Evolution and intelligent design are controversial, complicated topics. I am not a creationist, but I am sympathetic to some of the criticisms leveled at evolution (some from intelligent design theorists, some from within the field itself). Research on the origin of life, in particular, has some difficult conceptual issues to face. Here is one that I find interesting:
First, I want you to look at the computer that you are using right now to view this page. Whether it is a laptop or a desktop computer, you are looking at something rather special. Here's what I mean: your computer is made of ordinary elements. Silicon, aluminum, iron, carbon, etc. The elements in your computer are not any different from elements you could find anywhere else. But, in this case, they are allowing you to connect to a world-wide network of information and view things like this webpage. Why? Nothing in the elements themselves dictate this sort of function. One could have all the appropriate elements and compounds together, but there would be no tendency for a computer to emerge. This means that these elements, along with the physical and chemical laws that govern their behavior, allow a computer to be built out of them. However, the raw materials themselves along with the natural laws that govern them are not capable of creating a computer. We can therefore say that computers are contingent; that is, the existence of computers cannot be explained by any known scientific laws, and nothing in the universe requires that they exist. Contrast this to, say, the gravitational attraction between the Earth and the Sun. In our universe, the laws (of gravity, in this case) demand a force of attraction (in Newtonian terms) of a certain magnitude between these two masses. They have no choice in the matter. A computer, on the other hand, is entirely contingent. Your computer's elements have been organized in a complicated, specific arrangement that results in the functional computer you have in front of you, but nothing in the physics or chemistry of those elements led to this specific arrangement.
OK, so what does this have to do with the origin of life? It turns out that biological life, as we know it, is contingent. DNA, for example, contains the genetic information that you or I or any living thing are built according to. DNA can be said to contain the blueprints of an organism. It consists of long strands of what are called nucleotides, major parts of which are made of nucleobases such as adenine (A), cytosine (C), guanine (G), and thymine (T). The arrangement of these nucleobases is largely responsible for the information contained in the strands of DNA. For a simple analogy, think of the words in an encyclopedia, or in this post. The arrangement of the letters (of the English alphabet, in this case) determines the information contained in a written entry. Different arrangement, different meaning. Different arrangements of the four "letters" of the genetic language (A, C, G, T) are responsible for all of the different organisms we see on earth. Now, this is not merely an analogy; DNA is not like a language, it is a language. Contemporary biologists have come to realize that when they speak of genes, they are speaking of information, not something physical. Here's where it gets interesting: information transcends physics and chemistry.
I don't mean this in some mystical, metaphysical way. Think about it. When you write a journal entry in your notebook, you are using ink and paper to represent information. But the ink and paper itself cannot be responsible for the information content, nor can the ink and paper itself said to be the information. Rather, the ink and paper happen to be the physical medium for embodying that information. The information itself transcends the physical and chemical. At first the information was in your mind, then on your paper, and then could be typed into a computer document, then transferred onto a thumb drive, etc.
In each case the information was being physically stored in a medium, but it is clear that the physical media are not themselves responsible for the creation of the information. In fact, the physical medium must be neutral, or flexible, in order for it to usefully store information. Consider your ink and paper again. If whenever you wrote the letter "t", the physical properties of the ink and paper forced the next bit of ink to form the letter "s", then you would have a very difficult time writing anything meaningful. The physical and chemical properties would determine the content of the paper, and your journal entries would be limited to repetitive strings of letters. Physical and chemical laws are good at repetition (a rock will always fall the same way, bits of lava will crystallize again and again in the same pattern), but cannot explain the origin of complex, specific information.
Let's get back to biology: our DNA strands, with the arrangement of our bases (letters) A,C,G, and T, cannot be explained by any physical or chemical laws. Like the ink and paper, the physical laws of the DNA strand itself allow a near infinite variety of arrangements of DNA letters. How then, can the materialist explain the origin of genetic information? If one is a materialist (i.e. one who holds that the physical, or nature, is all that exists), one is stuck trying to explain the origin of genetic information in terms of the known physical and chemical laws of the genetic molecules themselves. This is very literally like trying to explain this essay by appealing to the physics and chemistry of the molecules in the computer screen, or like trying to explain a newspaper's information content in terms of the physical and chemical properties of the ink and paper. As we have seen, the physical medium itself must remain neutral on the arrangement of letters or else the medium is useless for information storage. A neutral physical medium cannot explain why we see one particular arrangement and not another.
