amino acid creation experiment

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Scientists recreated classic origin-of-life experiment and made a new discovery

1952 Miller-Urey experiment showed organic molecules forming from inorganic precursors.

In 1952, a University of Chicago chemist named Stanley Miller and his adviser, Harold Urey, conducted a famous experiment . Their results, published the following year, provided the first evidence that the complex organic molecules necessary for the emergence of life ( abiogenesis ) could be formed using simpler inorganic precursors, essentially founding the field of prebiotic chemistry. Now a team of Spanish and Italian scientists has recreated that seminal experiment and discovered a contributing factor that Miller and Urey missed. According to  a new paper published in the journal Scientific Reports, minerals in the borosilicate glass used to make the tubes and flasks for the experiment speed up the rate at which organic molecules form.

In 1924 and 1929, respectively, Alexander Oparin and J.B.S. Haldane had hypothesized that the conditions on our primitive Earth would have favored the kind of chemical reactions that could synthesize complex organic molecules from simple inorganic precursors—sometimes known as the " primordial soup " hypothesis. Amino acids formed first, becoming the building blocks that, when combined, made more complex polymers.

Miller set up an apparatus to test that hypothesis by simulating what scientists at the time believed Earth's original atmosphere might have been. He sealed methane, ammonia, and hydrogen inside a sterile 5-liter borosilicate glass flask, connected to a second 500-ml flask half-filled with water. Then Miller heated the water, producing vapor, which in turn passed into the larger flask filled with chemicals, creating a mini-primordial atmosphere. There were also continuous electric sparks firing between two electrodes to simulate lighting. Then the "atmosphere" was cooled down, causing the vapor to condense back into water. The water trickled down into a trap at the bottom of the apparatus.

That solution turned pink after one day and deep red after a week. At that point, Miller removed the boiling flask and added barium hydroxide and sulfuric acid to stop the reaction. After evaporating the solution to remove any impurities, Miller tested what remained via paper chromatography. All known life consists of just 20 amino acids. Miller's experiment produced five amino acids, although he was less certain about the results for two of them.

When Miller showed his results to Urey, the latter suggested a paper should be published as soon as possible. (Urey was senior but generously declined to be listed as co-author, lest this lead to Miller getting little to no credit for the work.) The paper appeared in 1953 in the journal Science. "Just turning on the spark in a basic pre-biotic experiment will yield 11 out of 20 amino acids," Miller said in a 1996 interview . The original apparatus has been on display at the Denver Museum of Nature and Science since 2013.

Miller died in 2007. Shortly before he passed, one of his students, Jeffrey Bada, now at the University of San Diego, inherited all his mentor's original equipment. This included several boxes filled with vials of dried residues from the original experiment. Those 1952 samples were re-analyzed the following year using the latest chromatography methods, revealing that the original experiment actually produced even more compounds (25) than had been reported at the time.

Miller had also performed additional experiments simulating conditions similar to those of a water-vapor-rich volcanic eruption, which involved spraying steam from a nozzle at the spark discharge. Bada and several colleagues re-analyzed the original samples from those experiments, too, and found this environment produced 22 amino acids, five amines, and several hydroxylated molecules. So the original experiments were even more successful than Miller and Urey realized.

There have been many, many more experiments on abiogenesis over the ensuing decades, but co-author Joaquin Criado-Reyes of the Universidad de Granada in Spain and his collaborators thought that one potential factor had been overlooked: the role of the borosilicate glass that comprised the flasks and tubes Miller had used. They noted that Miller's simulated atmosphere was highly alkaline, which should cause the silica to dissolve. "Therefore, it could be expected that upon contact of the alkaline water with the inner wall of the borosilicate flask, even this reinforced glass will slightly dissolve, releasing silica and traces of other metal oxides [into the vapor]," the authors wrote.

To test their hypothesis, Criado-Reyes et al . recreated three versions of the Miller-Urey experiment, mostly using the same chemicals and equipment. One version used the same borosilicate flasks Miller had used; another version used a Teflon flask; and a third version used a Teflon flask with pieces of borosilicate submerged in the water.

