Biology: Miller–Urey Experiment
Biology: Miller-Urey Experiment
In 1950s America, there was much speculation about outer space travel. President Dwight D. Eisenhower called for the United States to put a satellite in orbit in 1957–1958, the International Geophysical Year. But in a demonstration of cold war rivalry, the Soviet Union was the first to do so, launching Sputnik I atop a ballistic missile on October 4, 1957. On the ground, American scientists were also doing sets of experiments to understand the possibilities of extraterrestrial life, as well as the origins of life on earth. The most elegant and famous set of investigations into how life began on the planet was the Miller/Urey experiment, done by graduate student Stanley L. Miller and biochemist Harold C. Urey at the University of Chicago in 1953. The success of their work also spawned the scientific subspecialty of exobiology. Exobiology is the speculative study of life forms beyond earth, and, as Miller remarked in a 1996 interview by Sean Henahan for Access Excellence, “the study of pre-biotic Earth and what chemical reactions might have taken place as the setting for life's origin.”
Historical Background and Scientific Foundations
Background to the Experiment: The Search for the Origins of Life
Prior to Miller and Urey's work, there had been much scientific speculation about the origins of life. Contrary to popular opinion, the idea of a primordial soup from which life arose did not begin with Charles Darwin and his On the Origin of Species by Means of Natural Selection (1859). Rather, the ancient Greeks were pioneers in speculating about how life began.
In the sixth century BC, Anaximander (c.610–c.546 BC) deduced that life arose in a gradual development, beginning from moisture under the influence of warmth, which eventually lead to aquatic animals and then land animals. Aristotle (384–322 BC) originated the idea of spontaneous generation, which was the belief that life could arise suddenly by change from matter independently of a parent. Aristotle observed that eels seemed to be generated out of mud, fleas appeared on the bodies of humans and animals, and parasitic worms arose internally.
Christianity, which posited that the ultimate origin of life was divine, supplanted the ancient Greeks views of life's origins. However, spontaneous generation seemed a viable theory until the end of the seventeenth century, when it became incompatible with the new philosophies during the scientific revolution.
Empirical support against spontaneous generation came from the work of Italian physician Francesco Redi (1626–1697). Redi noticed that maggots tended to rise from rotting meat, but observed it was not because they were “spontaneously generated,” but because he observed fly eggs on the flesh. He therefore placed a tremendous variety of dead animal material in open boxes—snakes, pigeons, fish, sheep hearts, frogs, and meat from deer, dogs, lambs, rabbits, goats, ducks, geese, hens, swallows, and even lions, tigers, and buffalo—and the same sorts of meat in boxes covered with gauze. The covered boxes produced no flies, but in the open containers, Redi noticed in every case the occurrence of the same kind of flies hatching from maggots. This insight made, Redi concluded that all dead flesh, fish, plants, and fruits form a good breeding place for flies and other winged animals. French chemist Louis Pasteur (1822–1895) later proved that phenomena like molds or spoilt wine also did not arise spontaneously, but were caused by specific forms of bacteria.
Despite these breakthroughs, questions about the ultimate origins of life remained in the scientific community. In the nineteenth century, the most prevalent debates about the origins of life were between the vitalists and the materialists. It was believed that matter was divided into two types, depending on a substance's behavior when heated. Inorganic substances could be melted, but reconstituted if the heat source was removed. However, when heat changed the form of organic substances, the form could not be recovered. The vitalists, such as Henri Bergson (1859–1941) explained these differences by stating that inorganic materials did not contain an élan vital or a creative force in life. The vitalists believed only living tissue could produce organic compounds. Materialists did not make such assumptions, thinking that living tissue and organic compounds could be made of chemical elements just as inorganic compounds were.
In 1845, German chemist Adolph Wilhelm Hermann Kolbe (1818–1884) synthesized an organic substance, acetic acid (vinegar) from its elements, which was a significant blow to vitalism. Other scientists, such as English botanist Thomas Huxley (1825–1895), denounced vitalism. In his 1868 lecture “On the Physical Basis of Life,” Huxley argued that the composition of living material (which he termed “protoplasm”) was essentially identical in all living things and was composed of carbon, hydrogen, oxygen, and nitrogen. Huxley then remarked in his lecture:
These new compounds, like the elementary bodies of which they are composed, are lifeless. But when they are brought together, under certain conditions, they give rise to the still more complex body, protoplasm, and this protoplasm exhibits the phenomenon of life. I see no break in this series of steps in molecular complication.