[In case the reader is wondering, chance cannot explain the first bit of genetic information; the probabilities of accidents resulting in even the "simplest" self-replicating cell are incredibly low (a single cell in your body contains vastly more information than Encyclopedia Britannica). Origin of life researchers do not consider chance to be a viable explanation, but are investigating scenarios where unique chemical conditions can hopefully explain the emergence of the first self-replicating machine.]
I am not trying to suggest that materialists are stupid; I am merely drawing attention to an important conceptual problem. If you are familiar at all with origin of life research then you know that scientists have investigated a huge range of possible scenarios leading to the origin of the first cell with some genetic information. While this research is extensive and informative, it is also above all strongly inconclusive. See Shapiro's Origins for a devastating critique, or Schopf's Life's Origin for an optimistic but honest assessment of the research. I am not calling for origin of life researchers to quit and say "God did it." I am, however, suggesting that researchers should pay careful attention to criticisms like those discussed above. It is possible that new laws will be discovered that can shed light on this problem. Only time will tell if the origin of life will eventually be explicable in physical and chemical terms. I will enjoy following along to find out.
Showing posts with label origin of life. Show all posts
Showing posts with label origin of life. Show all posts
Monday, August 10, 2009
Saturday, December 13, 2008
The Miller-Urey Experiment
The Origin of Life
Though Darwin’s theory of evolution applies to the diversification of biological life, not the origin of biological life, scientists have nonetheless attempted to find a naturalistic explanation of life’s origin through recourse to Darwinian type events. Competition between the first self-replicating molecules is thought to have led to increasingly efficient and complicated bio-molecules until the first primitive cell emerged. The question then is: where did these first self-replicated molecules come from? Every living thing known to science utilizes the same set of bio-molecules to reproduce: DNA, RNA, and proteins. These are enormously complicated molecules that, respectively, contain genetic information, the ability to translate and transport genetic information, and the ability to construct molecular machines (including other proteins) based on that information. Without all three components in place and functioning, there is no self-replication. The difficult task facing origin of life scientists is to discover which bio-molecule came first, and how, and then to show how the other bio-molecules developed to form the first reproducing organism.
Darwin himself thought that life may have arisen from a “warm little pond,” and in the early 20th century the scientists J.B.S. Haldane and A.I. Oparin independently speculated that a pre-biotic organic soup must have arisen early in the Earth’s history. Haldane and Oparin postulated a reducing atmosphere for the early Earth, an atmosphere containing abundant hydrogen, methane, ammonia, and water vapor. It was a logical assumption, as hydrogen is the most abundant element in the universe, and methane and ammonia are both hydrogen containing compounds. In this environment organic molecules were thought to naturally accumulate into the organic soup, and eventually, into life itself.
Miller’s Experiment
In 1953 a graduate student name Stanley Miller decided to test what may be called the “Oparin-Haldane Hypothesis.” Working under his advisor, Harold Urey, Miller created an experimental set-up to explore whether synthesis of organic molecules was possible in the hypothetical atmosphere of the early Earth.
Miller’s device (see Figure 1) contained three main compartments filled with water, methane, ammonia, and hydrogen. The water was boiled and electrical charges were sent through the vapors, which then passed into the next compartment and cooled and condensed. He ran the device for a week and then analyzed the resulting compounds. Miller discovered that among the compounds were some amino acids, the building blocks of proteins. This result sent a ripple through the scientific community. Miller had uncovered experimental evidence demonstrating the first steps of how life could have arisen purely through natural means. The experiment has become a staple in science textbooks, often accompanied by words like "the Miller-Urey experiment has shown that biological molecules can accumulate through natural means, and events like these led to the formation of life on Earth.1" But just how significant were Miller’s results?
Figure 1
First, it had been known for a century that organic compounds could be synthesized from inorganic ones, so the fact that organic materials can result from reactions with non-biological materials had already been discovered (Schopf 2002).
