- Written by Aatmananda Nayak
Oftentimes, when discussing where life, as we know it, may have originated, the idea that the first repeating biological molecules “occurred” around hydrothermal vents at the bottom of the sea is a popular idea. This theory has garnered a significant amount of traction due to the fact that it elegantly ties together laws of physics and chemical phenomena to explain the complexity of biological molecules we observe today.
To first understand why this theory works so well, an understanding of the concept of entropy is required. Entropy, simply put, is the randomness of the arrangement of particles in a system. The higher the entropy of a system, the more spread out the particles in that system are. Energy is distributed more evenly in such a system. A higher entropy system can be thought of a less interesting, or less complex system. Such a system would be said to have a higher proportion of energy that can’t be used for work. This is because the energy is spread out evenly throughout the system. This is called unusable energy. Thus, high entropy systems have evenly dispersed particles and predominantly unusable energy. Low entropy systems, on the other hand, have a less random dispersion of particles. These particles are more ‘clumped’ together and are, thus, concentrated in a certain point more. They are not evenly distributed, and instead are closer together and can react to form complex structures. Within low entropy systems there is a greater proportion of usable energy, as it isn’t spread out much. As a result, low entropy systems can lend to more interesting situations, as they consist of more complex arrangements of particles. Low entropy systems have regions of concentrated particles and predominantly useful energy for work. They are ordered.
The 2 nd law of thermodynamics states that the entropy of a system must always increase. Energy must always be dissipated into the least useful form possible (heat) and the complexity of structures must always be reduced to the simplest state possible. Life, however, even in its simplest form (prokaryotes) is composed of several complex molecules held together by chemical bonds. Chemical bonds are connections between atoms and molecules that are formed by an attraction between those atoms and molecules. Breaking these bonds releases energy into the system. Thus, chemical bonds can be thought of as stores of potentially useful energy. This allows us to think of life as regions where particles are not randomly arranged and where useful energy is concentrated. This resembles a low entropy state.
This means that for the formation of “life”, particles need to re-arrange themselves into a more complex structure, and useful energy needs to be consolidated into a specific region. As a result, the entropy of the system must decrease, which seems to be an abject violation of the 2 nd law of thermodynamics. This would indeed be the case, if life (complex, repeating molecules) were to form in an isolated system, where no other exchange of energy was occurring simultaneously. However, if life were to form in a non-isolated environment where exchange of energies was taking place in the environment (that could influence the formation of certain structures), it wouldn’t necessarily violate the 2nd law of thermodynamics. Such an environment can be found at the ocean floor, where hydrothermal vents spew forth minerals and heated fluids into the massive,freezing body of water that is the ocean. The temperature and chemical composition of the fluid ejected from the hydrothermal vents, as well as the temperature differential, is what allows for the formation of complex, organic molecules.
Let me elucidate. The hydrothermal vents, and the underlying magma, have very high energy densities, whereas the ocean, a freezing body of water, has a low energy density. Thus, energy must flow from the hydrothermal vents to the ocean. This constant flow of energy, a consistent energy gradient, is what life depends on. This flow of energy occurs because the energy is “trying” to redistribute itself as randomly and evenly as possible and make the temperature uniform throughout. It’s simply obeying the 2 nd law of thermodynamics. Thus, the energy is “desperate” to redistribute itself into every possible form it can take, in order to reach an equilibrium between both the vents and the ocean. As stated before these forms can include chemical bonds between atoms and molecules. Thus the complexity of chemical structures can be developed. In any other conditions, where thermal equilibrium has been reached, the chemical bonds are broken as soon as they are formed, so as not to violate the 2 nd law of thermodynamics, thus limiting the complexity of molecules. However in the hydrothermal vent-ocean system, equilibrium is never reached, as the ocean is too vast of a reservoir. This allows for continuous formation of chemical bonds and the rise of complexity in molecules, as a byproduct of the energy trying to redistribute itself.
At a certain point, chemicals that catalyse the very reactions that create more of them are produced. Naturally such molecules are favoured, as they are much more efficient at redistributing the energy, by sapping it from the hydrothermal vents to create chemical bonds. Thus, more of these ‘self-catalysing’ molecules are produced, in order to increase entropy. As a result, these molecules would increase in complexity and become increasingly better at self replication, eventually laying down the blueprints for the formation of RNA.
Life is a complicated and beautiful mechanism that took billions of years of evolution to reach the stage it has today. It’s comforting to think that this would have happened, regardless of luck, and that the laws of physics deemed it inevitable. The idea that self-replicating systems (such as life) are the best distributors of energy, and hence were bound to develop, is indeed reassuring.
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