Although the molecular nature of tumours is well studied in vertebrates, their evolutionary origin remains unknown. In particular, there is no evidence for naturally occurring tumours in pre-bilaterian animals, such as sponges and cnidarians. We provide the first evidence for naturally occurring tumours in two species of Hydra, which show a transcriptome that mimics expression shifts in vertebrate cancers suggesting that spontaneous tumours have deep evolutionary roots and that early branching animals may be informative in revealing the fundamental mechanisms of tumorigenesis. I will also discuss the evolutionary roots of the complex vertebrate brain. A phylostratigraphic analysis of gene expression patterns in the developing zebrafish (Danio rerio) revealed three important periods in the evolutionary history of the vertebrate brain. The most striking is the finding that the extant vertebrate brain organization was already present in the chordate ancestor. This links the origin of the vertebrate brain with the evolutionary arms race between animal phyla that took place in the Cambrian.