Second, it turns out that Miller’s experiment has several problems. One major problem with the Miller experiment is the assumption of a reducing atmosphere. Electrical sparks in an oxidizing atmosphere (like our current atmosphere) do not lead to any organic compounds. As mentioned above, the atmosphere was originally thought to be reducing (composed of hydrogen, methane, and ammonia) because of the abundance of hydrogen in the universe. The problem is that hydrogen is too light for earth's gravity to hold it, and it escapes out of our atmosphere (Brinkman 1969)(Catling et al. 2001). Geochemists and atmospheric scientists currently believe the Earth’s original atmosphere was neutral, not reducing (Miyakawa et al. 2002)(Shapiro 1986)(Schopf 2002)(Stanley 1999). The atmosphere came from the interior of the earth through volcanic outgassing. Small amounts of oxygen also had to be present due to photodissociation2, though the exact levels at which oxygen was present remain unclear (Brinkman 1969)(Stanley 1999). What is clear is that oxygen is present as far back as the rock record goes (Rosing and Frei 2003). In any case, it is now recognized that the early atmosphere was neutral and consisted of nitrogen, carbon dioxide, and water vapor with trace amounts of hydrogen, oxygen, and other gases (Schopf 2002)(Shapiro 1986)3.
In addition to hydrogen not being present except in trace amounts, the existence of methane and ammonia on the early Earth are also problematic. Since there was little oxygen on the early Earth, there was no ozone (O3) layer to absorb ultra-violet light. In addition, the younger sun would have produced ultra-violet light levels 30 times stronger than current levels (Schopf 2002)4. Methane and ammonia are both rapidly decomposed by UV rays and plausible suppliers of large amounts of these gases on the early earth do not exist. Neither could have been present on the early earth except in trace amounts (Schopf 2002)(Shapiro 1986). So, hydrogen, methane, and ammonia could at best be trace gases in the early atmosphere, but Miller's experiment postulated an atmosphere containing only them and water vapor. Clearly this is a fundamental flaw. Neutral atmospheres when sparked create only the simplest biological molecules, and this only with considerable hydrogen sources (Schopf 2002.) Since there are no plausible significant hydrogen sources for the early Earth's atmosphere, Miller’s experiment is something of a non-starter. However, there are other problems still.
Though it may seem trivial, the spark itself in Miller’s experiment is problematic, as there is no natural counterpart to the type of spark Miller used. He actually has tried simulating a lightning-type spark and, in his own words "very few organic compounds were produced and this discharge was not investigated further” (Shapiro 1986).
Miller's apparatus also contained a crucial piece: a trap which separated some of the resulting compounds, saving them from further exposure to energy. In nature, there is no such convenient trap, and the same energy that caused any molecules to bond would just as quickly (and in fact more commonly) break down those molecules. Energy is far more likely to break things down than to build them up. Effectively harnessing energy requires delicate, complicated, specific processes carried out by appropriate molecules. These processes do not occur in organic soup, and the soup would have moved toward equilibrium (the breakdown of all biological molecules is energetically favored in water) (Shapiro 1986)(Schopf 2002).
Interestingly, the specific arrangement of the device itself is in part responsible for the result of biological molecules. Miller had previously done the same experiment, with the same chemicals and spark, but with the pieces of the apparatus in a different arrangement and no biological molecules formed. The design of the apparatus favored the production of certain types of organic molecules, but in nature the process would not be so ordered (Shapiro 1986).
Ignoring all these problems, let us consider Miller's results anyway. The majority (85%) of the result can be referred to as tar, or organic goo, bearing no relevance to biological life (Shapiro 1986)(Schopf 2002). Of the approximately 50 major small organic compounds relevant to life, two were produced in Miller's experiments in a meaningful amount. These were the two simplest amino acids, glycine and alanine. There are twenty amino acids relevant to life, and though six were produced in Miller's experiment, only the aforementioned two were present in more than a miniscule amount. In addition, around half of the already small amounts of amino acids that were present are irrelevant to life due to the chirality problem (Shapiro 1986)(Schopf 2002). The chirality problem is that amino acids come in two forms, mirror images of the other. Only one type (the left-handed ones) are relevant to biology, and amino acids which spontaneously form will end being about half one type and half the other.
