution.” The Oparin– cuss the experiment as an historically interest- Haldane model suggested that under the ing yet flawed exercise, or not discuss it at all. strongly reducing conditions theorized to have Wells concludes by saying that textbooks been present in the atmosphere of the early should replace their discussions of the Miller– earth (between 4.0 and 3.5 billion years ago), Urey experiment with an “extensive discus- inorganic molecules would spontaneously sion” of all the problems facing research into form organic molecules (simple sugars and the origin of life. amino acids). In 1953, Stanley Miller, along These allegations might seem serious; how- with his graduate advisor Harold Urey, tested ever, Wells’s knowledge of prebiotic chemistry this hypothesis by constructing an apparatus is seriously flawed. First, Wells’s claim that that simulated the Oparin-Haldane “early researchers are ignoring the new atmospheric earth.” When a gas mixture based on predic- data, and that experiments like the Miller– tions of the early atmosphere was heated and Urey experiment fail when the atmospheric given an electrical charge, organic compounds composition reflects current theories, is simply were formed (Miller, 1953; Miller and Urey, false. The current literature shows that scien- 1959). Thus, the Miller-Urey experiment tists working on the origin and early evolution demonstrated how some biological molecules, of life are well aware of the current theories of such as simple amino acids, could have arisen the earth’s early atmosphere and have found abiotically, that is through non-biological that the revisions have little effect on the processes, under conditions thought to be sim- results of various experiments in biochemical ilar to those of the early earth. This experiment synthesis. Despite Wells’s claims to the con- provided the structure for later research into trary, new experiments since the Miller–Urey the origin of life. Despite many revisions and ones have achieved similar results using vari- additions, the Oparin–Haldane scenario ous corrected atmospheric compositions remains part of the model in use today. The (Figure 1; Rode, 1999; Hanic et al., 2000). Miller–Urey experiment is simply a part of the Further, although some authors have argued experimental program produced by this para- that electrical energy might not have efficient- digm. ly produced organic molecules in the earth’s early atmosphere, other energy sources such as WELLS BOILS OFF cosmic radiation (e.g., Kobayashi et al., 1998), ells says that the Miller–Urey exper- high temperature impact events (e.g., iment should not be taught because Miyakawa et al., 2000), and even the action of Wthe experiment used an atmospheric waves on a beach (Commeyras et al., 2002) composition that is now known to be incorrect. would have been quite effective. Wells contends that textbooks don’t discuss Even if Wells had been correct about the 3 Icons of Evolution? Why Much of What Jonathan Wells Writes about Evolution is Wrong Alan D. Gishlick, National Center for Science Education

Researcher(s) Year Reactants Energy source Results Probability Miller 1953 CH4, NH3, H2O, H2 Electric discharge Simple amino acids, unlikely organic compounds Abelson 1956 CO, CO2, N2, NH3, H2, Electric discharge Simple amino acids, unlikely H2O HCN Groth and Weyssenhoff 1957 CH4, NH3, H2O Ultraviolet light Simple amino acids (low under special conditions (1470–1294 ?) yields) Bahadur, et al. 1958 Formaldehyde, Sunlight Simple amino acids possible molybdenum oxide (photosynthesis) Pavolvskaya and 1959 Formaldehyde, nitrates High pressure Hg lamp Simple amino acids possible Pasynskii (photolysis) Palm and Calvin 1962 CH4, NH3, H2O Electron irradiation Glycine, alanine, aspartic under special conditions acid Harada and Fox 1964 CH4, NH3, H2O Thermal energy 14 of the “essential” under special conditions (900–1200º C) amino acids of proteins Oró 1968 CH4, NH3, H2OPlasma jet Simple amino acids unlikely Bar-Nun et al. 1970 CH4, NH3, H2O Shock wave Simple amino acids under special conditions Sagan and Khare 1971 CH4, C2H6, NH3, H2O, Ultraviolet light (>2000 Simple amino acids (low under special conditions H2S ?) yields) Yoshino et al. 1971 H2, CO, NH3, Temperature of 700°C Glycine, alanine, unlikely montmorillonite glutamic acid, serine, aspartic acid, leucine, lysine, arginine Lawless and Boynton 1973 CH4, NH3, H2O Thermal energy Glycine, alanine, aspartic under special conditions acid, ?-alanine, N-methyl-?-alanine, ?-amino-n-butyric acid. Yanagawa et al. 1980 Various sugars, Temperature of 105°C Glycine, alanine, serine, under special conditions hydroxylamine, aspartic acid, glutamic inorganic salts, acid Kobayashi et al. 1992 CO, N2, H2O Proton irradiation Glycine, alanine, aspartic possible acid, ?-alanine, glutamic acid, threonine, ?