The results: far fewer organic compounds formed in the experiments using just the Teflon flasks. As geologist David Bressan wrote at Forbes :

Miller and Urey used equipment made from borosilicate glass as this special type of heat-resistant material is commonly used in chemical laboratories all over the world. But the new experiment shows how similar materials may have played a major role in the origin of life on Earth. More than 90 percent of Earth’s crust is made up of silicates, minerals composed predominantly of silicon-dioxide . Weathering of silicate minerals by the corrosive primordial atmosphere and water may have provided the right conditions for the assembly of the first building blocks of life on Earth.

This finding supports the authors' original hypothesis. Corrosion on the surface of the glass (due to the hot and caustic water circulating through it) plays a key role, since this releases silicon-dioxide molecules into the solution. This in turn acts as a catalyst to speed up the chemical reactions between the nitrogen, carbon, and hydrogen atoms that ultimately create organic molecules. In addition, they found that the corrosion on the glass also forms millions of tiny pits. The authors think those pits could serve as tiny reaction chambers, also speeding up the rate at which organic molecules form in the experiment.

These results are consistent with recent suggestions that it was the combination of a reduced atmosphere, electrical storms, silicate-rich rocky surfaces, and liquid water that led to the origin of life. "Miller recreated in his experiments the atmosphere and waters of the primitive Earth," the authors concluded. "The role of the rocks was hidden in the walls of the reactors."

DOI: Scientific Reports, 2021. 10.1038/s41598-021-00235-4  ( About DOIs ).

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Miller-Urey Revisited

Members of NAI ’s Carnegie Institution of Washington, Indiana University, and NASA Goddard Space Flight Center Teams and their colleagues have revisited the Miller-Urey experiments, and found some surprising results.

A classic experiment proving amino acids are created when inorganic molecules are exposed to electricity isn’t the whole story, it turns out. The 1953 Miller-Urey Synthesis had two sibling studies, neither of which was published. Vials containing the products from those experiments were recently recovered and reanalyzed using modern technology. The results are reported in this week’s Science .

One of the unpublished experiments by American chemist Stanley Miller (under his University of Chicago mentor, Nobelist Harold Urey) actually produced a wider variety of organic molecules than the experiment that made Miller famous. The difference between the two experiments is small — the unpublished experiment used a tapering glass “aspirator” that simply increased air flow through a hollow, air-tight glass device. Increased air flow creates a more dynamic reaction vessel, or “vapor-rich volcanic” conditions, according to the present report’s authors.

“The apparatus Stanley Miller paid the least attention to gave the most exciting results,” said Adam Johnson, lead author of the Science report. “We suspect part of the reason for this was that he did not have the analytical tools we have today, so he would have missed a lot.”

Johnson is a doctoral student in IU Bloomington’s Biochemistry Program. His advisor is biogeochemist Lisa Pratt, professor of geological sciences and the director of NASA ’s Indiana-Princeton-Tennessee Astrobiology team.

In his May 15, 1953, article in Science, “A Production of Amino Acids Under Possible Primitive Earth Conditions,” Miller identified just five amino acids: aspartic acid, glycine, alpha-amino-butyric acid, and two versions of alanine. Aspartic acid, glycine and alanine are common constituents of natural proteins. Miller relied on a blotting technique to identify the organic molecules he’d created — primitive laboratory conditions by today’s standards. In a 1955 Journal of the American Chemical Society paper, Miller identified other compounds, such as carboxylic and hydroxy acids. But he would not have been able to identify anything present at very low levels.

Johnson, Scripps Institution of Oceanography marine chemist Jeffrey Bada (the present Science paper’s principal investigator), National Autonomous University of Mexico biologist Antonio Lazcano, Carnegie Institution of Washington chemist James Cleaves, and NASA Goddard Space Flight Center astrobiologists Jason Dworkin and Daniel Glavin examined vials left over from Miller’s experiments of the early 1950s. Vials associated with the original, published experiment contained far more organic molecules than Stanley Miller realized — 14 amino acids and five amines. The 11 vials scientists recovered from the unpublished aspirator experiment, however, produced 22 amino acids and the same five amines at yields comparable to the original experiment.

“We believed there was more to be learned from Miller’s original experiment,” Bada said. “We found that in comparison to his design everyone is familiar with from textbooks, the volcanic apparatus produces a wider variety of compounds.”

