EXCERPT To understand the origin of these complex behaviours, we must take a closer look at the molecular structure of silk. First of all, silk is a protein, i.e. a chain—in chemistry we call it a polymer—of small molecular fragments called amino acids. In particular, major ampullate silk is obtained from a mixture of two different types of protein, which emerged at two successive moments in the spider’s evolutionary history. Amino acids, the molecular units that make up proteins, are very similar to one another: every amino acid has two ends, each capable of chemically bonding with the opposite end of another amino acid, as if they were Lego bricks or puzzle pieces. Each amino acid, however, is characterised by a molecular ‘pendant group’ different from the others, and which gives it particular properties: in living systems there are twenty in all, which together form a sort of alphabet of life. Combining these twenty fragments yields a surprising variety of proteins, each capable of performing its own specific function, from communication between the cells of an organism to the catalysis of metabolic reactions, to specific structural functions as in the case of spider silk…