He concluded that protoplasm, the matter of life, “is built up of ordinary matter and again resolved into ordinary matter when its work is done.” But where did protoplasm and thus life ultimately come from? Huxley used the analogy of water being made by passing an electric spark through a mixture of oxygen and hydrogen to suggest that basic elements that were electrified could give rise to protoplasm. Huxley's colleague Charles Darwin (1809–1882) also speculated about the origins of life, writing in a letter to botanist Joseph Dalton Hooker (1817–1911) on February 1, 1871, that life may have begun in a “warm little pond, with all sorts of ammonia and phosphoric salts, lights, heat, electricity, etc. present, [so] that a protein compound was chemically formed ready to undergo still more complex changes.”
Nineteenth-century German zoologist Ernst Haeckel (1834–1919) “epitomized the leading scientific beliefs after Darwin.” Haeckel argued that the first life forms were microorganisms that were capable of photosynthesis that had evolved directly out of nonliving matter.
However, in 1924 the Russian biochemist and evolutionary biologist Aleksandr I. Oparin (1894–1980) questioned Haekels' views, as he did not see how a complex organism capable of photosynthesis could suddenly spring from simple chemical elements. As Lazcano has shown, Oparin instead proposed “that a long period of abiotic synthesis on early Earth had caused organic compounds to accumulate in a prebiotic soup, which had preceded life. Oparin then described how organic molecules could have evolved, via simple, ubiquitous fermentation reactions, into precellular systems on the primitive Earth. Such systems, he maintained, could then have led to cells that survived without oxygen and fed on the prebiotic soup.” These simple fermentation reactions were done without oxygen (anaerobic) as the most primitive organisms today also operate by anaerobic respiration to produce their energy. Oparin also postulated likewise that the early earth had an anaerobic-reducing atmosphere with high hydrogen content, perhaps in the form of ammonia, methane, and water.
When Miller and Urey began their own search to understand the origins of life on earth, they had to have a reasonably complete knowledge of two things: first, the early history of the earth, including the chemical components of its atmosphere, the amount of sunlight, and the presence of water, and secondly, the chemical properties of living material.
Oparin provided the basis for their assumptions about the primordial earth. Miller remarked in an interview that Urey had also suggested in the 1950s that the early earth had a reducing atmosphere, “since all of the outer planets in our solar system—Jupiter, Saturn, Uranus and Neptune—have this kind of atmosphere.”
The discovery of the double helical structure of DNA (the molecule that comprises the genetic code of life) in 1953 by molecular biologists James Watson (1928–) and Francis Crick (1916–2004) helped them understand the chemistry of living material (The Miller-Urey experiment and Watson and Crick's results were only published one month apart). Watson and Crick discovered that DNA was made of a sequence of nucleic acids that coded for specific amino acids, which are the building blocks of proteins. In other words, DNA essentially provides instructions for the making of all the proteins that cells need to function and for genetic expression. Amino acids therefore are among the very basic building blocks of life. Miller and Urey therefore knew that in order for life to begin, at a minimum, amino acids had to be present.
First, Miller and Urey's experiment was designed to replicate the atmospheric conditions thought to exist on a primeval earth. In Miller and Urey's original paper about their work, Miller started with gases thought to comprise the early Earth's reducing atmosphere—methane (CH4), ammonia (NH3), hydrogen (H2), and water (H2O) and put them into a five liter closed glass receiver. Next, Miller ran a continuous electrical current through his system to simulate lightening storms and UV radiation (from the sun) on the early Earth. The resulting mixture was allowed to condense and collect in a lower flask, representing an early sea or ocean on the Earth. This lower flask was also heated to reproduce the water cycle of condensation in the atmosphere and rain.
After a week, the liquid mixture turned dark brown and was analyzed using high performance liquid chromatography (HPLC), which isolates and purifies chemical compounds according to their structure and size. Miller found that approximately 10–15% of the solution consisted of organic compounds, and two percent were amino acids that had formed. They detected 11 out of the 20 amino acids that comprise proteins and were essential to early cellular life. Miller and Urey thus indicated that amino acids could be made easily in conditions present on the primordial earth.
Questions and Controversies
When the paper was submitted to Science magazine, Miller recalled that the editor was so surprised at the results that he delayed publication. Other colleagues assumed there was bacterial contamination, and so they “filled the tank with gas, sealed it, and put it in an autoclave for 18 hours at 15 psi. The results were the same.”