Johnson added, “Many of these other amino acids have hydroxyl groups attached to them, meaning they’d be more reactive and more likely to create totally new molecules, given enough time.”

The results of the revisited experiment delight but also perplex.

What is driving the second experiment’s molecular diversity? And why didn’t Miller publish the results of the second experiment?

A possible answer to the first question may be the increased flow rate itself, Johnson explained. “Removing newly formed molecules from the spark by increasing flow rate seems crucial,” he said. “It’s possible the jet of steam pushes newly synthesized molecules out of the spark discharge before additional reactions turn them into something less interesting. Another thought is that simply having more water present in the reaction allows a wider variety of reactions to occur.”

An answer to the second question is relegated to speculation — Miller, still a hero to many scientists, succumbed to a weak heart in 2007. Johnson says he and Bada suspect Miller wasn’t impressed with the experiment two’s results, instead opting to report the results of a simpler experiment to the editors at Science.

Miller’s third, also unpublished, experiment used an apparatus that had an aspirator but used a “silent” discharge. This third device appears to have produced a lower diversity of organic molecules.

Research on early planetary geochemistry and the origins of life isn’t limited to Earth studies. As humans explore the Solar System, investigations of past or present extra-terrestrial life are inevitable. Recent speculations have centered on Mars, whose polar areas are now known to possess water ice, but other candidates include Jupiter’s moon Europa and Saturn’s moon Enceladus, both of which are covered in water ice. The NASA Astrobiology Institute, which supports these investigations, has taken a keen interest in the revisiting of the Miller-Urey Synthesis.

“This research is both a link to the experimental foundations of astrobiology as well as an exciting result leading toward greater understanding of how life might have arisen on Earth,” said Carl Pilcher, director of the NASA Astrobiology Institute, headquartered at NASA Ames Research Center in Mountain View, Calif.

Henderson Cleaves (Carnegie Institution for Science) also contributed to the report. It was funded with grants from the NASA Astrobiology Institute, the Marine Biological Laboratory in Woods Hole, Mass., and Mexico’s El Consejo Nacional de Ciencia y Tecnologia.

Scripps Institution of Oceanography is a research center of the University of California at San Diego.

The NASA Astrobiology Institute ( NAI ), founded in 1998, is a partnership among NASA , 16 U.S. teams and five international consortia. NAI ’s goal is to promote, conduct and lead interdisciplinary astrobiology research and to train a new generation of astrobiology researchers. For more information, see http://astrobiology.nasa.gov/nai.

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Scientists finish a 53-year-old classic experiment on the origins of life

In 1958, a young scientist called Stanley Miller electrified a mixture of simple gases, designed to mimic the atmosphere of our primordial lifeless planet. It was a sequel to one of the most evocative experiments in history, one that Miller himself had carried five years earlier. But for some reason, he never finished his follow-up. He dutifully collected his samples and stored them in vials but, whether for ill health or dissatisfaction, he never analysed them.

The vials languished in obscurity, sitting unopened in a cardboard box in Miller’s office. But possessed by the meticulousness of a scientist, he never threw them away. In 1999, the vials changed owners. Miller had suffered a stroke and bequeathed his old equipment, archives and notebooks to  Jeffrey Bada , one of his former students. Bada only twigged to the historical treasures that he had inherited in 2007. “Inside, were all these tiny glass vials carefully labeled, with page numbers referring Stanley’s laboratory notes. I was dumbstruck. We were looking at history,” he said  in a New York Times interview .

By then, Miller was completely incapacitated. He  died of heart failure  shortly after, but his legacy continues. Bada’s own student Eric Parker has finally analysed Miller’s samples using modern technology and published the results, completing an experiment that began 53 years earlier.

Miller conducted his  original 1953 experiment  as a graduate student, working with his mentor Harold Urey. It was one of the first to tackle the seemingly insurmountable question of how life began. In their laboratory, the pair tried to recreate the conditions on early lifeless Earth, with an atmosphere full of simple gases and laced with lightning storms. They filled a flask with water, methane, ammonia and hydrogen and sent sparks of electricity through them.

The result, both literally and figuratively, was lightning in a bottle. When Miller looked at the samples from the flask, he found five different amino acids – the building blocks of proteins and essential components of life.

