ORIGIN OF CELLS
Summary Bio-organic chemicals can be formed by simple inorganic processes involving basic C, H, O, S, and N compounds and a source of energy Concentration of substrates for synthetic reactions may involve membranes or adsorptive surfaces Self-reproducing systems can be envisioned as starting with either a replicator or with a complex metabolic system
Geological stratigraphy, together with radioactive dating, show the sequence of events in the history of the Earth. Note the entry for cyanobacteria and stromatolites only one billion years after the formation of the Earth and very soon after the last Impact heating. Where did the materials for making cells come from? Was heating necessary? Or deleterious? Were bacteria really the first life? Last impact heating ~3500
Life starts with chemistry Chemical experiments, initiated by Stanley Miller and Harold Urey, showed that some biochemicals could be formed naturally (but not all--where did the rest come from?)
The Miller-Urey experiment was repeated with several modifications: The gas mixture was varied Initially, H 2, NH 3, CH 4, H 2 O Later, less or no H 2, added CO 2, H 2 S, SO 2 The energy source was varied Spark (lightning) UV lamp (sunlight) Sonic energy (thunder)
The Miller experiment lives on New amino acids detected in old vials (Johnson et al., Science 322:404, 17 Oct 2008)
Applying the Miller experiment to extraterrestrial conditions Researchers used radio-frequency radiation a more convenient substitute for ultraviolet sunlight to turn methane, nitrogen, and carbon monoxide (the main constituents of Titan's atmosphere) into glycine and alanine, the two smallest amino acids, and cytosine, adenine, thymine, and guanine, the four most basic components of DNA, and uracil, a precursor of RNA. The researchers said that because they achieved the reactions without the presence of liquid water, it's possible life could have sprung forth on Earth not in the seas, as commonly assumed, but perhaps in the planet's early atmosphere a considerably thinner version of the fog enveloping Titan today (Science 330, 307, 2010)
Notice that the Miller-Urey compounds are similar to those found in the Murchison meteorite--extraterrestrial synthesis? (Murchison meteorite: Australia, 1969)
Life requires a boundary Hypothesis: Metabolism (energy, reproduction) needs a way of concentrating intermediates. Membranes Adsorptive surfaces
Protobionts: a step toward living cells? In 1957, Sidney Fox demonstrated that dry mixtures of amino acids could be encouraged to polymerize upon exposure to moderate heat. When the resulting polypeptides, or proteinoids, were dissolved in hot water and the solution allowed to cool, they formed small spherical shells about 2 μm in diameter microspheres. Under appropriate conditions, microspheres will bud new spheres at their surfaces.
A different kind of protobiont? Tiny compartments in mineral structures can shelter simple molecules, while mineral surfaces can provide the scaffolding on which those molecules assemble and grow. Beyond these sheltering and supportive functions, crystal faces of certain minerals can actively select particular molecules resembling those that were destined to become biologically Important [eg. L-amino acids, D-sugars]. The metallic ions in other minerals can jumpstart meaningful reactions like those that must have converted simple molecules into self-replicating entities. --R.M. Hazen, Life s Rocky Start Scientific American, April, 2001
Some have taken the term protobiont a step too far.
How does a replicating system arise from passive adsorption? Two hypotheses: Replicator first Metabolism first From R. Shapiro, A Simpler Origin of Life Scientific American June, 2007
From R. Shapiro, A Simpler Origin of Life Scientific American June, 2007 1. Boundary 2. Energy source 3. Energy coupled to chemical reaction 4. Network; adaptation; evolution 5. Growth; reproduction
Shapiro, in proposing the metabolism-first hypothesis, accepts general ideas about what life requires: 1. Boundary 2. Energy source 3. Energy coupled to chemical reaction 4. Network; adaptation; evolution 5. Growth; reproduction [From my first lecture: Catalysis Complex organization Requirement for energy Homeostasis Inheritance Natural selection] From R. Shapiro, A Simpler Origin of Life Scientific American June, 2007
Summary Bio-organic chemicals can be formed by simple inorganic processes involving basic C, H, O, S, and N compounds and a source of energy Concentration of substrates for synthetic reactions may involve membranes or adsorptive surfaces Self-reproducing systems can be envisioned as starting with either a replicator or with a complex metabolic system