The Origins of Life

As we contemplate the beginnings of a new year, an interesting paper by Wimmer et al. in Frontiers in Microbiology reexamines the beginnings of the first biochemical compounds. We know the Earth formed about 4.5 billion years ago and that life originated about 3.8 billion years ago, but a key question has always been how did the transition from the innate inorganic elements of our planet (carbon, nitrogen, oxygen, and hydrogen among others) to complex organic compounds and life forms occur?  In 1953, Urey and Miller published the first experimental approach to this question. They showed that an electrical spark (simulating lightning) inside a flask containing water (H2O), methane (CH4), ammonia (NH3), and hydrogen (H2) could produce amino acids, the building blocks of proteins. Other in vitro re-creation experiments over the last 70 years have likewise demonstrated that nucleotides and sugars (the components of RNA and DNA) as well as many other simple organic compounds can form from inorganic molecules under conditions mimicking prebiotic Earth. The conclusion from these experiments was that organic complexity could derive from the inorganic elements prevalent in the early Earth’s geosphere. Importantly though, most investigators who study and model the origins of life have speculated on the need for an energy source to drive the chemical reactions that created the first organic compounds that served as the precursors to primordial life. Proposed sources of energy have included lightning, UV radiation from the sun, volcanic activity, and even meteor strikes, though no one explanation has gained universal acceptance.

The Wimmer et al. paper reexamined this need for external energy sources to create organic compounds from inorganic materials. To consider this synthesis problem, the scientific group went back to metabolic reactions that were likely present in the Last Universal Common Ancestor (LUCA). Because all existing life on Earth, from bacteria to humans, uses the same genetic code for our DNA and the same amino acids for our proteins, we all must have evolved from the same ancestor, the unknown LUCA. Previous work identified around 400 key metabolic reactions that were likely present in LUCA, and this new work examined the energy requirements for each reaction under conditions similar to what is found in oceanic hydrothermal vents. These vents are rich in minerals, gases (hydrogen and carbon dioxide), and physical conditions that favor chemical reactions. Surprisingly, Wimmer et al. found that 97% of the key metabolic reactions would spontaneously occur under these conditions without any other energy input. This work adds considerable credibility to the hypothesis that organic molecules arose in the oceans at hydrothermal vents. The hydrothermal vent environment would have allowed a continuous and abundant process for organic molecule formation, conditions more likely to lead to the first organic life forms than sporadic events like a meteor strike or volcanic eruption. At some point, the ocean vents were likely teeming with numerous organic compounds and primitive life forms co-existing in the chaotic early bioorganic environment. Eventually, one organism dominated and became the LUCA that evolved into all life on Earth. While no study we can perform today can definitively prove how life originated, this new study offers an intriguing and plausible explanation consistent with much of what we know about the early Earth’s environment and the mechanisms of chemical reactivity.

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