The relevance of these results to the origins of life is debatable, but there’s no denying their influence. They kicked off an entire field of research, graced the cover of  Time  magazine and made a celebrity of Miller.  Nick Lane  beautifully describes the reaction to the experiment in his book,  Life Ascending : “Miller electrified a simple mixture of gases, and the basic building blocks of life all congealed out of the mix. It was as if they were waiting to be bidden into existence. Suddenly the origin of life looked easy.”

Over the next decade, Miller repeated his original experiment with several twists. He injected hot steam into the electrified chamber to simulate an erupting volcano, another mainstay of our primordial planet. The samples from this experiment were among the unexamined vials that Bada inherited. In 2008, Bada’s student Adam Johnson  showed that the vials  contained a wider range of amino acids than Miller had originally reported in 1953.

Miller also tweaked the gases in his electrified flasks. He tried the experiment again with two newcomers – hydrogen sulphide and carbon dioxide – joining ammonia and methane. It would be all too easy to repeat the same experiment now. But Parker and Bada wanted to look at the original samples that Miller had himself collected, if only for their “considerable historical interest”.

Using modern techniques, around a billion times more sensitive than those Miller would have used, Parker identified 23 different amino acids in the vials, far more than the five that Miller had originally described. Seven of these contained sulphur, which is either a first for science or old news, depending on how you look at it. Other scientists have since produced sulphurous amino acids in similar experiments, including  Carl Sagan . But unbeknownst to all of them, Miller had beaten them to it by several years. He had even scooped himself – it took him till  1972  to publish results where he produced sulphur amino acids!

The amino acids in Miller’s vials all come in an equal mix of two forms, each the mirror image of the other. You only see that in laboratory reactions – in nature, amino acids come almost entirely in one version. As such, Parker, like Miller before him, was sure that the amino acids hadn’t come from a contaminating source, like a stray bacterium that had crept into the vials.

Imagine then, a young and violent planet, wracked with exploding volcanoes, noxious gases and lightning strikes. These ingredients combined to brew a “primordial soup”, fashioning the precursors of life in pools of water. On top of that, meteorites raining down from space could have added to the accumulating molecules. After all, Parker found that the amino acid cocktail in Miller’s samples is very similar to that found on the  Murchison meteorite , which landed in Australia in 1969.

These are powerful images, so why aren’t people more excited? Echoing many sources I spoke to,  Jim Kasting , who studies the evolution of Earth’s atmosphere, said, “I am underwhelmed by it.” The main problem with the study is that Miller was probably wrong about the conditions on early Earth.

By analysing ancient rocks, scientists have since found that Earth was never particularly teeming in hydrogen-rich gases like methane, hydrogen sulphide or hydrogen itself. If you repeat Miller’s experiment with a more realistic mixture – heavy in carbon dioxide and nitrogen, with just trace amounts of other gases – you’d have a hard time finding amino acids in the resulting brew.

Parker accepts the problem, but he suggests that a few specific places on the planet may have had the right conditions. Exploding volcanoes, for example, throw up masses of sulphurous compounds, as well as methane and ammonia. These gases,  belched forth into lightning storms , could have produced amino acids that rained out and gathered in tidal pools. But Kasting still isn’t convinced. “Even then the reduced gases would not be as concentrated as they are in this experiment.”

Even if our young planet had the right conditions to produce amino acids, that’s a less impressive feat than it appeared in the 1950s. “Amino acids are old hat and are a million miles from life,” says Nick Lane. Indeed, as Miller’s experiments showed, it’s not difficult to create amino acids. The far bigger challenge is to create nucleic acids – the building blocks of molecules like RNA and DNA. The origin of life lies in the origin of these “replicators”, molecules that can make copies of themselves. Lane says, “Even if you can make amino acids (and nucleic acids) under soup conditions, it has little if any bearing on the origin of life.”

The problem is that replicators don’t spontaneously emerge from a mixture of their building blocks, just as you wouldn’t hope to build a car by throwing some parts into a swimming pool. Nucleic acids are innately “shy”. They need to be strong-armed into forming more complex molecules, and it’s unlikely that the odd bolt of lightning would have been enough. The molecules must have been concentrated in the same place, with a constant supply of energy and catalysts to speed things up. “Without that lot, life will never get started, and a soup can’t provide much if any of that,” says Lane.

