Chemical evolution — One-pot wonder
Evolution has a history: Before life could be formed on the then still young planet, the first simple building blocks must have been created some four billion years ago that set its formation in motion. Under what conditions and in what ways did such molecules come together to form more complex genetic polymers that were able to replicate themselves — precursors of today’s DNA? Scientists working with Professor Thomas Carell at the Ludwigs-Maximilian-Universitaet (LMU) Munich, are now able to explain another, if not the decisive, step in this chemical evolution that preceded biological phylogeny. They report about it in the journal Science.
In their new work, Thomas Carell and his team propose a cascade of chemical reactions in which the four different components of the hereditary molecule RNA can all be produced under identical early Earth conditions: the primordial soup — cooked in one pot, so to speak. So far, there have been two competing pathways that required different geochemical settings on early Earth. One leads to the construction of the so-called pyrimidines, the letters C (cytosine) and U (uracil) in the RNA alphabet, the other to A (adenine) and G (guanin), the purines. Carell’s team had already described the reaction path to the latter molecules in a previous paper. Now the Munich scientists have finally created all four genetic building blocks that might have jump-started life.
Accordingly, the simplest chemical ingredients and reaction conditions, such as those found on Earth millions of years ago on geothermal fields with subsoil volcanic activity or in shallow ponds for example, were sufficient to keep the synthesis of the RNA building blocks, going over a whole series of reaction steps. Starting materials for the experiments, which were intended to simulate prebiotic conditions, were substances as simple as ammonia, urea and formic acid. It also needed salts such as nitrites and carbonates as well as metals such as iron and zinc, which are present in large quantities in the Earth’s crust. The chain of chemical reactions was driven only by wet-dry cycles, such as those caused by hydrothermal sources or periods of drought or rain.
Thomas Carell calls it a “breakthrough.” It is interesting to see how comparatively homogeneous the reaction conditions are for the individual steps of synthesis. Even small fluctuations of physical parameters such as mild warming or cooling or the change between a slightly acidic and a slightly alkaline reaction environment are sufficient. “There are few complex molecules that can be produced in such narrow reaction bands,” says the LMU chemist. Such simple framework conditions, he concludes, made it all the more plausible that these reaction cascades and thus a decisive step in chemical evolution could have taken place on early Earth.