Insulin or a closely related molecule is native to Escherichia coli.

Insulin which is found in large amounts in the @cells of the pancreas in vertebrates has also been shown to be present, in the most. primitive of vertebrates and in complex invertebrates, which are evolutionarily about 500 million years old (see Refs. 1-8 of Ref. 1). We reported evidence for the existence of material extremely similar to insulin in three unicellular eukaryotes, uiz. Tetrahymena pyriformis, a ciliated protozoan, and two fungi, Neurospora crassa and Aspergillus fumigatus (1). This finding suggested that the insulin molecule may have had its origin about 1 billion years ago. In this study, we present evidence for the presence of similar material with both immunological and biological similarities to mammalian (or hagfish) insulin in a prokaryote, Escherichia coli K12, which suggests that insulin may have origins even earlier than previously thought, namely 2 billion years ago (z).'