Miller’s results consisted of small amounts of a few of the molecules needed for life.5 The majority of a simple organism like a bacterium is composed of proteins (microscopic molecular machines), nucleic acids (DNA and RNA), polysaccharides (sugars), and lipids (fatty membranes). "None have been detected, in any amount, in a Miller-Urey experiment" (Shapiro 1986).
Conclusion
In short, even if the Miller experiment had turned out to simulate realistic conditions on the early Earth, its results are truly a most insignificant step toward figuring out how life arose. It is clear that the significance of this experiment has been grossly exaggerated. Miller himself has said as much. The severe problems with the experiment have led to interest in a variety of other ideas to explain the origin of life, including hydrothermal vents, meteorite seeding, and even panspermia (the idea that extraterrestrials planted the first life forms here).
So then, students should be taught that the Miller experiment is what it appears to be: a historically important but outdated and flawed experiment.
A further note: even if Miller’s experiment had generated every last biological molecule known to man, present in exactly the right proportions, we still would not have solved the origin of life. One could have all the right pieces together, in any conditions, and still “life” would not emerge. Scientists are unable to re-create any kind of cell even with all the right materials and controlled conditions. How then did it happen by accident on the early Earth?
1. Some modern textbooks do acknowledge the deficiencies in Miller’s experiments, but the Miller experiment remains an icon of pre-biotic evolution.
2. Photodissociation is the breakdown of H2O into hydrogen and oxygen by sunlight.
3. Scientists estimate that oxygen levels just 1% of current levels would prevent organic molecules from forming at all on the early earth.
4. The strong UV rays would have also instantly destroyed any biological molecules that had formed.
5. Subsequent Miller-type experiments have resulted in the production of almost all of the 20 amino acids found in proteins. While interesting, see the rest of this essay.
Works Cited
Brinkman, R.T., 1969, The photodissociation of water vapor, evolution of oxygen and escape of hydrogen in the earth’s atmosphere. PhD. Diss., California Institute of Technology, http://resolver.caltech.edu/CaltechETD:etd-10062004-120013
Catling et al., 2001, Biogenic Methane, Hydrogen Escape, and the Irreversible Oxidation of the Early Earth. Science, 839-843
Miyakawa et al. 2002. Prebiotic synthesis from CO atmospheres: implications for the origin of life. PNAS, 14628-14631
Rosing, M.T. and Frei, R., 2003. U-rich Archaean sea-floor sediments from Greenland – indications of > 3700 Ma oxygenic photosynthesis. Earth and Planetary Science Letters, 237-244.
Schopf, W., ed. 2002. Life’s Origin: the beginnings of biological evolution, California, UCAL Press
Shapiro, R. 1986. Origins: a skeptic’s guide to the creation of life on earth, New York, Simon & Schuster Inc.
Stanley, S. 1999. Earth System History, New York, W.H. Freedman and Company
Though Darwin’s theory of evolution applies to the diversification of biological life, not the origin of biological life, scientists have nonetheless attempted to find a naturalistic explanation of life’s origin through recourse to Darwinian type events. Competition between the first self-replicating molecules is thought to have led to increasingly efficient and complicated bio-molecules until the first primitive cell emerged. The question then is: where did these first self-replicated molecules come from? Every living thing known to science utilizes the same set of bio-molecules to reproduce: DNA, RNA, and proteins. These are enormously complicated molecules that, respectively, contain genetic information, the ability to translate and transport genetic information, and the ability to construct molecular machines (including other proteins) based on that information. Without all three components in place and functioning, there is no self-replication. The difficult task facing origin of life scientists is to discover which bio-molecule came first, and how, and then to show how the other bio-molecules developed to form the first reproducing organism.
Darwin himself thought that life may have arisen from a “warm little pond,” and in the early 20th century the scientists J.B.S. Haldane and A.I. Oparin independently speculated that a pre-biotic organic soup must have arisen early in the Earth’s history. Haldane and Oparin postulated a reducing atmosphere for the early Earth, an atmosphere containing abundant hydrogen, methane, ammonia, and water vapor. It was a logical assumption, as hydrogen is the most abundant element in the universe, and methane and ammonia are both hydrogen containing compounds. In this environment organic molecules were thought to naturally accumulate into the organic soup, and eventually, into life itself.
