A new analysis from a research team at the University of Illinois provides further proof for the hypothesis that viruses are living entities. Until now, the classification of viruses has been a subject of confusion, and the consensus was that all viruses originated from an ancestral virus. The study focused on structural building blocks of proteins, called “folds,” that are found on all cells and viruses. Viral genomes are prone to rapid mutations, making it hard to find evolutionary relationships in them. Protein folds, on the other hand, can maintain their three-dimensional structures despite mutations and thus are better markers of evolutionary events. By comparing all of the known folds in organisms representing every branch of the tree of life to those in viruses, the researchers identified 442 folds shared between cells and viruses. The viruses were also found to have unique components not shared with cells, contradicting the old idea that all viral genetic materials are captured from cells. These data suggest that viruses originated from ancient cells and later evolved to gain their own parasitic characteristics. With these newfound evidence to support them, researchers argue that viruses should be given their own branch on the tree of life.