It is still unclear as to what were the exact environmental conditions which led to the origin of life on Earth. Today we shall look at one of the experiments which attempted to create such conditions. Chemists tell a joke among themselves when nobody else is around. Life is impossible, they say: we’ve put simple chemical ingredients in water, we’ve added energy in the form of heat or electricity, and all we ever get is an organic sludge. We never see replicating molecules. We never make a cell.
The astronomer Fred Hoyle once said that the act of assembling the simplest living organism from simple molecular ingredients was as unlikely as a tornado whipping through a junkyard and assembling a jumbo jet.
Yet somehow it happened. Was it blind luck? And if it somehow happened here, could it happen somewhere else?
In 1953, Stanley Miller, a young University of Chicago graduate student, flipped a switch and sent an electric current through a glass flask containing water and a mixture of methane, ammonia, and hydrogen (fig. 32). His advisor, Harold Urey, suggested the experiment but then tried to dissuade Miller from going ahead with it, thinking that it might take months or years to yield a useful result. Both men were amazed by what happened.
Within a few weeks, 4 percent of the contents of the flask had been converted into thirteen of the twenty amino acids used in life on Earth..
Within hours, the liquid turned distinctly pink and then very red. Within a day, there was a brown film on the inside of the flask. Within a few weeks, 4 percent of the contents of the flask had been converted into thirteen of the twenty amino acids used in life on Earth—amino acids are the building blocks of proteins, which combine to create the genetic code. Any high school chemistry lab can repeat the original result.
If God didn’t do it this way, he missed a good bet.
Miller presented his results in a seminar; their reception was predictably electric. When the noted physicist Enrico Fermi asked if this process could actually have taken place on the primitive Earth, Urey leaped up and said, “If God didn’t do it this way, he missed a good bet.” The scientific community was surprised and skeptical. When Miller and Urey’s paper was submitted, one reviewer refused to believe the result and delayed publication. (By chance, it appeared within a month of Watson and Crick’s description of the DNA molecule.)
But there’s a catch to this classic experiment. Miller and Urey took their cue from planetary scientists, using a gas mixture with a composition comparable to that of the giant planets at the time of the Solar System’s formation. 12 Gases rich in hydrogen are able to combine with carbon and build more complex molecules.
However, on a terrestrial planet such as Earth, volcanoes produced an early atmosphere rich in carbon dioxide. In alternative Miller-Urey experiments where carbon dioxide replaces hydrogen-bearing molecules, there’s a lower yield of organic compounds. The original experiment also used an electrical discharge that was unrealistically strong—an attempt to simulate the effect of lightning.
Regardless of the nature of the energy source, all “life in a bottle” experiments produce similar results if gases rich in hydrogen are used.
However, there were a number of other energy sources on the early Earth, such as ultraviolet radiation, volcanoes, seismic shocks, deep-sea vents, and even impacts from space. Regardless of the nature of the energy source, all “life in a bottle” experiments produce similar results if gases rich in hydrogen are used.
In addition to organic compounds that formed in the primordial soup of Earth’s atmosphere and oceans, life probably had assistance from space. Raw organic materials are deposited on the Earth’s surface at a rate of ten million kilograms per year. The rate was one hundred thousand times higher during the bombardment that preceded the earliest evidence of life when debris left over from planet formation was abundant. Both comets and meteors can transport complex molecules to Earth.
when scientists simulate impacts, amino acids not only survive the crashes, many join to form polypeptides or miniproteins—another step along the road to life.
More than seventy amino acids were found in the Murchison meteorite after it landed in 1969, eight of which are among the twenty amino acids used by terrestrial life. Surprisingly, when scientists simulate impacts, amino acids not only survive the crashes, many join to form polypeptides or miniproteins—another step along the road to life.
Meteorites also deliver a vital ingredient: phosphorus. This reactive element is the fifth most important biological element after carbon, hydrogen, oxygen, and nitrogen, but stars make very little of it (fig. 22). Phosphorus is five times rarer in the cosmos and eight times rarer in ocean water than it is in bacteria. Phosphorus is critical to life because it forms the backbone of DNA, and it’s also the major ingredient in ATP (adenosine triphosphate), life’s fundamental fuel.
Phosphorus is critical to life because it forms the backbone of DNA..
The most common phosphorus-bearing mineral on Earth, apatite, does not give up its phosphorus easily. However, meteorites contain the metallic mineral schreibersite, which releases phosphorus-rich compounds into water. Many variations of the Miller-Urey, or “life in a bottle,” experiments have been carried out since 1953. None has produced any living organism or even a replicating molecule, but they’ve generated most of the simple building blocks of life.
In addition to producing amino acids, the experiments have yielded sugars, fatty acids, and all of the bases used by DNA and RNA. By adding carbonyl sulfide, which is found in volcanic gases and deep-sea vent emissions, one group at Scripps Institute saw amino acids combine four at a time into polypeptides, a level of complexity not observed by Miller and Urey. Perhaps we shouldn’t be surprised that a few weeks in the lab can’t duplicate a process that probably took millions of years on Earth.
..the results of the “life in a bottle” experiments seem a little disappointing
Yet the results of the “life in a bottle” experiments seem a little disappointing. They take us from simple molecules with three to five atoms up to organic molecules with a few dozen atoms. Compare this to the simplest proteins, which have thousands of atoms, and DNA strands from the most primitive organisms, which have tens of millions of atoms.
Hence, just as the primordial soup still lingers in uncertainty, so does this experiment proves little to the actual events which may have taken place for the necessary factors and environment which led to the origin of life on Planet Earth.
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