Methane is a gas , but other simple, organic compounds made from it are liquid and would have rained down on the early Earth. Because there was no oxygen in the atmosphere , the early Earth lacked a layer of ozone to block out powerful ultraviolet radiation from space. Haldane hypothesized that the ultraviolet radiation from space, along with lightning constantly hitting the primordial organic soup, delivered energy to the various simple organic compounds.
This caused chemical bonds between the atoms of the molecules to break and reform, creating new and different molecules, leading to extremely large, complex organic molecules. Molecules that could copy better than their neighbors multiplied and gradually dominated the soup. Some of these self-copying molecules became surrounded by a kind of barrier, the precursor to what we call a membrane. This happened by accident, so it was very rare, but when it did happen, Haldane explained, the enclosed, self-copying molecules had an enormous survival advantage.
So they came to dominate, ate up the soup, and life had begun. Moreover, it was perfectly consistent with the state of science in the s and 30s regarding the chemistry of the early Earth.
It turned out that he was proposing almost the same thing as Haldane, so the idea became known as the Oparin-Haldane hypothesis. As for testing the Oparin-Haldane hypothesis , that role fell into the hands of a graduate student, Stanley Miller. In the early s, Miller was looking for a thesis project in the Department of Chemistry at the University of Chicago. In , his academic mentor, Professor and Nobel laureate Harold Urey, suggested that he try putting the origins of living molecules to a test.
That meant recreating the kind of atmosphere that scientists thought had existed on primordial Earth: hydrogen, methane, ammonia, and water. It also meant providing what Haldane thought set the stage for creating more complicated molecules needed for life: lightning and ultraviolet light.
Once the ancient atmosphere was created and contained in a flask, Miller and Urey exposed the mixture to powerful ultraviolet light. They also put electrodes inside the flask and sent an electric current through the apparatus, creating sparks to simulate lightning, which interacted with the gases in the flask. After several days, they checked the contents of the liquid that accumulated at the bottom of the apparatus Figure 7. They found that different molecules had been created, including various important biological molecules, such as the amino acids glycine, alanine, and valine.
They ran the experiment over and over and, depending on how they changed around the gas mixture, different varieties of amino acids and other biological molecules were created. This showed that it was possible for biologically important molecules to form on a planet under abiotic conditions.
Perhaps it had not been dominated by methane, hydrogen, and ammonia, and possibly it could have been more oxidized as opposed to reduced. But as theories about the ancient atmosphere were refined, Miller tried variations of his original experiment with the adjusted gas mixtures.
Although chemical products changed with each new mixture, in each case they included compounds that were vital to life, such as amino acids , or nitrogenous bases , the building blocks needed to make DNA and RNA.
The emerging answer seemed to be that, almost regardless what the precise mixture and conditions were, complex organic molecules would result. What formed when electric sparks were created in a flask of hydrogen, methane, ammonia, and water? This too produced important biological compounds.
Thus, today, the moon Titan is a prime focus for astrobiology studies in the Solar System. It may have exotic life forms, or it may be a model of how Earth was prior to life. Several years after the original Miller-Urey experiment , another investigator, Sidney Fox, ran experiments showing that some of the Miller-Urey compounds — the amino acids — could join together to form polymers , bigger molecules known as peptides , or small proteins.
This happened when amino acids made through a Miller-Urey mechanism were splashed onto surfaces of clays and other materials, under hot, dry conditions. On the ancient Earth, such conditions would have occurred at the boundary between ancient ponds or seas and ancient land. Given enough time, complex proteins could arise. Other researchers later found that spheres of lipids the class of organic molecules that includes fats also could form under conditions thought to exist on the ancient Earth.
This would create a water environment inside the sphere that was separated from the outside. In other words, crude membranes can form spontaneously under the same conditions in which biological compounds like amino acids and small proteins can form. The fact that membranes can form spontaneously is key to origins of life research.
This is because to move from non-living chemistry to biology, very complex networks of chemical reactions need to emerge. Like a car being made on an assembly line, biological molecules are put together section by section. They also are converted into different molecules section by section, so there is a series of intermediate chemicals in addition to a starting molecule called a substrate and final product of each reaction. But a membrane would enclose all of the chemicals within a compartment.
That compartment would then act as a chemical laboratory, holding inside any reactions that happened to emerge. Since we know that membrane spheres can spontaneously form, the primordial soup of early Earth must have had billions of these little chemical laboratories in which the chemistry of life was sputtering along.
Demonstration that biological molecules and membranes can arise in an abiotic environment is not a demonstration of the emergence of life. The prize was claimed in by Louis Pasteur, as he published the results of an experiment he did to disproved spontaneous generation in these microscopic organisms.
Observation s : From Needham's and Spallanzani's experiments, it was known that soup that was exposed to the air spoiled — bacteria grew in it. Containers of soup that had been boiled for one hour, and then were sealed, remained sterile.
