Thursday, July 27, 2017

The Physics of Life's Origins(?)

The biophysicist Jeremy England made waves in 2013 with a new theory that cast the origin of life as an inevitable outcome of thermodynamics. His equations suggested that under certain conditions, groups of atoms will naturally restructure themselves so as to burn more and more energy, facilitating the incessant dispersal of energy and the rise of “entropy” or disorder in the universe. England said this restructuring effect, which he calls dissipation-driven adaptation, fosters the growth of complex structures, including living things. The existence of life is no mystery or lucky break, he told Quanta in 2014, but rather follows from general physical principles and “should be as unsurprising as rocks rolling downhill.”

Concerning the PNAS study:
The paper strips away the nitty-gritty details of cells and biology and describes a simpler, simulated system of chemicals in which it is nonetheless possible for exceptional structure to spontaneously arise — the phenomenon that England sees as the driving force behind the origin of life. “That doesn’t mean you’re guaranteed to acquire that structure,” England explained. The dynamics of the system are too complicated and nonlinear to predict what will happen.

The simulation involved a soup of 25 chemicals that react with one another in myriad ways. Energy sources in the soup’s environment facilitate or “force” some of these chemical reactions, just as sunlight triggers the production of ozone in the atmosphere and the chemical fuel ATP drives processes in the cell. Starting with random initial chemical concentrations, reaction rates and “forcing landscapes” — rules that dictate which reactions get a boost from outside forces and by how much — the simulated chemical reaction network evolves until it reaches its final, steady state, or “fixed point.”

Often, the system settles into an equilibrium state, where it has a balanced concentration of chemicals and reactions that just as often go one way as the reverse. This tendency to equilibrate, like a cup of coffee cooling to room temperature, is the most familiar outcome of the second law of thermodynamics, which says that energy constantly spreads and the entropy of the universe always increases. [...]

But for some initial settings, the chemical reaction network in the simulation goes in a wildly different direction: In these cases, it evolves to fixed points far from equilibrium, where it vigorously cycles through reactions by harvesting the maximum energy possible from the environment.
Of the PRL paper:
In the PRL paper, England and his coauthors Tal Kachman and Jeremy Owen of MIT simulated a system of interacting particles. They found that the system increases its energy absorption over time by forming and breaking bonds in order to better resonate with a driving frequency. “This is in some sense a little bit more basic as a result” than the PNAS findings involving the chemical reaction network, England said.
But even if the fine-tuned fixed points can be observed in settings that are increasingly evocative of life and its putative beginnings, some researchers see England’s overarching thesis as “necessary but not sufficient” to explain life, as Walker put it, because it cannot account for what many see as the true hallmark of biological systems: their information-processing capacity. From simple chemotaxis (the ability of bacteria to move toward nutrient concentrations or away from poisons) to human communication, life-forms take in and respond to information about their environment.

To Walker’s mind, this distinguishes us from other systems that fall under the umbrella of England’s dissipation-driven adaptation theory, such as Jupiter’s Great Red Spot. “That’s a highly non-equilibrium dissipative structure that’s existed for at least 300 years, and it’s quite different from the non-equilibrium dissipative structures that are existing on Earth right now that have been evolving for billions of years,” she said. Understanding what distinguishes life, she added, “requires some explicit notion of information that takes it beyond the non-equilibrium dissipative structures-type process.” In her view, the ability to respond to information is key: “We need chemical reaction networks that can get up and walk away from the environment where they originated.”

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