Deep-sea vents  are a better location for the origins of life. Deep under the ocean’s surface, these rocky chimneys spew out superheated water and hydrogen-rich gases. Their rocky structures contain a labyrinth of small compartments that could have concentrated life’s building blocks into dense crowds, and minerals that would have catalysed their get-togethers. Far away from visions of languid soups, these churning environments are the current best guess for the site of life’s hatchery.

So Miller’s iconic experiment, and its now-completed follow-ups, probably won’t lay out the first steps of life. As  Adam Rutherford,  who is writing a book on the origin of life, says, “It’s really a historical piece, like finding that Darwin had described a  Velociraptor  in one of his notebooks.”

If anything, the analysis of Miller’s vials is a testament to the value of meticulous scientific work. Here was a man who prepared his samples so cleanly, who recorded his notes so thoroughly, and who stored everything so carefully, that his contemporaries could pick up where he left off five decades later.

Reference:  Parker, Cleaves, Dworkin, Glavin, Callahan, Aubrey, Lazcano & Bada. 2011. Primordial synthesis of amines and amino acids in a 1958 Miller H2S-rich spark discharge experiment. PNAS  http://dx.doi.org/10.1073/pnas.1019191108

Photos  by Carlos Gutierrez and  Marco Fulle

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Possible Key to Life's Chemistry Revealed in 50-Year-Old Experiment

An old experiment, rediscovered after more than 50 years, may demonstrate how volcanoes – and possibly chemical reactions far from primitive Earth in outer space – played a role in creating the first amino acids, the building blocks of life.

In 1953, chemists Harold Urey and Stanley Miller performed a landmark experiment intended to mimic the primordial conditions that created the first amino acids, by exposing a mix of gases to a lightning-like electrical discharge. Five years later, in 1958, Miller performed another variation on this experiment. This time he added hydrogen sulfide, a gas spewed out by volcanoes, to the mix. [ Scientists Hunt for Signs of Earliest Life on Earth ] But for some reason, Miller never analyzed the products of the hydrogen sulfide reaction. About half a century later, Miller's former student Jeffrey Bada, a marine chemist at the Scripps Institution of Oceanography in California, discovered the old samples in a dusty cardboard box in Miller's laboratory, which Bada had inherited. (Miller passed away in 2007.)

Old experiment, new analysis

Using modern analytical techniques, Bada and his team, which included Eric Parker, then at Scripps, analyzed the products of the reaction, which were housed in small vials. They found an abundance of promising molecules: 23 amino acids and four amines, another type of organic molecule. The addition of hydrogen sulfide had also led to the creation of sulfur-containing amino acids, which are important to the chemistry of life. (One of these, methionine, initiates the synthesis of proteins.)

The results of the experiment – which exposed a mix of volcanic gases, including hydrogen sulfide, methane, ammonia and carbon dioxide gas to an electrical discharge – tell us that volcanic eruptions coinciding with lightning may have played a role in synthesizing large quantities and a variety of biologically crucial molecules on the primitive Earth, Parker, now a graduate student at Georgia Institute of Technology, told LiveScience.

"The gas mixture Miller used in this experiment was likely not ubiquitous throughout early Earth's atmosphere on a global scale, but it may have been common on a more local scale where there was heavy volcanic activity," Parker said.

Parallel to the Urey-Miller experiment

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By comparison, the famous Urey-Miller experiment in 1953 exposed hydrogen, steam, methane and ammonia to an electrical discharge. The initial results included far fewer organic molecules – only five amino acids. However, Bada and his team reanalyzed these old samples along with previously unpublished results with modern techniques, revealing a much greater variety of biologically important products. 

The results of the 1958 experiment, however, show that adding hydrogen sulfide to the reaction enriches the mixture of organic molecules produced, according to Bada.

The 1958 reaction – which also incorporated carbon dioxide, a gas not included in the earlier experiment – created a mix more like that which geoscientists now believe made up the atmosphere of primordial Earth, Parker said.

From outer space?