-aminobutyric acid, serine Hanic, et al. 1998 CO2, N2, H2O Electric discharge Several amino acids possible Figure 1. A table of some amino acid synthesis experiments since Miller–Urey. The “probability” column reflects the likelihood of the environmental conditions used in the experiment. Modified from Rode, 1999. Miller–Urey experiment, he does not explain since 1961 (see Oró, 1961; Whittet, 1997; that our theories about the origin of organic Irvine, 1998). Wells apparently missed the vast “building blocks” do not depend on that exper- body of literature on organic compounds in iment alone (Orgel, 1998a). There are other comets (e.g. Oró, 1961; Anders, 1989; Irvine, sources for organic “building blocks,” such as 1998), carbonaceous meteorites (e.g., Kaplan meteorites, comets, and hydrothermal vents. et al., 1963; Hayes, 1967; Chang, 1994; All of these alternate sources for organic mate- Maurette, 1998; Cooper et al., 2001), and con- rials and their synthesis are extensively dis- ditions conducive to the formation of organic cussed in the literature about the origin of life, compounds that exist in interstellar dust clouds a literature that Wells does not acknowledge. (Whittet, 1997). In fact, what is most striking about Wells’s Wells also fails to cite the scientific litera- extensive reference list is the literature that he ture on other terrestrial conditions under which has left out. Wells does not mention extrater- organic compounds could have formed. These restrial sources of organic molecules, which non-atmospheric sources include the synthesis have been widely discussed in the literature of organic compounds in a reducing ocean 4 Icons of Evolution? Why Much of What Jonathan Wells Writes about Evolution is Wrong Alan D. Gishlick, National Center for Science Education (e.g., Chang, 1994), at hydrothermal vents troversy is really over why it took so long for (e.g., Andersson, 1999; Ogata et al., 2000), and oxygen levels to start to rise. Current data in volcanic aquifers (Washington, 2000). A show that oxygen levels did not start to rise cursory review of the literature finds more than significantly until nearly 1.5 billion years after 40 papers on terrestrial prebiotic chemical syn- life originated (Rye and Holland, 1998; thesis published since 1997 in the journal Copley, 2001). Wells strategically fails to clar- Origins of life and the evolution of the bios- ify what he means by “early” when he discuss- phere alone. Contrary to Wells’s presentation, es the amount of oxygen in the “early” atmos- there appears to be no shortage of potential phere. In his discussion, he cites research sources for organic “building blocks” on the about the chemistry of the atmosphere without early earth. distinguishing whether the authors are refer- Instead of discussing this literature, Wells ring to times before, during, or after the period raises a false “controversy” about the low when life is thought to have originated. Nearly amount of free oxygen in the early atmos- all of the papers he cites deal with oxygen lev- phere. Claiming that this precludes the sponta- els after 3.0 billion years ago. They are irrele- neous origin of life, he concludes that vant, as chemical data suggest that life arose “[d]ogma had taken the place of empirical sci- 3.8 billion years ago (Chang, 1994; Orgel, ence” (Wells, 2000:18). In truth, nearly all 1998b), well before there was enough free researchers who work on the early atmosphere oxygen in the earth’s atmosphere to prevent hold that oxygen was essentially absent during Miller–Urey-type chemical synthesis. the period in which life originated (Copley, Finally, the Miller–Urey experiment tells us 2001) and therefore oxygen could not have nothing about the other stages in the origin of played a role in preventing chemical synthesis. life, including the formation of a simple genet- This conclusion is based on many sources of ic code (PNA or “peptide”-based codes and data, not “dogma.” Sources of data include RNA-based codes) or the origin of cellular fluvial uraninite sand deposits (Rasmussen and membranes (liposomes), some of which are Buick, 1999) and banded iron formations discussed in all the textbooks that Wells (Nunn, 1998; Copley, 2001), which could not reviewed. The Miller–Urey experiment only have been deposited under oxidizing condi- showed one possible route by which the basic tions. Wells also neglects the data from pale- components necessary for the origin of life osols (ancient soils) which, because they form could have been created, not how life came to at the atmosphere–ground interface, are an be. Other theories have been proposed to excellent source to determine atmospheric bridge the gap between the organic “building composition (Holland, 1994). Reduced pale- blocks” and life. The “liposome” theory deals osols suggest that oxygen levels were very low with the origin of cellular membranes, the before 2.1 billion years ago (Rye and Holland, RNA-world hypothesis deals with the origin of 1998). There are also data from mantle chem- a simple genetic code, and the PNA (peptide- istry that suggest that oxygen was essentially based genetics) theory proposes an even sim- absent from the earliest atmosphere (Kump et pler potential genetic code (Rode, 1999). Wells al., 2001). Wells misrepresents the debate as doesn’t really mention any of this except to over whether oxygen levels were 5/100 of 1%, suggest that the “RNA world” hypothesis was which Wells calls “low,” or 45/100 of 1%, proposed to “rescue” the Miller–Urey experi- which Wells calls “significant.” But the con- ment. No one familiar with the field or the evi- 5 Icons of Evolution? Why Much of What Jonathan Wells Writes about Evolution is Wrong Alan D. Gishlick, National