Single-cell organisms were already in existence 500 million years ago, with several thousand genes providing different cellular functions. Further developments seemed dependent on producing even more genes. For a highly developed organism like a human, this form of evolution would have resulted in several million genes. Researchers were therefore surprised to learn, following publication of the human genome, that a human only has around 25,000 genes ? not many more than a fruit fly or a worm with approximately 15,000 to 20,000 genes. It would appear that, over the last 500 million years, other ways to produce highly complex organisms have evolved. Evolution has simply found more efficient ways to use the genes already there. But what could have made this possible? New light has now been shed on the purpose of an unusual structure in RNA polymerase II.