Exactly 150 years ago on 1st February 1871, Charles Darwin wrote to a colleague :
"But if (and oh what a big if) we could conceive in some warm little pond with all sorts of ammonia and phosphoric salts,—light, heat, electricity &c. present, that a protein compound was chemically formed, ready to undergo still more complex changes, at the present day such matter wd be instantly devoured, or absorbed, which would not have been the case before living creatures were formed."
Eighty years later in 1952 Stanley Miller and Harold Urey translated Darwin's thoughts into an experiment. Using water, methane, ammonia, hydrogen and electric spark to mimic lightning, he synthesised amino acids, the building blocks of proteins. The hypothesis of oceanic primordial soup containing all possible chemicals subjected to Sun's ultraviolet rays and occasional lightning giving rise to life's molecules gained wider acceptance.
But almost immediately everything changed. The 1953 Nature paper by Watson and Crick established deoxyribonucleic acid, DNA for short, as the macromolecule of life with the genetic code encrypted in a unique way in the DNA chain using 4 nucleobases: adenine, guanine, thymine and cytosine. In 2017, a group of scientists repeated the Miller-Urey experiment with suitable modifications and demonstrated that abiotic synthesis of nucleobases is also possible. And then RNA replaced DNA as the original molecule!
The prebiotic soup model has thermodynamic as well as kinetic inconsistencies. Issue is not the formation of the building blocks, but the process of linking them and sustaining these linkages in a vast and seemingly limitless waterbody. The peptide bonds, and the phosphodiester bonds, which form the backbone of the proteins and nucleic acids respectively, are both extremely susceptible to water. Biochemist Robert Shapiro a vehement critic of the primordial soup hypothesis stated: "And of course the apparatus itself has no resemblance whatsoever to the primitive Earth. One of the popular magazines said that if this apparatus had been left on for a million years, something like the first living creature might have crawled out of it. And I say, if he'd left his apparatus on for a million years, he would have run up one hell of an electric bill " .
Nonetheless the primordial soup model prevails, of course with modifications. For example Professor John Sutherland feels that small shallow ponds filled with primordial soup, and which have a tendency to dry out and refill might be a possibility. The clay bed of such a pond subjected to periodic wet and dry cycles together with light and dark cycles of day and night would coax/catalyse molecules to form, organise, hold together and grow. If the clay is rich in minerals such as quartz, then the issue of chirality could also be somewhat settled. Because as professors Hazen and Sverjensky point out "such surfaces may have contributed centrally to the linked prebiotic problems of containment and organization by promoting the transition from a dilute prebiotic “soup” to highly ordered local domains of key biomolecules".
Anthropogenic activities have irreversibly contaminated all possible terrestrial sites hence
Professor Sutherland is pinning hopes on Perseverance, the rover heading towards Mars. Perseverance is programmed to land in the Jezero crater in Mars and collect and bring back soil and rock samples. The assumption is that 3.5 billion years ago this crater could have been a water body that underwent wet-dry cycles and hence a faint probability that it could have supported life. Scientists hope that Martian rocks and soil samples might hold secrets of life, that the ancient biosignatures inscribed in there might still be decipherable.
So you are back from the lockdown. Am cooped up here. did you see my new year post in FB?
ReplyDeleteHappy to see your new article and hope to see more.
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