Boiling for only a few minutes was not enough to sterilize the soup. Pasteur had previously demonstrated that the dust collected by drawing air through a cotton ball contained large numbers of bacteria, hence he knew that bacteria were present in the air and could be filtered out by using a cotton ball. He also knew that bacteria would settle out on the walls of a long, bent, glass tube as air was passed through it.
Question: Is there indeed a "life force" present in air or oxygen that can cause bacteria to develop by spontaneous generation?
Is there a means of allowing air to enter a container, thus any life force, if such does exist, but not the bacteria that are present in that air?
Hypothesis: There is no such life force in air, and a container of sterilized broth will remain sterile, even if exposed to the air, as long as bacteria cannot enter the flask. Prediction: If there is no life force, broth in swan-neck flasks should remain sterile, even if exposed to air, because any bacteria in the air will settle on the walls of the initial portion of the neck.
Broth in flasks plugged with cotton should remain sterile because the cotton is able to filter bacteria out of the air. Testing: Pasteur boiled broth in various-shaped flasks to sterilize it, then let it cool. As the broth and air in the containers cooled, fresh room air was drawn into the containers. None of the flasks were sealed — all were exposed to the outside air in one way or another. This allowed air to enter these flasks, but the long, swan neck or the cotton balls filtered out any bacteria present in that air.
He subsequently broke the long necks off some of the swan-neck flasks. According to one freshman biology text, some of his original flasks, on display in France , still are sterile. Data: Broth in flasks with necks opening straight up spoiled as evidenced by a bad odor, cloudiness in previously clear broth, and microscopic examination of the broth confirming the presence of bacteria , while broth in swan-neck flasks did not, even though fresh air could get it.
Broth in flasks with cotton plugs did not spoil, even though air could get through the cotton. If the neck of a swan-neck flask was broken off short, allowing bacteria to enter, then the broth became contaminated.
Conclusion s : There is no such life force in air, and organisms do not arise by spontaneous generation in this manner. To quote Louis Pasteur, "Life is a germ, and a germ is Life. Never will the doctrine of spontaneous generation recover from the mortal blow of this simple experiment.
One very important point to note here is that Pasteur did not seek to find an answer to the broad question, "Has spontaneous generation ever occurred? Spontaneous Generation Background — Spontaneous Generation Today, we take many things in science for granted.
Redi left meat in each of six containers Figure 1. Two were open to the air, two were covered with gauze, and two were tightly sealed. His hypothesis was supported when maggots developed in the uncovered jars, but no maggots appeared in either the gauze-covered or the tightly sealed jars. He concluded that maggots could only form when flies were allowed to lay eggs in the meat, and that the maggots were the offspring of flies, not the product of spontaneous generation.
Figure 1. Maggots only appeared on the meat in the open container. However, maggots were also found on the gauze of the gauze-covered container. In , John Needham — published a report of his own experiments, in which he briefly boiled broth infused with plant or animal matter, hoping to kill all preexisting microbes. After a few days, Needham observed that the broth had become cloudy and a single drop contained numerous microscopic creatures.
He argued that the new microbes must have arisen spontaneously. In reality, however, he likely did not boil the broth enough to kill all preexisting microbes. This suggested that microbes were introduced into these flasks from the air. Any subsequent sealing of the flasks then prevented new life force from entering and causing spontaneous generation Figure 2.
Figure 2. The debate over spontaneous generation continued well into the nineteenth century, with scientists serving as proponents of both sides. To settle the debate, the Paris Academy of Sciences offered a prize for resolution of the problem.
Louis Pasteur, a prominent French chemist who had been studying microbial fermentation and the causes of wine spoilage, accepted the challenge. This certainly excluded spontaneous generation as a viable theory. Except it was noted by scientists of the day that Spallanzani had deprived the closed bottle of air, and it was thought that air was necessary for spontaneous generation.
So although his experiment was successful, a strong rebuttal blunted his claims. Pasteurization originally was the process of heating foodstuffs to kill harmful microorganisms before human consumption; now ultraviolet light, steam, pressure, and other methods are available to purify foodsin the name of Pasteur.
Louis Pasteur, the notable French scientist, accepted the challenge to re-create the experiment and leave the system open to air. He subsequently designed several bottles with S-curved necks that were oriented downward so gravity would prevent access by airborne foreign materials. He placed a nutrient-enriched broth in one of the goose-neck bottles, boiled the broth inside the bottle, and observed no life in the jar for one year. He then broke off the top of the bottle, exposing it more directly to the air, and noted life-forms in the broth within days.
He noted that as long as dust and other airborne particles were trapped in the S-shaped neck of the bottle, no life was created until this obstacle was removed. He reasoned that the contamination came from life-forms in the air.
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