A fine yeasty head on a fermentation vessel at the former Whitbread brewery in Chiswell Street, London about 1948
Around 80 million years ago, when Triceratops still browsed the plains of what wasn’t yet North America, some flowering plant species developed a new strategy to spread their seeds, encasing them in a soft, fleshy cover – fruit – that became sweet and tasty as everything ripened. The fruit was then eaten by animals, which would subsequently deposit the seeds far away from the mother plant, and with the addition of some useful fertiliser as well.
It did not take long, however, for enterprising funguses to start exploiting the sugar in the ripe fruits for their own growth and development, using oxygen to break the sugar down into carbon dioxide and water, releasing energy at the same time. If there was no oxygen about they would turn the sugar, via acetaldehyde, into alcohol, and make energy that way, although they very much preferred not to: alcohol was poisonous.
These funguses mostly reproduced by budding – dividing into two, and giving each daughter cell a complete copy of her mother’s genes. However, on one occasion, at least, a daughter received two copies of her mother’s genome instead of one. The daughter’s descendants evolved this spare set of genes so that, unlike their ancestors, they could make alcohol all the time, not just in the absence of oxygen. The spare copy of the gene that created the enzyme that turned acetaldehyde into alcohol also evolved so that it could do this trick in reverse – turn alcohol into acetaldehyde.
What these changes enabled the clever little fungus – the ancestor of brewing yeast – to do when it landed on sugary fruit was to quickly flood its environment with alcohol, which was toxic to most of its microbial rivals (our double-genome yeast had, of course, also evolved greater tolerance to alcohol, to cope with the extra alcohol it now created). Once it had swamped the area with alcohol, and thus seen off rival funguses, it could then win even more energy by turning the alcohol into acetaldehyde.
The alcohol content of over-ripe fruit attacked by yeast has been tested at as high as 4.5 per cent, and there is a theory that humans like alcohol in part because our early primate ancestors learned to associate its smell with the presence of ripe, sugary fruit – a sort of ancient alcopop. Appreciating alcohol, therefore, looks to be something deep in humanity’s genes, going back to when we lived in trees and were still covered in fur.