One of the smaller mysteries of life is that it is built out of asymmetric building blocks. The amino acids
that are chained together to make proteins are asymmetric, as are the sugars that are chained together to make starch, cellulose, and other useful polysaccharides
. The standard way to show this, is by making a solution of the specific molecule, shining polarized light
through it, and demonstrating that the "plane of polarization" has rotated. Those that rotate the light counter-clockwise are "left-handed" and those that rotate it clockwise are "right-handed". Using the letters "l" for left, and "d" for right, we now can label the mirror-image molecules with their appropriate chirality
. (The amino acids are labelled L or D not by their rotation of light, but by their chemical synthesis from l- or d-glyceraldehyde. All L-amino acids do not rotate light the same direction, but they can all be derived from l-glyceraldehyde.)
A science fair project
that deeply impressed me as a junior high student, was to take two plane polarizers (such as those found in sunglasses) and put different thicknesses of ordinary cellophane tape in between, by taping it to a clear sheet of acrylic (overhead transparency for those old enough to remember). Each thickness of tape (D-sugar!) would rotate the light more, and when viewed through "crossed" polarizers, different colors would appear as if by magic. The explanation was even more magical, and I spent an enjoyable hour playing with tape, polarizers and white Karo syrup.
Evidently, this also impressed chemists in the early 1800's, since inorganic crystals did not show these colors. They took this as evidence of one more way that living things were different from non-living things--they turned colors under polarized light, as well as multiplied, fermented, decomposed, and otherwise did miracles with common substances. Which is why they separated "inorganic" from "organic" chemistry. Darwin, of course, refused to acknowledge any separation between organic and inorganic chemistry, and this intransigence has plagued his theory ever since.
The actual explanation is more mundane, but still amazing. In order to affect polarized light, a molecule has to have a fundamentally asymmetric shape, but with the same chemistry. Suppose we have an inorganic salt, Na-Cl. Can we distinguish it from Cl-Na? No, because a simple rotation brings it back to itself again, and in a water solution, the molecules are spinning rapidly. How about something like water, H
, which looks like a bent paper clip? Nope, once again, rotation brings it back to itself. Okay, what about three different atoms like Br-Ca-Cl or the triangle molecule F-BClBr
, are they different from Cl-Ca-Br or F-BBrCl
? No, we can still rotate them back into the other kind. What it takes to be rotationally distinct is a tetrahedron, a chemical with 4 bonds that has different elements on each bond. Halon (a fire-extinguishing gas) is a good candidate: H-F
-Br is different from H-F
-Cl, and no amount of rotation will make the two molecules the same despite the same chemical formula. Only reflection in a mirror will get them to look the same, hence the name "stereochemistry
". Now coincidentally, the only atoms in the periodic table that regularly take 4 bonds are C, Si, and Ge. Since silicon chemistry is relatively rare, and germanium even rarer, only carbon is routinely capable of optical stereo-isomers
, confirming the idea that only life shows polarization effects.
At age 29, Louis Pasteur
, one of my all time hero scientists, was working
with tartaric acid, a byproduct sediment of wine production, and discovered that he could get tartaric acid to rotate polarized light but identical "paratartaric acid" didn't. Being an objective scientist, he got out his microscope and discovered that a sample of crystallized paratartaric acid had two kinds of crystals that appeared to be mirror images of each other. He painfully separated them into two piles with a tweezers and a microscope, and when he dissolved the two piles, lo-and-behold, one rotated light "l-" and the other "d-".
So in fact, paratartaric acid _did
_ rotate light, but being an equal mixture of L and D ("racemic"), it appeared to do nothing, that is, until it was manually separated. Today we have separation columns (chiral beads packed into a big cylinder) that can slow down the transport of D more than L, so we run the solution through a separator and the first stuff out is enriched in L. Keep doing this separation, and we get pure L without the painful approach used by Pasteur.
But the peculiar point of all this nomenclature, is that life always makes L-type amino acids and it always makes D-type sugars. All the inorganic chemistry in the world--cooking with acids, neutralizing with bases--makes equal amounts of L and D. Nothing in inorganic chemistry excepting carbon itself, is stereoactive. So despite the great advances of 20th century chemistry into the synthesis of organic molecules, none of these syntheses produce pure L-amino acids or pure D-sugars the way life does. The Miller-Urey experiment
, for all the organic material it made with electric sparks and methane, produced only trace amounts of racemic amino acids.
Why is life different? Because the chemistry of life is made possible by catalysts, by enzymes, which speed up the reactions millions and billions of times over what inorganic chemistry can do. And these enzymes are made up proteins that are made up of L-amino acids, so the enzymes select for L-type amino acid formation. So this becomes a chicken-and-egg problem: the stereoactivity of life's chemicals is caused by their formation by stereo-active enzymes which are in turn made by stereo-active life....
