For nearly a century, the origin of life has been traced back to a “primordial soup“:
Under the conventional theory, life supposedly began when lightning or UV rays caused simple molecules to join together into more complex compounds. This culminated in the creation of information-storing molecules similar to our own DNA, housed within the protective bubbles of primitive cells. Laboratory experiments confirm that trace amounts of molecular building blocks that make up proteins and information-storing molecules can indeed be created under these conditions. For many, the primordial soup has become the most plausible environment for the origin of first living cells.
But life isn’t just about replicating information stored within DNA. All living things have to reproduce in order to survive, but replicating the DNA, assembling new proteins and building cells from scratch require tremendous amounts of energy.
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This process works a bit like a hydroelectric dam. Instead of directly powering their core metabolic reactions, cells use energy from food to pump protons (positively charged hydrogen atoms) into a reservoir behind a biological membrane. This creates what is known as a “concentration gradient” with a higher concentration of protons on one side of the membrane than other. The protons then flow back through molecular turbines embedded within the membrane, like water flowing through a dam. This generates high-energy compounds that are then used to power the rest of cell’s activities.
Life could have evolved to exploit any of the countless energy sources available on Earth, from heat or electrical discharges to naturally radioactive ores. Instead, all life forms are driven by proton concentration differences across cells’ membranes. This suggests that the earliest living cells harvested energy in a similar way and that life itself arose in an environment in which proton gradients were the most accessible power source.
Recent studies based on sets of genes that were likely to have been present within the first living cells trace the origin of life back to deep-sea hydrothermal vents. These are porous geological structures produced by chemical reactions between solid rock and water. Alkaline fluids from the Earth’s crust flow up the vent towards the more acidic ocean water, creating natural proton concentration differences remarkably similar to those powering all living cells.
The studies suggest that in the earliest stages of life’s evolution, chemical reactions in primitive cells were likely driven by these non-biological proton gradients. Cells then later learned how to produce their own gradients and escaped the vents to colonise the rest of the ocean and eventually the planet.
When I was taking microbiology and biochemistry in the late 60′s, the proton gradient was just being proposed. Biologists assumed that fermentation came first, especially as it was known that the early Earth had no free oxygen. But fermentation is enormously more complicated than respiration, and most of the fermentation pathways also double (in reverse) as synthesis pathways. Once the mechanics of respiration were known (thanks Peter Mitchell, 1961), the question of which came first still persisted. Then in the later part of the 20th Century microbiologists discovered all sort of metal-based oxidation/reduction reactions that drove proton gradients, and it was clear that respiration without oxygen was possible, and indeed common.