Center for Science Education dence could make such a fatuous and inaccu- deep-sea hydrothermal vents (Figure 2). No rate statement. The Miller–Urey experiment is textbook claims that these experiments conclu- not relevant to the RNA world, because RNA sively show how life originated; and all text- was constructed from organic “building books state that the results of these experi- blocks” irrespective of how those compounds ments are tentative. came into existence (Zubay and Mui, 2001). It is true that some textbooks do not mention The evolution of RNA is a wholly different that our knowledge of the composition of the chapter in the story of the origin of life, one to atmosphere has changed. However, this does which the validity of the Miller–Urey experi- not mean that textbooks are “misleading” stu- ment is irrelevant. dents, because there is more to the origin of WHAT THE TEXTBOOKS SAY life than just the Miller–Urey experiment. Most textbooks already discuss this fact. The ll of the textbooks reviewed contain a textbooks reviewed treat the origin of life with section on the Miller–Urey experi- varying levels of detail and length in “Origin Ament. This is not surprising given the of life” or “History of life” chapters. These experiment’s historic role in the understanding chapters are from 6 to 24 pages in length. In of the origin of life. The experiment is usually this relatively short space, it is hard for a text- discussed over a couple of paragraphs (see book, particularly for an introductory class like Figure 2), a small proportion (roughly 20%) of high school biology, to address all of the the total discussion of the origin and early evo- details and intricacies of origin-of-life research lution of life. Commonly, the first paragraph that Wells seems to demand. Nearly all texts discusses the Oparin-Haldane scenario, and begin their origin of life sections with a brief then a second outlines the Miller–Urey test of description of the origin of the universe and that scenario. All textbooks contain either a the solar system; a couple of books use a dis- drawing or a picture of the experimental appa- cussion of Pasteur and spontaneous generation ratus and state that it was used to demonstrate instead (and one discusses both). Two text- that some complex organic molecules (e.g., books discuss how life might be defined. simple sugars and amino acids, frequently Nearly all textbooks open their discussion of called “building blocks”) could have formed the origin of life with qualifications about how spontaneously in the atmosphere of the early the study of the origin of life is largely hypo- earth. Textbooks vary in their descriptions of thetical and that there is much about it that we the atmospheric composition of the early earth. do not know. Five books present the strongly reducing atmosphere of the Miller–Urey experiment, WELLS’SEVALUATION whereas the other five mention that the current s we will see in his treatment of the geochemical evidence points to a slightly other “icons,” Wells’s criteria for judg- reducing atmosphere. All textbooks state that Aing textbooks stack the deck against oxygen was essentially absent during the peri- them, ensuring failure. No textbook receives od in which life arose. Four textbooks mention better than a D for this “icon” in Wells’s eval- that the experiment has been repeated success- uation, and 6 of the 10 receive an F. This is fully under updated conditions. Three text- largely a result of the construction of the grad- books also mention the possibility of organic ing criteria. Under Wells’s criteria (Wells, molecules arriving from space or forming at 2000:251–252), any textbook containing a pic- 6 Icons of Evolution? Why Much of What Jonathan Wells Writes about Evolution is Wrong Alan D. Gishlick, National Center for Science Education

Figure 2. Textbook treatments of the Miller–Urey experiment. Textbooks are listed in order of increasing detail (AP/College textbooks highlighted; note that Futuyma is an upper-level college/graduate textbook). ture of the Miller–Urey apparatus could pictures. Wells’s criteria would require that receive no better than a C, unless the caption of even the intelligent design “textbook” Of the picture explicitly says that the experiment Pandas and People would receive a C for its is irrelevant, in which case the book would treatment of the Miller–Urey experiment. receive a B. Therefore, the use of a picture is In order to receive an A, a textbook must the major deciding factor on which Wells eval- first omit the picture of the Miller–Urey appa- uated the books, for it decides the grade irre- ratus (or state explicitly in the caption that it spective of the information contained in the was a failure), discuss the experiment, but then text! A grade of D is given even if the text state that it is irrelevant to the origin of life. explicitly points out that the experiment used This type of textbook would be not only scien- an incorrect atmosphere, as long as it shows a tifically inaccurate but pedagogically defi- picture. Wells pillories Miller and Levine for cient. exactly that, complaining that they bury the correction in the text. This is absurd: almost all textbooks contain pictures of experimental apparatus for any experiment they