Although the chemistry involved was ultimately accepted, there were arguments over what the atmosphere of a primordial earth was actually like, and subsequently the validity of their experiment. The methane and ammonia gases that Miller and Urey used are now believed not to have existed on the ancient Earth; rather the primordial earth's atmosphere had a mixture of carbon dioxide (CO2) and nitrogen. And when Miller and Urey repeated the experiment with CO2 and nitrogen, no amino acids were produced, throwing their hypothesis into doubt.
Exobiologists began to postulate that the materials for amino acids may have come from an outside source—perhaps chemical elements in meteorites or passing comets. For instance, comets have a good deal of hydrogen cyanide, which is important to the synthesis of amino acids, as well as some of the nucleic acids, one of the building blocks of DNA. Some hydrogen cyanide likely came into the earth's atmosphere from comets.
Modern Cultural Connections
Recent work by Jeffrey Bada at the Scripps Institution of Oceanography has added renewed validity to Miller and Urey's results. Bada realized that the reactions involving CO2 and nitrogen were producing chemicals called nitrites that destroy amino acids by turning the water acidic. But, as Fox explained, “primitive Earth would have contained iron and carbonate minerals that neutralized nitrites and acids. So, Bada added chemicals to the experiment to duplicate these functions. When he reran it, he still got the same watery liquid as Miller did in 1983, but this time it was full of amino acids.”
Bada's work indicates that perhaps the primitive earth had all the chemical elements it needed for the origin of life to occur, without the need for extraterrestrial help. The debate about the life's evolutionary beginnings thus continues.
Primary Source Connection
As science journalist Michelle Thaller notes, the saga of meteorite ALH 84001 reveals a fundamental hurdle in exobiology research—what does life from other worlds look like, and how do we recognize it?
STICKS AND STONES: THE MARTIAN METEORITE DEBATE RAGES ON
PASADENA, CA —Mars has always been a provocateur. The planet has a long history of making us uneasy, from the portents of violence our ancestors associated with its red glow, to our science-fiction nightmares of malicious, technologically superior alien invaders.
And Mars is still stirring things up in the scientific community. For several years now, there has been an ongoing debate as to whether a meteorite from Mars contains the fossilized remnants of microbial life. Some scientists think we no longer have to wonder about whether there is other life in the universe; we have the remains of tiny Martian cousins in our laboratories at this very moment. Others remain skeptical, claiming that every structure and chemical in the meteorite could have been formed by natural processes that have nothing to do with life, like chemical weathering and heating. Despite the controversy, the Martian Meteorite debate has already taught us a lot about what kind of questions to ask the next time we get our hands on a sample of Martian soil, as well as shown us how little we understand about the threshold of life itself.
Backing up a little, how in the world did a piece of Mars find its way to Earth? Would you recognize a Martian rock if it were sitting in your backyard? The idea isn't as outlandish as it seems. We know of 24 meteorites that were originally part of Mars.
The first was recovered after it boomed down into a field outside of Chassigny, France, in 1815 (although the people of the time had no way of knowing that it came from Mars). The famous Martian Meteorite (designated ALH 84001) that has spurred all the debate was found more recently, lying on the Antarctic ice in 1984. Antarctica turns out to be a rich ground for meteorite hunters, as all of the indigenous rocks are buried beneath thousands of feet of glacial ice. If you find a rock sitting on top of the ice, there's a good chance it landed there from somewhere else. One of the meteorite-hunting teams even has a mascot of a penguin standing with a baseball glove aimed at the sky. Deserts like the Sahara and the Mojave are also a good bet, as the meteorites stand out from the sandy, eroded rocks around them.
So now that you've found a rock from space, how do you know it came from Mars? We've never brought a rock back from Mars, and our robotic landers have only been able to do crude chemical analysis of the rocks and soil there. Interestingly, that issue is not part of the debate. Scientists are almost certain that these meteorites are bits of the planet Mars due to careful chemical analysis of bits of the Martian atmosphere trapped in the rocks. We know the chemical composition of Mar's atmosphere very well (from spectroscopic measurements), and the rocks match it exactly. Really, there's no where else they could have come from.
Which leaves the next big question: How did they get here?
That might be the most amazing part of the whole story. The only way a rock could get to here from there is to be blasted off the surface of Mars at 11,000 miles per hour (that's the speed needed to escape Mars' gravity). There's no physical process on any planet that we know of that can achieve such speeds. Even rocks hurled out of giant volcanic explosions don't go anywhere near that fast. So, in fact, the only thing that can create a Martian meteorite is another meteorite. Probably a very big one, as big as a mile across.
