Based on the fact that it is just as easy for left-handed amino acids to form in the laboratory as right-handed amino acids, and just as easy for left-handed sugars to form in the laboratory as right-handed sugars, we would expect for there to be a symmetry in the handedness of amino acids, with an equal amount of left-handed and right-handed amino acids. We would also would expect a symmetry in the handedness of sugars, with equal amounts of left-handed sugars and right-handed sugars. But what we see is an asymmetry, with living things having only left-handed amino acids and right-handed sugars in DNA.
I can give an analogy for why homochirality is such a mystery. Let us imagine a very large box filled with 5000 cards, each displaying one of the letters in the alphabet. On one side of each card is a letter. For example:
On the back side of each card is the mirror image of the letter on the front side of the card. For example:
We can calculate this probability using a large exponents calculator. The probability of you getting 5000 amino acids that were all left-handed from a random set of amino acids that was equally left-handed and right-handed is 1 in 2 to the 5000th power. As we can see in the screen below (using this large exponents calculator), such a probability is less than 1 in 10 to the 1500th power, less than 1 in 101500.
This probability of less than 1 in 101500 is only one of two miscroscopic probabilities involved in the origin of life. There is an equally tiny probability involved in having all of the amino acids spelling out functional fine-tuned proteins. That probability is not as easy to exactly calculate. But you can a rough idea of that probability by realizing that the probability of 5000 amino acids randomly forming into 50 functional proteins is comparable to the probability of 5000 letter cards dropped from a tall building falling to the ground and forming into a useful 5000-letter instruction (such as an instruction on exactly how to build a car or a human).
I haven't even discussed the improbability of getting all the sugars in RNA and DNA molecules to be right-handed, which is an additional explanatory nightmare every bit as bad as the one just discussed. In fact, the improbability is far worse if we consider things from a DNA perspective. The DNA to specify 5000 amino acids requires three nucleotides to specify each amino acid, and each one of those nucleotides involves a sugar that is always right-handed.
To get 15,000 sugars that are all right-handed, given random sugars that can have an equal likelihood of being left-handed or right-handed, you need some luck with a probability of 1 in 2 to the fifteenth thousandth power. That is a likelihood of less than 1 in 104500, as we can see below.
The attempt to explain homochirality was made in the paper "The Chiral Puzzle of Life" by Noemie Globus and Roger D. Blandford (and another similar paper by the same authors). The authors imagine two different types of life existing in the early Earth: what they call "live" life (like the life that we know), and what they call "evil" life. The "evil" life they imagine is the mirror image of known life, with an inverse homochirality. So instead of having sugars that are all right-handed, the "evil" life would have sugars that are only left-handed. And instead of having amino acids that are only left-handed, the "evil" life would have amino acids that are only right-handed. There is zero evidence that any such "evil" life ever existed, and scientists do not typically maintain or believe that any such "evil" life (with an inverse homochirality) ever existed.
The authors discuss some far-fetched scenario by which cosmic rays might have caused the "live" life to have an advantage over the imagined "evil" life. In a nutshell, this attempt at an explanation is the story that "homochiral life defeated its mirror-image competitor, with some help from cosmic rays."
There are two gigantic problems with this attempt at an explanation. The first is that it does nothing at all to explain the gigantically improbable appearance of the "live" life with homochirality. The second is that the attempt at an explanation requires us to believe that there was also this "evil" life existing with an inverse homochirality that was the mirror image of life's homochirality, something for which there is no evidence. The probability of the appearance of such "evil life" with inverse homochirality is just as microscopic as the probability of the appearance of the "live life" with regular homochirality.
So instead of requiring us to believe in one accidental homochirality miracle of luck with a likelihood of less than 1 in 101500 , the Globus and Blandford scenario requires us to believe in two accidental homochirality miracles of luck, each with a likelihood of less than 1 in 101500 (less than 1 in 10 to the 1500th power). Instead of making biological homochirality easier to explain, the Globus and Blandford scenario makes homochirality more than 101500 times harder to explain. Under the Globus and Blandford scenario, the improbability of homochirality becomes less than 1 in 101500 multiplied by less than 1 in 101500, which is less than 1 in 103000. Talk about marching in the wrong direction.
It does not help at all if you consider only the origin of a DNA molecule, without even considering proteins made from the genes in such a molecule. A DNA or RNA molecule could not be the beginning of life if it had fewer than 5000 nucleotides (enough to specify about 1700 amino acids). Each one of those nucleotides includes a sugar that might randomly be either left-handed or right-handed (all sugars in DNA are right-handed). The chance of you getting sugar homochirality (nothing but right-handed sugars) with 5000 random nucleotides (each having a sugar of random handedness) is less than 1 in 101500, and the chance of you getting some "evil life" with inverse homochirality is also less than 1 in 101500. The chance of having both the homochirality and the inverse homochirality (as in the Globus and Blandford scenario) would be less than 1 in 103000.