discuss. It is WHY WE SHOULD STILL TEACH the text that is important pedagogically, not the MILLER–UREY 7 Icons of Evolution? Why Much of What Jonathan Wells Writes about Evolution is Wrong Alan D. Gishlick, National Center for Science Education he Miller–Urey experiment represents of scientific understanding of the origin of life. one of the research programs spawned This is the kind of “good science” that we want Tby the Oparin-Haldane hypothesis. to teach students. Even though details of the model for the origin Finally, the Miller–Urey experiment should of life have changed, this has not affected the still be taught because the basic results are still basic scenario of Oparin–Haldane. The first valid. The experiments show that organic mol- stage in the origin of life was chemical evolu- ecules can form under abiotic conditions. Later tion. This involves the formation of organic experiments have used more accurate atmos- compounds from inorganic molecules already pheric compositions and achieved similar present in the atmosphere and in the water of results. Even though origin-of-life research has the early earth. This spontaneous organization moved beyond Miller and Urey, their experi- of chemicals was spawned by some external ments should be taught. We still teach Newton energy source. Lightning (as Oparin and even though we have moved beyond his work Haldane thought), proton radiation, ultraviolet in our knowledge of planetary mechanics. radiation, and geothermal or impact-generated Regardless of whether any of our current theo- heat are all possibilities. ries about the origin of life turn out to be com- The Miller–Urey experiment represents a pletely accurate, we currently have models for major advance in the study of the origin of life. the processes and a research program that In fact, it marks the beginning of experimental works at testing the models. research into the origin of life. Before Miller– Urey, the study of the origin of life was mere- HOW TEXTBOOKS COULD IMPROVE THEIR PRESENTATIONS OF ly theoretical. With the advent of “spark exper- THE ORIGIN OF LIFE iments” such as Miller conducted, our understanding of the origin of life gained its first extbooks can always improve discus- experimental program. Therefore, the Miller– sions of their topics with more up-to- Urey experiment is important from an histori- Tdate information. Textbooks that have cal perspective alone. Presenting history is not already done so should explicitly correct good pedagogy because students understand the estimate of atmospheric composition, and scientific theories better through narratives. accompany the Miller–Urey experiment with a The importance of the experiment is more than clarification of the fact that the corrected just historical, however. The apparatus Miller atmospheres yield similar results. Further, the and Urey designed became the basis for many wealth of new data on extraterrestrial and subsequent “spark experiments” and laid a hydrothermal sources of biological material groundwork that is still in use today. Thus it is should be discussed. Finally, textbooks ideally also a good teaching example because it shows should expand their discussions of other stages how experimental science works. It teaches in the origin of life to include PNA and some students how scientists use experiments to test of the newer research on self-replicating pro- ideas about prehistoric, unobserved events teins. Wells, however, does not suggest that such as the origin of life. It is also an interest- textbooks should correct the presentation of ing experiment that is simple enough for most the origin of life. Rather, he wants textbooks to students to grasp. It tested a hypothesis, was present this “icon” and then denigrate it, in reproduced by other researchers, and provided order to reduce the confidence of students in new information that led to the advancement the possibility that scientific research can ever 8 Icons of Evolution? Why Much of What Jonathan Wells Writes about Evolution is Wrong Alan D. Gishlick, National Center for Science Education establish a plausible explanation for the origin Organic compounds in stony meteorites. Geochimica et of life or anything else for that matter. If Cosmochimica Acta. 27:805–834. Wells’s recommendations are followed, stu- Kobayashi, K., T. Kaneko, T. Saito, and T. Oshima. dents will be taught that because one experi- 1998. Amino acid formation in gas mixtures by high energy particle irradiation. Origins of Life and the ment is not completely accurate (albeit in hind- Evolution of the Biosphere 28:155–165. sight), everything else is wrong as well. This is Kump, L. R., J. F. Kasting, M. E. Barley. 2001. Rise of not good science or science teaching. atmospheric oxygen and the “upside-down” Archean mantle. Geochemistry, Geophysics, Geosystems –G3, 2, References paper number 2000GC000114. Anders, E. 1989. Pre-biotic organic matter from comets Maurette, M. 1998. Carbonaceous micrometeorites and and asteroids. Nature 342:255–257. the origin of life. Origins of Life and the Evolution of the Andersson, E. and N. G. Holm. 2000. The stability of Biosphere 28: 385–412. some selected amino acids under attempted redox con- Miller, S. 1953. 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