Amino acids, which combine to form proteins, which, in turn, form cellular structures and control reactions in living things, are not unique to Earth. They have been found on meteorites, mainly from samples acquired from asteroids and from one comet , according to Scott Sandford, a research scientist at NASA 's Ames Research Center in California.   

Bada's team compared the amino acids produced by the 1958 experiment with those contained in a type of carbon-rich meteorite, known as a carbonaceous chrondite. These meteorites are believed to provide snapshots of the types of organic reactions that took place in the early solar system , Bada told LiveScience in an email.

The researchers compared the amino acids produced by the hydrogen sulfide experiment with those contained by several carbonaceous chrondites. Some matched well, while others did not, suggesting that hydrogen sulfide played a role in the synthesis of amino acids in certain environments within our early solar system, but not in others, Bada wrote. Although the meteorites are all from our solar system, the same results would be expected in other solar systems elsewhere in the universe, he said.

There is a theory that life on Earth got a jump start from organic molecules when they arrived on the planet from space, Sandford told LiveScience. There is no doubt that space delivers much of the molecular building blocks for terrestrial life, but the question is the role the molecules played in getting life started, he added.

"In the end, if life was trying to get started, my guess is the process wasn't very picky about where the molecules came from," Sandford said. "[Early life] didn't care if that amino acid was formed in space or a lightning strike in Earth's atmosphere or came out of a hydrothermal vent … So in the end, it is possible life got started from acquiring building blocks from a wide variety of sources."

Sandford's work involves simulating ices found in many environments in space – including comets – that contain molecules similar to those used in the Urey-Miller experiment, and bombarding them with ionizing radiation. And like the reactions believed to have taken place on primordial Earth, these simulated cosmic ice reactions synthesize amino acids.

"At some level, the universe seems to be hard-wired to create amino acids, provided you have the right elements present and energy," he said.

A smelly piece of science history

It's not clear why Miller never analyzed the samples he produced with the hydrogen-sulfide experiment, but Parker speculates that it may have had something to do with the rotten-egg odor of hydrogen sulfide .

"When I was working with them by hand I could smell them myself," Parker said. "It wasn't so strong that it was overpowering, but it was strong enough to convince me to not stick my nose in front of it again."

But, unpleasant odors aside, the experience was a memorable one.

"It is sort of surreal to hold the sample vial in your hands and look at Stanley Miller's handwriting on the label," Parker said. "It was a very unique opportunity to go back in time and look at what he did and be able to use modern analysis techniques to be able to analyze samples produced over 50 years and see what they still contain today."

Their work is published this week in the journal Proceedings of the National Academy of Sciences.

You can follow LiveScience writer Wynne Parry on Twitter @Wynne_Parry .

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November 26, 2021

Redo of a Famous Experiment on the Origins of Life Reveals Critical Detail Missed for Decades

The Miller-Urey experiment showed that the conditions of early Earth could be simulated in a glass flask. New research finds the flask itself played an underappreciated, though outsize, role.

By Sarah Vitak

Chemist Stanley Miller re-creates the Miller-Urey experiment using the original laboratory equipment in the 1980s.

Roger Ressmeyer/Corbis/VCG/Getty Images

Sarah Vitak: This is Scientific American ’s 60 Second Science. I’m Sarah Vitak.

The question of how life came to be has captivated humans for millennia. The prevailing theory now is that, on a highly volatile early Earth, lightning struck mineral-rich waters and that the energy from lighting strikes turned those minerals into the building blocks of life: organic compounds like amino acids—something we often refer to as the “primordial soup.” The wide acceptance of this theory is in large part due to the very famous Miller-Urey experiment. You surely encountered this in a science textbook at some point. But to refresh your memory: in 1952 Stanley Miller and Harold Urey simulated the conditions of early Earth by sealing water, methane, ammonia and hydrogen in a glass flask. Then they applied electrical sparks to the mixture. Miraculously, amino acids came into existence amid the roiling mixture. It was a big deal.

But recently a team of researchers realized that—much like that first primordial soup sitting in a bowl of Earth—the experiment’s container played an underappreciated role—that perhaps it was also critical to the creation of organic building blocks inside their laboratory life soup.

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I talked to someone from the team.

Saladino: I am Raffaele Saladino from University of Tuscia in Italy.