Miller’s Experiment
In 1953 a graduate student name Stanley Miller decided to test what may be called the “Oparin-Haldane Hypothesis.” Working under his advisor, Harold Urey, Miller created an experimental set-up to explore whether synthesis of organic molecules was possible in the hypothetical atmosphere of the early Earth.
Miller’s device (see Figure 1) contained three main compartments filled with water, methane, ammonia, and hydrogen. The water was boiled and electrical charges were sent through the vapors, which then passed into the next compartment and cooled and condensed. He ran the device for a week and then analyzed the resulting compounds. Miller discovered that among the compounds were some amino acids, the building blocks of proteins. This result sent a ripple through the scientific community. Miller had uncovered experimental evidence demonstrating the first steps of how life could have arisen purely through natural means. The experiment has become a staple in science textbooks, often accompanied by words like "the Miller-Urey experiment has shown that biological molecules can accumulate through natural means, and events like these led to the formation of life on Earth.1" But just how significant were Miller’s results?
Figure 1
First, it had been known for a century that organic compounds could be synthesized from inorganic ones, so the fact that organic materials can result from reactions with non-biological materials had already been discovered (Schopf 2002).
Second, it turns out that Miller’s experiment has several problems. One major problem with the Miller experiment is the assumption of a reducing atmosphere. Electrical sparks in an oxidizing atmosphere (like our current atmosphere) do not lead to any organic compounds. As mentioned above, the atmosphere was originally thought to be reducing (composed of hydrogen, methane, and ammonia) because of the abundance of hydrogen in the universe. The problem is that hydrogen is too light for earth's gravity to hold it, and it escapes out of our atmosphere (Brinkman 1969)(Catling et al. 2001). Geochemists and atmospheric scientists currently believe the Earth’s original atmosphere was neutral, not reducing (Miyakawa et al. 2002)(Shapiro 1986)(Schopf 2002)(Stanley 1999). The atmosphere came from the interior of the earth through volcanic outgassing. Small amounts of oxygen also had to be present due to photodissociation2, though the exact levels at which oxygen was present remain unclear (Brinkman 1969)(Stanley 1999). What is clear is that oxygen is present as far back as the rock record goes (Rosing and Frei 2003). In any case, it is now recognized that the early atmosphere was neutral and consisted of nitrogen, carbon dioxide, and water vapor with trace amounts of hydrogen, oxygen, and other gases (Schopf 2002)(Shapiro 1986)3.
In addition to hydrogen not being present except in trace amounts, the existence of methane and ammonia on the early Earth are also problematic. Since there was little oxygen on the early Earth, there was no ozone (O3) layer to absorb ultra-violet light. In addition, the younger sun would have produced ultra-violet light levels 30 times stronger than current levels (Schopf 2002)4. Methane and ammonia are both rapidly decomposed by UV rays and plausible suppliers of large amounts of these gases on the early earth do not exist. Neither could have been present on the early earth except in trace amounts (Schopf 2002)(Shapiro 1986). So, hydrogen, methane, and ammonia could at best be trace gases in the early atmosphere, but Miller's experiment postulated an atmosphere containing only them and water vapor. Clearly this is a fundamental flaw. Neutral atmospheres when sparked create only the simplest biological molecules, and this only with considerable hydrogen sources (Schopf 2002.) Since there are no plausible significant hydrogen sources for the early Earth's atmosphere, Miller’s experiment is something of a non-starter. However, there are other problems still.
Though it may seem trivial, the spark itself in Miller’s experiment is problematic, as there is no natural counterpart to the type of spark Miller used. He actually has tried simulating a lightning-type spark and, in his own words "very few organic compounds were produced and this discharge was not investigated further” (Shapiro 1986).
Miller's apparatus also contained a crucial piece: a trap which separated some of the resulting compounds, saving them from further exposure to energy. In nature, there is no such convenient trap, and the same energy that caused any molecules to bond would just as quickly (and in fact more commonly) break down those molecules. Energy is far more likely to break things down than to build them up. Effectively harnessing energy requires delicate, complicated, specific processes carried out by appropriate molecules. These processes do not occur in organic soup, and the soup would have moved toward equilibrium (the breakdown of all biological molecules is energetically favored in water) (Shapiro 1986)(Schopf 2002).