Well that explains that all life will remain the same chirality, (Greek for handedness), but where did that homochirality
come from originally? Why from the first life of course. So in typical Darwinian reductionist fashion, we say that all life descended from one common ancestor who established the chirality of biochemistry. This ancestor was the first cell, the OOL
who first showed up (never say "created"!) some 3.8 billion years ago.
It all sounds neat and pretty, and fully consistent with Darwinian materialist philosophy.
There's just one problem. OOL is still highly unlikely.
, again, demonstrated this by building a flask
full of "pasteurized" nutrient broth that did not spontaneously generate
life. His experiments finally silenced all the critics as well as began the canning industry, and biology accepted his conclusion: life only comes from life. But a short decade or so later, we find Darwin arguing that the origin of life must be accidental, spontaneous. Which is it?
When we consider the very simplest cell, the most stripped down, bare-bones, hardly recognizable cell, we still have at least 100 protein machines working away in this Darwinian factory. The numbers vary, since no one has actually made such a stripped down cell, but estimates come from examining, for example, Craig Venter
's Mycoplasma laboratorium
which began with a very small parasitic bacteria M. genitalium
of 482 genes in 580,000 DNA base pairs. By removing those that seemed unnecessary, Venter reduced the count to 382. Three hundred and eighty two genes are at least 382 proteins, (more likely 1000 proteins since the cell performs lots of after-market modifications on proteins) which by dividing the base pairs into codons, demonstrate that these proteins average some 401 amino acids long. So in Venter's "artificial life" we are talking at a minimum 153,000 amino acid sequences, chosen from a menu of 21 possible amino acids. Mathematically this is 21^153000 power, which is 10^200,000.
But life doesn't require one long protein, but a minimum of 100 separate
proteins in any order, so the odds are more like (21^401
)^100=21^501. (We note that this is a best case using the average protein length and in a minimal 100-protein cell, because if some of the
proteins are longer than 401 amino acids or more than 100 in number, the odds go up rapidly to approach our worst case above.) Converting to base 10, we have 10^1107 is the number of possibilities or 1 in 10^1107 odds.
Just how small is this chance? Well suppose we built a computer out of every atom in the known universe, 10^80 atoms. And suppose we did a computer calculation, a clock cycle, at the fastest physically possible speed, a Planck time of 10^34 per second (which is much faster than any known process, but anything faster would destroy quantum mechanics). Then we would have only completed 10^130 trials since the beginning of the known universe. That is, we would still have 10^977 more possible lifeforms to calculate before we could be certain of randomly discovering our hypothetical "stripped down" cell. Even the "cosmic landscape" of baby universes being formed at the fastest possible speed at every possible location in the known universe and making babies at the same rate since the Big Bang only knocks off 500 zeroes. That leaves us 10^477 more tries to go, and we've filled up every possible baby universe with massive computers all trying to make that hypothetical simplest cell.
Nope, we aren't even close.
One objection is that we don't need _exactly
_ the right combination of amino acids, that there are many _nearly
_ similar solutions that will construct a simple cell and work just fine. (BTW, this requires that life not be "optimal", which is contrary to experiment.) For example, there is no law of physics that says the mirror image amino acids wouldn't also make viable life. Life could evolve L or D, whichever it randomly hit first. So the "density" of viable stripped down cells in the landscape of possible combinations might be high enough that in the 1 billion years after the Earth was formed, life showed up with probability 1.
That sounds reasonable until Sir Fred Hoyle
demonstrated why this solution doesn't work
. All inorganic reactions have a speed that depends upon the concentrations of the reagents. Rate = constant * concentration_n * concentration_m, or in chemical notation, R=k [n][m], So when life showed up, what was the expected concentration in the "pond" of all the important amino acids needed? A solution with parts per thousand, parts per million, parts per billion? Miller-Urey
suggests that a realistic concentration would be at the very best parts per billion, though maybe, just maybe, we could get parts per million in a dehydrated pond somewhere, sometime.
Okay, take all your favorite ingredients. Put them in a flask at the parts per hundred level. This is somewhere between ten thousand and ten million times more concentrated than anything available in the primitive earth. Since there are two ingredients that have to react, this means the rate of the reaction is somewhere between 100 million and 100 trillion times faster than on the primitive Earth. It took less than a billion years (some would argue less than 100My) for life to show up after the Earth cooled down to room temperature from a molten blob (the Hadean), so that means our reaction should produce life in about, oh, 10 minutes to 10 years. Add in the fact that many of the reactions involve a third or fourth component, and this calculation suggests life should form in microseconds in our flask. Did it? Hoyle gives the obvious answer
So the fact that Pasteur could make nutrient broth that didn't spontaneous produce life in 10 years is real proof that spontaneous generation is not so easy.