But our hype-heaving science news sites have promoted the Globus and Blandford scenario as a "possible explanation for homochirality." This merely proves that the goofiest kind of attempt at an explanation can be trumpeted by our yearning-to-say-some-progress-was-made science news media, which has a ravenous appetite for clickbait teaser headlines that can increase web page clicks and advertising revenue. But at least one of these summary stories gives us a hint that "there's no there there" in the Globus and Blandford account for homochirality. After giving us a misleading headline of "Cosmic Rays May Explain Life’s Bias for Right-Handed DNA," the story in Quanta magazine confesses, "The theory doesn’t address, for example, how 'live' organisms and 'evil' organisms managed to materialize from a primordial smoothie containing both right- and left-handed building blocks." Which means it does nothing to solve the homochirality problem.
Homochirality in living things is something so unlikely to occur by chance that we can rightly call it accidentally unachievable. The only way to naturally explain it would be to discover some causal factors which excluded all right-handed amino acids in the early earth, and excluded all left-handed sugars. No one has presented any such explanation. Whenever sugars or amino acids are created in the laboratory or during experiments simulating the early earth, left-handed sugars appear as frequently as right-handed sugars, and left-handed amino acids appear as frequently as right-handed amino acids.
On the back side of each card is the mirror image of the letter on the front side of the card. For example:
Now, let us suppose that someone dumped this large box of cards from the top of a tall building. Imagine that the cards fell to the ground, forming a set of useful instructions that was 5000 letters long, and that none of those letters were the mirror images of the letters in the alphabet.
We would have two gigantic difficulties in explaining this outcome. The first problem would be in explaining how we accidentally got a useful and intelligible set of instructions 5000 characters long. The second problem would be in explaining how the 5000 cards all ended up showing the card side with the regular English letter, with none of them showing the mirror image of the letter on the opposite side of the card.
The origin of life is as hard-to-explain as the falling cards event just described. A team of 9 scientists wrote a scientific paper entitled, “Essential genes of a minimal bacterium.” It analyzed a type of bacteria (Mycoplasma genitalium) that has “the smallest genome of any organism that can be grown in pure culture.” The paper concluded that 382 of this bacteria's protein-coding genes (72 percent) are essential. So we can conclude that any self-reproducing cell would need a minimum of at least 50 types of proteins (each coded by a separate gene), and that such proteins would have an average amino acid length of at least 100 (the average amino acid length of the proteins in the most primitive prokaryotic cells is more than 150). 50 times 100 is 5000.
So you need at least 5000 amino acids arranged in just the right way to get a self-reproducing cell. The chance of such a thing occurring accidentally would be comparable to the chance of someone dumping a very large box with 5000 characters cards from a tall building, and those cards accidentally forming into a useful, coherent 5000-character set of instructions. And just as it would be fantastically improbable that dumping such a set of 5000 character cards from a tall building would ever result in the cards only showing the regular version of a letter on the front of each card, and never showing the mirror image of a letter on the back of each card, it would be fantastically improbable that you would end up with a cell containing about 5000 amino acids that were all left-handed, as in living things.
So you need at least 5000 amino acids arranged in just the right way to get a self-reproducing cell. The chance of such a thing occurring accidentally would be comparable to the chance of someone dumping a very large box with 5000 characters cards from a tall building, and those cards accidentally forming into a useful, coherent 5000-character set of instructions. And just as it would be fantastically improbable that dumping such a set of 5000 character cards from a tall building would ever result in the cards only showing the regular version of a letter on the front of each card, and never showing the mirror image of a letter on the back of each card, it would be fantastically improbable that you would end up with a cell containing about 5000 amino acids that were all left-handed, as in living things.
We can calculate this probability using a large exponents calculator. The probability of you getting 5000 amino acids that were all left-handed from a random set of amino acids that was equally left-handed and right-handed is 1 in 2 to the 5000th power. As we can see in the screen below (using this large exponents calculator), such a probability is less than 1 in 10 to the 1500th power, less than 1 in 101500.
This probability of less than 1 in 101500 is only one of two miscroscopic probabilities involved in the origin of life. There is an equally tiny probability involved in having all of the amino acids spelling out functional fine-tuned proteins. That probability is not as easy to exactly calculate. But you can a rough idea of that probability by realizing that the probability of 5000 amino acids randomly forming into 50 functional proteins is comparable to the probability of 5000 letter cards dropped from a tall building falling to the ground and forming into a useful 5000-letter instruction (such as an instruction on exactly how to build a car or a human).
