Figure 1: Spontaneously occurring tumours are not reported in animals outside the bilaterian clade.
From: Naturally occurring tumours in the basal metazoan Hydra
Schematic phylogenetic tree showing main branches of metazoan evolution and position of Cnidaria inside the group of pre-bilaterian metazoans. The clades where cases of spontaneously occurring bona fide tumours have been documented are marked by + sign. Phylostratigraphic analysis of genes associated with cancer in humans indicates their early evolutionary emergence and predicts the evolutionary origin of animal tumours to lie in the root of metazoan tree6. This does not preclude the possibility that tumour suppressor-related genes in early metazoan evolution were involved in the control of cell proliferation to adjust to environmental changes7.
