Cosmology and our View of the World
Weak Forces, Chaos, and Evolution
Lead: Tom Laue
Summary by Ethan Cline
Weak Forces, Chaos, and their Influence on Evolution
Kaufmann, S., Approaches to the Origin of Life on Earth
In this presentation, we discussed the chemical requirements for life and the implications of these requirements for the existence of life. Moreover, we discussed how structure in chemical bonds influenced life and how this structure could be modeled and observed to see behavior on the molecular level. We were specifically shown how all of the forces present on the chemical level for life are well understood and that modeling these forces can be accomplished. However any model will be incredibly complex and chaotic resulting in unpredictable results
We initially chemical requirements for life. These requirements were listed by Tom from his time at JPL where he was involved in round-table discussions to determine the necessary environmental characteristics for lif.
It was posited that for life to exist it must arise in a continuous medium, e.g. a solid, liquid, or gas. A vacuum is, fairly obviously, not suited for the development of life. The medium must also be a good solvent, and have a high dielectric constant D, which allows for a moderating influence on electrostatic forces. Water, the medium through which life is commonly assumed to have begun, is an excellent solvent and has a dielectric constant of D=80. This large dielectric constant prevents the polar nature of water from having too large an effect on the other chemical constituents of life and allows covalent bonds to form. We also require that directional bonding occurs. Otherwise, we would have amorphous blobs instead of clearly organized and regimented molecular structure. This structure allows for the control of rotation and flexibility of molecules, the importance of which will also be illuminated shortly.
Most of the atoms in the molecular makeup of living beings, (C)arbon, (O)xygen, (H)ydrogen, (N)itrogen, (P)hosphorous, and (S)ulfur, form strong, stable covalent bonds. With the exception of H, which forms only one bond, all of these atoms will form multiple bonds. These bonds share electrons between atoms, which result in hybridized electron orbitals, such as sp2 or sp3 having specific geometric symmetry that provide directional bonding and restrict the allowed movements of the involved atoms. Orbitals of the sp2 variety allow three planar bonds, which would form a flat surface. On the other hand sp3 orbitals allow four bonds but instead of planar bonds they create tetrahedral bonds, a pyramidal shape. For example the sp3 orbital only allows rotation about the bond and the sp^2 2 orbital prevents any type of rotation. With these characteristics it appears that covalent bonds satisfy some of our requirements for the chemical structure of life.
Given the directional nature of covalent bonds we can now begin to investigate the bio-molecular structures that form as a result of such bonding. Amino acids are held together via covalent bonds, the nature of this particular type of bond restricts the amino acids to only have two degrees of rotational freedom. This allows the angles of the bond to play a critical role in determining the structure of the protein. These simple proteins can form more complicated secondary, tertiary, and quaternary structures required by cells and organisms for proper functioning. As an example, the liver is able to collect Fe2+ using a ferritin, a large protein composed of 24 separate proteins, each with defined primary structure sp bonds, secondary, and tertiary structure, which form a hollow ball as the quaternary structure
Once structures and their uses were established the non-covalent forces that govern molecular interactions were described. All of the numerous forces that act on molecules are well understood. There is no additional mystery force that may act on molecules that could be responsible for life. All possible interactions are described by known physical laws. Typically, these forces are described in terms of potentials, which are of the form U ~ 1/(D*r^n), where D is the dielectric constant and r is distance . A potential of this form has a “potential hill” or an “activation energy”, which in our situation translates to a minimum energy molecules must have in order to form bonds. As water has a large dielectric constant of D = 80, the minimum energy required for a bond is significantly increased. This fact allows greater control of the non-covalent energies holding molecules together in water.
As we now know that atoms and molecules can form multiple bonds, which can cause large-scale structure, and as the constituents bond they create a predictable pattern of new molecule. It is possible to create a very simplified computer simulationof atomic and molecular bonding, an example of which was demonstrated during the presentation. In this model a series of 2278 adjacent squares in a two-dimensional array were simulated and every square was randomly set to on or off. Between any one “output” square and two of its adjacent “input” squares a random boolean logic gate was implemented. Once all logic gate inputs and outputs were established the program was run continuously allowing the various logic gates to turn various squares on or off as the logic gate required. These logic gates were intended to replicate how structure affects chemical bonding in a simplified manner. A graph of the number of squares changing from on to off or vice versa was displayed, and some very obvious atterns of on-off switches could be seen over iterations of the logic gates activating. Regardless of the initial configuration of switches, a similar pattern would occur each time the program was run. This indicates that behavior and patterns could potentially arise simply due to structure and simplified boolean logic. We then discussed the possibility of running the model with three inputs controlling a single output. The idea was that life is far more complicated than a simple two-input one-output model so what happens if we add an additional input? It turns out that an additional input creates chaos within the system and removes any observable repetitive pattern. The system is very sensitive to the starting conditions of which switches are on or off.
This introduced the concept of chaos, in that sufficiently complicated behavior cannot be analytically modeled. And any model you try to develop to be a useful approximation of a chaotic system is heavily dependent on the starting conditions of the model. This means that in any system as complex and chaotic as life, any kind of mathematical model to describe how we may come to be alive or have apparent cellular order in our bodies, will be a very approximate model. From the initial conditions, which are potentially quite varied for life on earth, we can find stable patterns that arise. These patterns could result in the rise of complicated behavior and given the multiplicity of these potential starting conditions there is a greater potential for one of the starting conditions to give rise to life. The idea of mathematically modeling life was discussed and it quickly became apparent that while this model is useful to think about, it is woefully inadequate for determining the actual structural characteristics of life. However, it is a useful thought experiment in that it investigated the requirements for life and how those requirements and their physical implications could be modeled, and whether or not forced structure can create behavior.