March 16, 2015
Abiogenesis is the name given to the process by which life arose from non-living matter. Today, we have a greater understanding of how abiogenesis can happen, although the details of how it happened on Earth are still a mystery. When scientists are faced with mystery, they respond with theories.
Panspermia is the theory that life was seeded on the Earth by cosmic spores arriving on a lifeless Earth by the infall of things such as comets. If you believe this theory, you still need to explain the abiogenesis that caused the original life. I wrote about abiogenesis in two previous articles (Catalytic Abiogenesis, January 26, 2011, and The Miller Experiment, July 30, 2014).
The idea of abiogenesis has been around for a long time. The Roman author, Ovid, retold a Greek myth about abiogenesis in his Metamorphoses. After Zeus destroyed Earth by a flood, Deucalion and Pyrrha were instructed by the goddess, Themis, on a method to repopulate the Earth. Quoting Ovid,
Mota dea est sortemque dedit: 'discedite templo et velate caput cinctasque resolvite vestes ossaque post tergum magnae iactate parentis!'
After an initial confusion, the couple realized that their "great mother" was the Earth, and her bones were stones. The stones they threw behind them became people, Deucalion's stones became men, and Pyrrha's stones became women.
Darwin, himself, thought that early in the long history of the Earth, self-replicating, living matter would have appeared in some "warm little pond." As Darwin wrote in an 1871 letter to botanist, Joseph Hooker,
The Goddess was moved, and gave this response: 'Depart from my temple, cover your heads, loosen your garments, and throw behind your backs the bones of your great mother!'
"It is often said that all the conditions for the first production of a living organism are now present, which could ever have been present. 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 proteine compound was chemically formed ready to undergo stillmore complex changes, at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed."
In a classic 1953 experiment, Stanley Miller recreated a version of Darwin's "warm little pond" in his laboratory. Miller put ammonia, methane, hydrogen and water, the supposed constituents of Earth's early atmosphere, into a closed reaction vessel. He excited this mixture with artificial lightning, producing amino acids, an essential building block for living organisms.
Amino acids are a good start, but they're still a long way from the complex chemicals of life. One way to produce a greater concentration of desired chemical product is to start with concentrated reactants. Beginning chemistry students learn this as Le Chatelier's principle, named after French chemist, Henry Louis Le Châtelier (1850-1936). This principle expresses the idea that products and reactants are in equilibrium with each other, and adding more reactant will give more product.
Scientists concerned with the conditions for creation of life on Earth and other planets, commonly called exobiologists or astrobiologists, have considered possible mechanisms of how sufficient concentrations of the proper chemicals for abiogenesis could have appeared on the early Earth.
A recent paper in Nature Chemistry by biophysicists at the Center for Nanoscience of the Ludwig-Maximilians-Universität München (Munich, Germany) examines how temperature gradients in water-filled micropores in hot rock may have provided a suitable environment for the creation and cyclical replication of nucleic acids.[3-4]
For comparatively simple biomolecules to have had the opportunities to form stable, complex structures, such as those needed for storage and reproduction of genetic information, there must have been some means of concentrating precursor molecules in solution. The concentration of suitable compounds would have been very small in Earth's primordial oceans.
The team of Munich physicists, led by Dieter Braun, conducted experiments in which they demonstrate that seafloor pore systems, heated by volcanic activity, could have been effective reaction chambers for RNA synthesis. Says Braun, "The key requirement is that the heat source be localized on one side of the elongated pore, so that the water on that side is significantly warmer than that on the other.” The mechanism involved is thermophoresis, in which charged molecules will migrate from a warmer to a cooler region. The cooler regions are maintained at a lower temperature by the higher thermal conductivity of the rock.
The experiments simulated such pores with glass capillary tubes. Heated water containing dissolved fragments of DNA was allowed to travel into the tubes, where the fragments became trapped. Small molecules in the hot region are transported by convection to the colder regions of the pores, where they encounter complementary template molecules with which they can attach. In this way, the template strands can replicate. Replicated molecules can leave the pores in which they were made, and then colonize other pores.[3-4]
Strands of 75 nucleotides survived in the experiments, while strands half that length decomposed. In this way, longer strands will predominate, and the pores become factories for creation of complex biomolecules. As Dieter Braun explains,
“Life is fundamentally a thermodynamic non-equilibrium phenomenon. That is why the emergence of the first life-forms requires a local imbalance driven by an external energy source – for example, by a temperature difference imposed from outside the system... That this can be achieved in such a simple and elegant way was surprising even to us.”
- Ovid, "Metamorphoses," Book I, ll. 381-384; English translation.
- As quoted by Darwin's son, Francis, in The life and letters of Charles Darwin, 1887.
- Moritz Kreysing, Lorenz Keil, Simon Lanzmich, and Dieter Braun, "Heat flux across an open pore enables the continuous replication and selection of oligonucleotides towards increasing length," Nature Chemistry, Advanced Online Publication, January 26, 2015, doi:10.1038/nchem.2155.
- Labyrinths as crucibles of life, Ludwig-Maximilians-Universität München Press Release, January 27, 2015.
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