That's right, scientists think these meteorites were chunks of Mars that got blasted into space after a violent (think dinosaurs) impact from a meteor or a comet. Judging from our maps of the Martian surface, this hasn't happened for a while. We have no way of knowing exactly when this impact took place, but we do have some idea how long ALH 84001 stayed drifting around between Earth and Mars. High energy particles called cosmic rays irradiate anything in space, leaving radioactive traces. ALH 84001 seems to have been exposed to these particles for about sixteen million years, although if it was the inner part of a larger meteor that broke up, it could have been in space much longer. And we didn't find it the moment it fell to Earth either, not by a long shot. Judging by other radioactive decay processes, the rock had been cooling its heels in Antarctica for about 13,000 years.
Right away, planetary scientists knew they'd found something interesting, as the rock showed signs of having been flooded with liquid water at least a billion years ago, perhaps as much as three billion. This piqued everyone's imagination, as the meteorite seemed to come from a lost age on Mars, when life might have taken hold.
Billions of years ago, Mars was a very different place, with a thick atmosphere and liquid water either on, or very near, the surface. Liquid water seems to have changed the chemistry of the rock, dissolving parts away and leaving globs of carbon-rich minerals. The globs were also rich in organic compounds called PAH (polycyclic aromatic hydrocarbons). It's actually not unheard of to find complex organic molecules in a meteorite. Many meteorites contain them, and some scientists think that may be how organic chemistry came to Earth in the first place. Still, the rock proved to be intriguing.
When the scientists turned an electron microscope on these carbon globs, they got the shock of their lives. Inside, clustered tightly together, were hundreds of tiny, wormy shapes. Only about 100 billionths of a meter across, the wormy things looked alarmingly similar to the fossilized remains of ancient Earth bacteria. They certainly didn't look like anything that had been seen inside a meteorite before.
And it wasn't only their shapes that were surprising; all around the “worms” were pure strings of iron crystals, called magnetite. Similar magnetite deposits are left behind when Earth bacteria die and decay. And at that point, scientists knew of no natural process that could produce pure magnetite crystals in the shapes and sizes observed in the meteorite. In fact, up until then, similar magnetite deposits had been used as a tracer to find bacteria in rocks. Did that still hold true? Had they, in fact, found the first example of life outside Earth?
At this point there was a bit of a media circus, and a lot of facts got distorted. In truth, no scientist had ever claimed that the meteorite definitely contained life; there were just a lot of tantalizing loose ends, and no good way of explaining them. Nature (and, it seems, the publicity machine) abhor a vacuum, so in the absence of any conclusions, many people got the idea that we had, in fact, discovered ancient Martian bacteria. But, as is often the problem with front-page news, any subsequent detractions seem to get buried somewhere on the back page.
In the last few months, scientists have done a bit of back-pedaling. What happened, in the best of scientific tradition, is that people went back to their labs and got to work. Was it possible, they wondered, to re-create everything in the mysterious meteorite by natural geologic processes? The wormy shapes were the first to go. There are plenty of ways to create similar shapes from minerals embedded in the meteorite, no life needed. And as stated before, PAH's, although highly-complex organic molecules, exist in abundance in space. There is still some arguing back and forth as to exactly what flavors of PAH's are commonly found in meteorites as opposed to those in ALH 84001, but that particular debate has reached no closure.
In March of 2002, a team of scientists announced a discovery that may turn out to be the last nail in the coffin for the Martian Meteorite. The team had, in their laboratory, created very similar magnetite crystals to the ones in ALH 84001, using nothing but repeated heating and shocking. From what little we know of the meteorite's history, it seems to have undergone plenty of both. Other scientists countered that the artificially created magnetite didn't have the exact same structure as bacteria-produced crystals, which may prove to be true. The crystals are so tiny that much of the discussion has centered on inventing better ways to probe the chemical structure of the crystals.
In the end, no one has proved beyond a shadow of a doubt that the shapes and chemicals in ALH 84001 are due to the presence of fossilized bacteria. But no one has disproved it either, and the whole debate brings up a fundamental issue that NASA will have to face as it begins to search for life outside the Earth: how do you recognize ancient life when you see it?
Billions of years ago, when ALH 84001 was forming inside some long-extinct Martian volcano, life in our Solar System had just barely taken hold. We're not just talking about Mars, either. Even on Earth, only the very first bacteria were emerging. So much of the chemistry of primitive life is indistinguishable from natural changes in rocks and minerals. That's no coincidence; that's what early life had to work with. What was the subtle change in chemistry, somewhere deep inside a rock or miles underneath the oceans, that allowed life to begin? We just don't know. So, it seems that before we can pass judgment on life elsewhere, we may need to get to know ourselves, and our origins, a whole lot better.
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Anna Marie Roos