I haven't even discussed the improbability of getting all the sugars in RNA and DNA molecules to be right-handed, which is an additional explanatory nightmare every bit as bad as the one just discussed. In fact, the improbability is far worse if we consider things from a DNA perspective. The DNA to specify 5000 amino acids requires three nucleotides to specify each amino acid, and each one of those nucleotides involves a sugar that is always right-handed.
To get 15,000 sugars that are all right-handed, given random sugars that can have an equal likelihood of being left-handed or right-handed, you need some luck with a probability of 1 in 2 to the fifteenth thousandth power. That is a likelihood of less than 1 in 104500, as we can see below.
Recently in the science news there was mention of a new attempt to explain the problem of homochirality, the problem of how we got life that uses only left-handed amino acids and right-handed sugars. The attempt is very goofy, because instead of making homochirality easier to explain, the attempt makes homochirality many, many times harder to explain.
The attempt to explain homochirality was made in the paper "The Chiral Puzzle of Life" by Noemie Globus and Roger D. Blandford (and another similar paper by the same authors). The authors imagine two different types of life existing in the early Earth: what they call "live" life (like the life that we know), and what they call "evil" life. The "evil" life they imagine is the mirror image of known life, with an inverse homochirality. So instead of having sugars that are all right-handed, the "evil" life would have sugars that are only left-handed. And instead of having amino acids that are only left-handed, the "evil" life would have amino acids that are only right-handed. There is zero evidence that any such "evil" life ever existed, and scientists do not typically maintain or believe that any such "evil" life (with an inverse homochirality) ever existed.
The authors discuss some far-fetched scenario by which cosmic rays might have caused the "live" life to have an advantage over the imagined "evil" life. In a nutshell, this attempt at an explanation is the story that "homochiral life defeated its mirror-image competitor, with some help from cosmic rays."
There are two gigantic problems with this attempt at an explanation. The first is that it does nothing at all to explain the gigantically improbable appearance of the "live" life with homochirality. The second is that the attempt at an explanation requires us to believe that there was also this "evil" life existing with an inverse homochirality that was the mirror image of life's homochirality, something for which there is no evidence. The probability of the appearance of such "evil life" with inverse homochirality is just as microscopic as the probability of the appearance of the "live life" with regular homochirality.
So instead of requiring us to believe in one accidental homochirality miracle of luck with a likelihood of less than 1 in 101500 , the Globus and Blandford scenario requires us to believe in two accidental homochirality miracles of luck, each with a likelihood of less than 1 in 101500 (less than 1 in 10 to the 1500th power). Instead of making biological homochirality easier to explain, the Globus and Blandford scenario makes homochirality more than 101500 times harder to explain. Under the Globus and Blandford scenario, the improbability of homochirality becomes less than 1 in 101500 multiplied by less than 1 in 101500, which is less than 1 in 103000. Talk about marching in the wrong direction.
It does not help at all if you consider only the origin of a DNA molecule, without even considering proteins made from the genes in such a molecule. A DNA or RNA molecule could not be the beginning of life if it had fewer than 5000 nucleotides (enough to specify about 1700 amino acids). Each one of those nucleotides includes a sugar that might randomly be either left-handed or right-handed (all sugars in DNA are right-handed). The chance of you getting sugar homochirality (nothing but right-handed sugars) with 5000 random nucleotides (each having a sugar of random handedness) is less than 1 in 101500, and the chance of you getting some "evil life" with inverse homochirality is also less than 1 in 101500. The chance of having both the homochirality and the inverse homochirality (as in the Globus and Blandford scenario) would be less than 1 in 103000.
But our hype-heaving science news sites have promoted the Globus and Blandford scenario as a "possible explanation for homochirality." This merely proves that the goofiest kind of attempt at an explanation can be trumpeted by our yearning-to-say-some-progress-was-made science news media, which has a ravenous appetite for clickbait teaser headlines that can increase web page clicks and advertising revenue. But at least one of these summary stories gives us a hint that "there's no there there" in the Globus and Blandford account for homochirality. After giving us a misleading headline of "Cosmic Rays May Explain Life’s Bias for Right-Handed DNA," the story in Quanta magazine confesses, "The theory doesn’t address, for example, how 'live' organisms and 'evil' organisms managed to materialize from a primordial smoothie containing both right- and left-handed building blocks." Which means it does nothing to solve the homochirality problem.
Homochirality in living things is something so unlikely to occur by chance that we can rightly call it accidentally unachievable. The only way to naturally explain it would be to discover some causal factors which excluded all right-handed amino acids in the early earth, and excluded all left-handed sugars. No one has presented any such explanation. Whenever sugars or amino acids are created in the laboratory or during experiments simulating the early earth, left-handed sugars appear as frequently as right-handed sugars, and left-handed amino acids appear as frequently as right-handed amino acids.
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