Early in the history of our Galaxy, a few hundred million years after their appearance, the first heaviest stars ended their days with an impressive explosion: the supernova explosion. In doing so, they expelled its contents into space, enriching the interstellar medium with the material synthesized inside: carbon, oxygen, nitrogen, iron …
With oxygen and carbon, carbon monoxide was formed, the most abundant two-atom molecule in the Universe. A small amount of oxygen, of the order of 0.5%, ended up forming water vapor, the most abundant triatomic molecule in the Universe; water is found in every corner of the Cosmos.
As the temperature dropped, the metal oxides formed silicates and condensed into ultramicroscopic solid grains that the water vapor coated into an ice crust. Solid matter had just appeared, for the first time in the history of the Universe.
In this environment the first triatomic molecule to form was hydrocyanic acid (HCN). Meanwhile, from carbon and hydrogen, polyaromatic hydrocarbons (PAH) appeared. The Universe was taking its first steps towards Life.
But classical chemical reactions cannot happen in severe interstellar cold. Instead, unusual reactions occurred, only occurring near absolute zero. Where did complex molecules appear? Everything happened inside the densest nebulae, where the most abundant molecules are hydrogen and carbon dioxide. Inside we have discovered more than a hundred different molecules, of which two thirds are organic.
Interestingly, it is on the surface of the dust grains in these clouds that most reactions take place: the molecules stick together and stay there long enough for them to combine with each other. That is why prebiotic molecules, the molecules necessary for the emergence of life, exist in abundance in space. Among the most crucial are water, formaldehyde (H2CO) and hydrocyanic acid, very abundant in our Galaxy and, presumably, throughout the Universe.
The evolutionary path that leads to life appears to have chosen a path with the least number of obstacles and the most abundant building materials available.
