Understanding the origin of life on Earth may lead to the discovery of life outside of Earth

Was life on Earth inevitable or implausible?

Mario Livio & Jack W. Szostak

Two of the most interesting questions in science are: How did life on Earth begin? and Is there life anywhere in the universe?

Given the interest in these questions, you might think that the basic concepts would already be settled. But in fact, many ideas about key aspects of life are plagued by uncertainty and misunderstanding. We need to know more about how life exists before we can make rough estimates of the likelihood of life outside of Earth. On The Earth began.

For example, there is the idea that life on Earth arose from something called a “primordial soup” – a liquid mixture of organic compounds that contained the ingredients necessary for biology. This idea gained popularity after a famous 1952 experiment by chemists Stanley Miller and Harold Urey. By subjecting a simple gas mixture to an electric spark discharge, the researchers showed that they could produce molecules very similar to amino acids. This experiment seemed to replicate how life on Earth began. But a closer look at the results shows that those sparks produced thousands of different chemicals in minute quantities. However, recent studies have shown that high concentrations of just a few key chemicals are really needed to build both RNA (responsible for storing operating instructions and information that must be passed from generation to generation) and amino acids (the building blocks of proteins).

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Another theory, which is often featured in scientific journals, is that life on Earth began in relatively cool hydrothermal vents on the ocean floor. These vents are known to spew alkaline water into highly acidic seawater, an energy source that is often efficiently exploited by the abundant life forms near the vents. However, the fact that such environments are heavily colonized by modern life does not mean that they were also home to ancient life. origin The beginning of life. There is no evidence to date of precursor chemical reactions occurring in deep-sea hydrothermal vent environments that could lead to the synthesis of the building blocks of RNA (nucleotides), proteins (amino acids), and cell membranes (lipids).

Furthermore, cutting-edge origin-of-life research points to the need for high concentrations of starting materials, which cannot be achieved in the vast oceans, where the necessary chemicals would be diluted and lost. Furthermore, many of the processes that turn out to be essential for the origin of life are known to require ultraviolet radiation (emitted by the young Sun) to provide energy. Such radiation would not have been able to penetrate the ocean floor.

Finally, the origin of life seems to have required different chemical reactions to occur in different places and times, and only then to combine in the right sequence. These conditions may have been possible in small lakes and ponds on the Earth’s surface, where geothermal heat (for example, from lava flows) and cycles of wetting and drying, freezing and thawing created the various conditions needed, but not in the deep ocean.

The third misconception (or at least an unresolved issue) concerns opinions on the existence of extraterrestrial life. Astronomical discoveries over the past 30 years have shown that our Milky Way galaxy may be teeming with Earth-sized planets within the so-called “habitable zone” of its host star – a “Goldilocks” range of distances from the central star where the surface temperature of rocky planets is not too hot, but not too cold, to support liquid water (and therefore potentially life) on their surface. The fact that there could be a billion such planets in the Milky Way alone leads many people to assume that they do not exist. Must Exoplanets that could support life may exist. However, this conclusion is premature at best. Extraterrestrial life has not yet been discovered. probability Even if conditions were favorable, life could not emerge on a planet. In particular, the latest insights from origin-of-life research suggest that different geochemical scenarios must come together in the right order for the building blocks of life to form in the same place at the same time.

To take one example, we know that iron and cyanide (ferrocyanide) are essential for the production of the nucleotide building blocks of RNA. Such reservoirs of ferrocyanide salts could have undoubtedly formed on the early Earth (iron in the Earth’s crust combined with cyanide in the Earth’s atmosphere, and ferrocyanide salts precipitated above a certain concentration). However, these reservoirs could have easily been washed away or even destroyed altogether. Thus, it may have been highly unlikely that a local accumulation of ferrocyanide salts would have survived long enough to participate in the subsequent steps required to make life molecules. Moreover, other necessary compounds had to be delivered in a specific order to an environment in which the subsequent steps could occur. At this point, we cannot even estimate the probability of the entire sequence of steps occurring, so it is impossible to conclude whether the emergence of life on Earth was inevitable or a highly unlikely chemical accident. Thus, the question of whether extraterrestrial life exists in the Milky Way galaxy may only be answered by an active astronomical search for life itself.

This is an opinion and analysis article and the views of the author are not necessarily those of Scientific American.