Vitak: Then, much like today, when a researcher goes to start an experiment, often one of the first things they do is reach for their glassware. Well, today, actually, we use a lot of plastic as well.

Saladino:  But 20 years ago in the lab, only glass containers because, in the mind of the researcher, glass is inert.

Vitak: He said inert, meaning that it doesn’t react with the chemicals you put inside it. But in reality, that is not necessarily always the case.

Most of the time glass is pretty inert. When you’re baking with Pyrex (which is made of borosilicate glass, the same type of glass most labware is made out of) the cookware isn’t going into your brownies. But when you’re baking, whatever is in the pan is usually mostly water, so it will come at a pH of around 7 or so.

But the pH of the Miller-Urey experiment is much higher. In the original experiments, they used a pH of 8.7, which is more alkaline, or basic.

Saladino: Why alkaline environment is an important topic? Since under alkaline condition borosilicate can be impacted through blinds in the reaction menu, it is not inert it became a reagent. 

Vitak: In fact, this was actually noted by Miller in his original experiments--that the alkaline conditions caused the silica to dissolve. But it was overshadowed by the discovery of the synthesis of organic compounds. And as future researchers carried on they missed that point in Miller’s notes.

Saladino: The attention was concentrated on modifying the atmosphere, on modifying the energy, the intensity, and modifying the analytical tools.

Vitak: And the role of the silica got forgotten entirely. 

Dr. Saladino’s team wanted to see if the glass was doing anything in the reaction. To test this they set up three different versions of the original experiment where everything was the same except the containers. For comparison they chose teflon which does not dissolve when holding an alkaline solution, the way the glass does.

Saladino: There is the experiment only glass, the experiment only Teflon, and in the middle, there is the experiment in teflon with some pieces of glass added inside.

Vitak: Then they used a technique called mass spectrometry to analyze what each reaction produced. Mass spectrometry is great for figuring out what kinds of molecules are in a complex mixture.

They found that teflon produced very few organic compounds. There were more compounds in the teflon with glass pieces. But the glass container, by far, created the greatest number and largest variety of organic molecules.

The mechanism of exactly how the silica helps catalyze the reaction is not clear yet--but it is very clearly does.

The obvious question then is: Was there silica available in the early earth environment?

Saladino:  The water is not suspended in a vacuum. No? The water is in geochemistry, it is surrounded by minerals. Borosilicate and silica are the most abundant minerals surrounding the water.

Vitak: The team has two next major objectives in mind. First, to try updating the experiment to model more closely the amount of silica that would have been available in the early Earth.

Second, they want to try replacing the silica with extraterrestrial minerals like, pieces of meteorite or rocks from other planets. Apart from just being very cool, that could give a more concrete idea of how to look for life in space. 

But here on Earth, coming one step closer to fully understanding why we exist is that much more satisfying. Even after nearly 70 years, a key discovery in our complex origin story still carries new revelations. As the authors say in the paper: "The role of the rocks was hidden in the walls of the reactors."

Thanks for listening. For Scientific American’s 60 Second Science, I’m Sarah Vitak. 

[ The above text is a transcript of this podcast .]

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• Gold Foil Experiment
• Faraday Cage
• Oil Drop Experiment
• Magnetic Monopole
• Why Do Fireflies Light Up
• Types of Blood Cells With Their Structure, and Functions
• The Main Parts of a Plant With Their Functions
• Parts of a Flower With Their Structure and Functions
• Parts of a Leaf With Their Structure and Functions
• Why Does Ice Float on Water
• Why Does Oil Float on Water
• How Do Clouds Form
• What Causes Lightning
• How are Diamonds Made
• Types of Meteorites
• Types of Volcanoes
• Types of Rocks

Miller-Urey Experiment

The Miller-Urey Experiment was a landmark experiment to investigate the chemical conditions that might have led to the origin of life on Earth. The scientist Stanley Miller, under the supervision of the Nobel laureate scientist Harold Urey conducted it in 1952 at the University of Chicago. They tried to recreate the conditions that could have existed in the first billion years of the Earth’s existence (also known as the Early Earth) to check the said chemical transformations.