Can we wiggle out of this? Can we suggest that life is harder to make than it looks, but not so hard as to prevent it from happening on Earth? Let's suppose that it take 100 years for our nutrient broth to generate life, wouldn't that still be consistent with Pasteur and laboratory experiments?
Okay, let's accept this hypothesis. Life, by whatever means we have yet to discover, is likely to form from non-life because there are many routes to viable cells. One of those viable routes involves mirror image amino acids. That is, before there was life on Earth, all production of amino acids through non-living or "abiotic" Miller-Urey methods, would produce racemic mixtures of L and D amino acids. Since no physical or chemical or biological barrier exists to mirror life, we would expect a 50% chance that life is like ours, and 50% chance that it is the mirror image.
So if life has spontaneously been created twice, there is a 50% chance of finding mirror life. If it has been spontaneously created three times, there is a 75% chance of finding mirror life. If it happened four times, we are up to 88%. And quickly you can see that if life spontaneously arose anywhere close to 10 times or more, we have virtually a 100% chance of finding mirror life.
We looked. Pasteur looked. Richard Hoover and Paul Davies
looked. Have they found it? No.
Well, maybe the fact that life is flooding the environment with L-amino acids prevents mirror life from forming. In that case, we should find mirror life on isolated locations, such as deep rocks or islands, or geothermal vents. Nope. How about if life were spontaneously created on other planets? (See Stephen Hawking's new video
Now we're talking. But we _have
_ actually looked at extraterrestrial samples that fall to Earth. They are called meteorites. And some meteorites that are black and crumbly and full of carbon, called "carbonaceous chondrites
", are also full of amino acids
. And these amino acids are predominantly L, that is, they always have more L than D in them. (The D variety appears as heat and cosmic rays "scramble" the L-variety, but all such random processes can never turn majority-L into majority-D, at best they leave it 50/50 racemic.)
That is, either there's an abiotic process on carbonaceous chondrites that makes chiral amino acids, (which if demonstrated, would win a chemistry Nobel prize) or there's chiral life on those meteorites that is always L-amino. This could arise from contamination from Earth, except that those amino acids are million or more years old, and are made from non-Earth carbon (with too much C-13 for Earth). So it looks like there is non-Earth life
that is always L-amino.
So far our prediction about mirror life is not working out too well.
Okay, maybe we were too quick to jump from physics to chemistry. Perhaps there is some "emergent" chemistry law that prevents D-amino acids from forming life. Say, the D-amino is less soluble than the L-amino acid. (Actually, we can check that out and it doesn't work.) But if it were true, there might be a reason why spontaneous generation always produces L-amino life.
A recent paper
came out in an American Chemical Society journal
that essentially made that claim. Homochirality is a consequence of chemistry, it argued. L-type life is simply a result of spontaneous generation in an abiotic homochiral chemistry.
Whew! That was a close one. We can go back to our random OOL, and keep our Darwinian belief in spontaneous generation
Or can we? A closer look at the paper reveals that it is playing word games with "racemic" so as to achieve the impression that it is a solution to the homochirality problem. What they actually discovered was that there are two different kinds of a racemic mixture of an essential amino acid--aspartic acid. One type of racemic mixture is formed the way Pasteur did, mixing crystals of L and D aspartic acid into a solution. The other kind of racemic mixture is a curious "twinned" crystal "dimer" made of one L and one D molecule that cling together in a LD arrangement. They had expected that solutions made from the Pasteur piles and the dimer pile would lead to the same result, since they thought water would tear apart the dimers into individual aspartic acid molecules. But contrary to all expectations, the dimer preferentially crystallized from solution as a dimer, and the Pasteur mixture crystallized as separate L and D crystals unless they cooked the solution at high temperature (and tore apart the dimers). The simplest conclusion for this result is that the dimer is more stable in solution than expected.
But does this mean that non-racemic crystals can form from a racemic solution? Of course, which is what Pasteur did when he was 29, separating L-tartaric from D-tartaric acid after crystallizing "paratartaric" acid. Does this mean that abiotic chemistry can make a solution non-racemic? No, unless that chemistry includes a pair of tweezers and the patience of Pasteur. Whether one can or cannot start with a dimer and end up with tweezer piles is irrelevant, because there is no abiotic path from a racemic solution to a stereo-active solution of amino acid that doesn't involve a biotic chiral agent, be it chiral beads or Louis Pasteur himself. Like many critiques of ID, the problem with these "Darwinist" solutions is that they always smuggle in some information, in this case, chiral agents.
If those 12 carbonaceous chondrites represent 12 separate OOL events, then the probability of finding at least one with D-amino acid overabundance should have been 99.97%. Does not finding a single D-amino mean OOL is disproven? Not if one is willing to overlook 1 in 10^1107 odds in the first place. After all, what are a mere three more zeroes? We've merely moved it up to 1 in 10^1110 odds.