Interestingly, the specific arrangement of the device itself is in part responsible for the result of biological molecules. Miller had previously done the same experiment, with the same chemicals and spark, but with the pieces of the apparatus in a different arrangement and no biological molecules formed. The design of the apparatus favored the production of certain types of organic molecules, but in nature the process would not be so ordered (Shapiro 1986).
Ignoring all these problems, let us consider Miller's results anyway. The majority (85%) of the result can be referred to as tar, or organic goo, bearing no relevance to biological life (Shapiro 1986)(Schopf 2002). Of the approximately 50 major small organic compounds relevant to life, two were produced in Miller's experiments in a meaningful amount. These were the two simplest amino acids, glycine and alanine. There are twenty amino acids relevant to life, and though six were produced in Miller's experiment, only the aforementioned two were present in more than a miniscule amount. In addition, around half of the already small amounts of amino acids that were present are irrelevant to life due to the chirality problem (Shapiro 1986)(Schopf 2002). The chirality problem is that amino acids come in two forms, mirror images of the other. Only one type (the left-handed ones) are relevant to biology, and amino acids which spontaneously form will end being about half one type and half the other.
Miller’s results consisted of small amounts of a few of the molecules needed for life.5 The majority of a simple organism like a bacterium is composed of proteins (microscopic molecular machines), nucleic acids (DNA and RNA), polysaccharides (sugars), and lipids (fatty membranes). "None have been detected, in any amount, in a Miller-Urey experiment" (Shapiro 1986).
Conclusion
In short, even if the Miller experiment had turned out to simulate realistic conditions on the early Earth, its results are truly a most insignificant step toward figuring out how life arose. It is clear that the significance of this experiment has been grossly exaggerated. Miller himself has said as much. The severe problems with the experiment have led to interest in a variety of other ideas to explain the origin of life, including hydrothermal vents, meteorite seeding, and even panspermia (the idea that extraterrestrials planted the first life forms here).
So then, students should be taught that the Miller experiment is what it appears to be: a historically important but outdated and flawed experiment.
A further note: even if Miller’s experiment had generated every last biological molecule known to man, present in exactly the right proportions, we still would not have solved the origin of life. One could have all the right pieces together, in any conditions, and still “life” would not emerge. Scientists are unable to re-create any kind of cell even with all the right materials and controlled conditions. How then did it happen by accident on the early Earth?
1. Some modern textbooks do acknowledge the deficiencies in Miller’s experiments, but the Miller experiment remains an icon of pre-biotic evolution.
2. Photodissociation is the breakdown of H2O into hydrogen and oxygen by sunlight.
3. Scientists estimate that oxygen levels just 1% of current levels would prevent organic molecules from forming at all on the early earth.
4. The strong UV rays would have also instantly destroyed any biological molecules that had formed.
5. Subsequent Miller-type experiments have resulted in the production of almost all of the 20 amino acids found in proteins. While interesting, see the rest of this essay.
Works Cited
Brinkman, R.T., 1969, The photodissociation of water vapor, evolution of oxygen and escape of hydrogen in the earth’s atmosphere. PhD. Diss., California Institute of Technology, http://resolver.caltech.edu/CaltechETD:etd-10062004-120013
Catling et al., 2001, Biogenic Methane, Hydrogen Escape, and the Irreversible Oxidation of the Early Earth. Science, 839-843
Miyakawa et al. 2002. Prebiotic synthesis from CO atmospheres: implications for the origin of life. PNAS, 14628-14631
Rosing, M.T. and Frei, R., 2003. U-rich Archaean sea-floor sediments from Greenland – indications of > 3700 Ma oxygenic photosynthesis. Earth and Planetary Science Letters, 237-244.
Schopf, W., ed. 2002. Life’s Origin: the beginnings of biological evolution, California, UCAL Press
Shapiro, R. 1986. Origins: a skeptic’s guide to the creation of life on earth, New York, Simon & Schuster Inc.
Stanley, S. 1999. Earth System History, New York, W.H. Freedman and Company
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