Miller-Urey Experiment And The Primordial Soup Theory

The experiment tested the primordial or primeval soup theory developed independently by the Soviet biologist A.I. Oparin and English scientist J.B.S. Haldane in 1924 and 1929 respectively. The theory propounds the idea that the complex chemical components of life on Earth originated from simple molecules occurring naturally in the reducing atmosphere of the Early Earth, sans oxygen. Lightning and rain energized the said atmosphere to create simple organic compounds that formed an organic “soup”. The so-called soup underwent further changes giving rise to more complex organic polymers and finally life.

The Miller-Urey Experiment In Support Of Abiogenesis

From what was explained in the previous paragraph, it can undoubtedly be considered as a classic experiment to demonstrate abiogenesis. For those who are not conversant with the term, abiogenesis is the process responsible for the development of living beings from non-living or abiotic matter. It is thought to have taken place on the Earth about 3.8 to 4 billion years ago.

Miller-Urey Experiment Apparatus and Procedure

The groundbreaking experiment used a sterile glass flask of 5 liters attached with a pair of electrodes, to hold water (H 2 O), methane (CH 4 ), ammonia (NH 3 ) and hydrogen (H 2 ), the major components of primitive Earth. This was connected to another glass flask of 500 ml capacity half filled with water. On heating it, the water vaporized to fill the larger container with water vapor. The electrodes induced continuous electrical sparks in the gas mixture to simulate lightning. When the gas was cooled, the condensed water made its way into a U-shaped trap at the base of the apparatus.

After electrical sparking had continued for a day, the solution in the trap turned pink in color. At the end of a week, the boiling flask was removed, and mercuric chloride added to prevent microbial contamination. After stopping the chemical reaction, the scientist duo examined the cooled water collected to find that 10-15% of the carbon present in the system was in the form of organic compounds. 2% of carbon went into the formation of various amino acids, including 13 of the 22 amino acids essential to make proteins in living cells, glycine being the most abundant.

Though the result was the production of only simple organic molecules and not a complete living biochemical system, still the simple prebiotic experiment could, to a considerable extent, prove the primordial soup hypothesis.

Miller-Urey Experiment Animation

Chemistry of the miller and urey experiment.

The components of the mixture can react among themselves to produce formaldehyde (CH 2 O), hydrogen cyanide (HCN) and other intermediate compounds.

CO 2 → CO + [O] (atomic oxygen)

CH 4 + 2[O] → CH 2 O + H 2 O

CO + NH 3 → HCN + H 2 O

CH 4 + NH 3 → HCN + 3H 2

The ammonia, formaldehyde and HCN so produced react by a process known as Strecker synthesis to form biomolecules including amino acids.

CH 2 O + HCN + NH 3 → NH 2 -CH 2 -CN + H 2 O

NH 2 -CH 2 -CN + 2H 2 O → NH 3 + NH 2 -CH 2 -COOH (glycine)

In addition to the above, formaldehyde and water can react by Butlerov’s reaction to produce a variety of sugars like ribose, etc.

Though later studies have indicated that the reducing atmosphere as replicated by Miller and Urey could not have prevailed on primitive Earth, still, the experiment remains to be a milestone in synthesizing the building blocks of life under abiotic conditions and not from living beings themselves.

https://www.bbc.co.uk/bitesize/guides/z2gjtv4/revision/1

https://www.juliantrubin.com/bigten/miller_urey_experiment.html

Article was last reviewed on Thursday, February 2, 2023

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One response to “Miller-Urey Experiment”

This experiment is currently seen as not sufficient to support abiogenesis. See Stephen C. Meyer, James Tour.

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• Published: 18 August 2023

Endogenous amino acid production

Miller’s spark

• Kensei Kobayashi   ORCID: orcid.org/0000-0003-2951-1341 1 &
• Yoko Kebukawa   ORCID: orcid.org/0000-0001-8430-3612 1  

Nature Reviews Chemistry volume  7 ,  pages 598–599 ( 2023 ) Cite this article

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Seventy years ago, Stanley L. Miller described the synthesis of amino acids from a simple mixture of gases, spurring investigations into the chemical origins of life. Here we discuss the rise, fall and renaissance of endogenous amino acid production.

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Kobayashi, K., Kebukawa, Y. Miller’s spark. Nat Rev Chem 7 , 598–599 (2023). https://doi.org/10.1038/s41570-023-00530-w

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