Homochirality: Computers Are Not the Real World
If you solve a vexing problem in a computer model, you have not solved it in the real world.
It’s a long-standing problem for origin of life research: how do you get the ingredients to be all of one hand? The amino acids in proteins and the sugars in DNA are all homochiral, composed of one hand out of two otherwise-identical “isoforms” (chemicals with the same composition that differ only in shape). Collections made outside of life tend to be random mixtures of left- and right-handed isoforms. Unless there is a yet-undiscovered natural law that can amplify one hand over the other, a prebiotic protein or genetic molecule would have to form by chance (see online book). An article in PhysOrg explains the problem:
“Imagine you’ve got a coin, and it’s perfectly made, so it’s not biased at all, and you start flipping the coin. Each time you flip it, it keeps coming up heads,” he said. “So then you say, something must be operating that’s causing this to happen . . . you get the same puzzle with these biological molecules, and that’s the problem of homochirality.”
Yet this same article offers a new explanation. Three researchers at the University of Illinois came up with a new idea. Will it work?
The Illinois team wanted to develop a simpler model, one based on only the most basic properties of life: self-replication and disequilibrium. They showed that with only these minimal requirements, homochirality appears when self-replication is efficient enough.
“There are other models, and they may be correct for the origin of homochirality on earth, if you can prove that those prerequisites existed during the emergence of life,” said Jafarpour. “But whether those foundations exist or not, for life that emerged anywhere in the universe, you’d expect that it would have self-replication, and our model says that’s enough to get homochirality.”
They knew about the ideas of Sir Charles Frank in 1953. He proposed that one chiral molecule able to replicate itself might repress the formation of the opposite hand.
The model relies on mathematical and computational techniques that were not available in Frank’s time. It takes into account the chance events involving individual molecules—which chiral self-replicator happens to find its next substrate first. The detailed statistics built into the model reveal that if self-replication is occurring efficiently enough, this incidental advantage can grow into dominance of one chirality over the other.
This article’s optimism exceeds its realism. Did they try this with actual chemicals in the lab? Of course not. They would have to start with what they needed to demonstrate: a chiral molecule able to make accurate copies of itself out of a soup of random ingredients. Unless the copying process was accurate and efficient enough, errors would accumulate and all gains would be lost (“error catastrophe”). But how probable is it to get such a replicator by chance? That’s a high bar to overcome at the beginning.
Presumably their replicator would be some RNA ribozyme able to catalyze its own replication. To demonstrate its ability to follow this scenario, they would have to see it self-organize under plausible prebiotic conditions. What are plausible prebiotic conditions? That question is highly theory-laden. Unless initial ingredients are agreed on, damaging cross-reactions prevented, pH controlled, temperature ranges monitored and oxygen excluded, nothing of interest would emerge. If something unlikely did emerge, charges of investigator interference would be impossible to dismiss.
That’s why their idea only works in the computer (and only on a computer programmed by intelligent human beings already made up of homochiral molecules). It’s puzzling they compared their scenario with patterns in the foraging activity of ants, which are also living and made up of homochiral molecules. The challenge is to get non-living natural conditions to achieve homochirality.
The only graphic in the article is a “computer simulation of the emergence of homochirality” as one hand grows over time in some kind of competition in the spatial domain. The diagram is highly contrived and unrealistic, but it didn’t stop the author of the press release from making sweeping generalities from it:
The work leads to a key conclusion: since homochirality depends only on the basic principles of life, it is expected to appear wherever life emerges, regardless of the surrounding conditions.
“For me, the most exciting thing is that this mechanism shows that homochirality is really a biosignature of life, a 100% signature, and should be expected anywhere life emerges,” said Goldenfeld. “So for example, we just learned that there is a global ocean of liquid water under the ice of Enceladus … I think that looking for homochirality in the organic molecules that have been detected there would be a fantastic way to look for life there.”
But it’s not a mechanism. It’s a simulation. Unless their simulation works in the real world, no such inference can be made, because it begs the question that real molecules can do this outside the computer. Until it can be demonstrated that blind, unguided molecules can produce homochirality (and anything less than 100% is doomed to failure), the simulation has nothing to say about life on Earth, Enceladus, or exoplanets.
There is, however, one cause now in operation that is capable of sorting otherwise identical objects by handedness. That cause is intelligence.
Speaking of Enceladus, Cassini flies by it today (Oct 14) in the first of three final encounters of the geysering moon (see NASA schedule). On October 28, the spacecraft will make a daring plunge through one of the plumes to collect samples of the dust and vapors from an altitude of 30 miles. Since another Enceladus mission didn’t make the final cut of NASA’s Discovery program (see Nature), these two encounters and the Dec. 19th flyby are likely to be the last opportunities for decades to gather data from this intriguing, unexpectedly active body.
We get excited every time there’s a new proposal to solve the homochirality problem. Excited, that is, for a few seconds, until we see more false optimism and cheating. This proposal is all fluff. Go into the chemistry lab, guys, and demonstrate your process! You can’t walk onto a football field with an iPad, show a computerized play that wins a touchdown under contrived circumstances, and declare victory.
The only value of this article (published uncritically by PhysOrg with no hard pushback questions) is that it makes a clear statement of the problem. The problem remains as unsolved now as it was in 1953. It remains evidence of life’s uniqueness, as it was to Louis Pasteur. The conclusion from our online book* remains: “We find that there is no lessening of confusion until one accepts the logic that ‘intelligent’ systems could not arise without an intelligent Designer.”
*This book, published first in 1973, was among the first to use the term “intelligent design” explicitly. It was the author’s inference from the extreme improbability of chance to achieve the high degree of complex information seen in even the simplest conceivable living cell.