The Ancestor's Tale: A Pilgrimage to the Dawn of Evolution

by Richard Dawkins

Polyps often reproduce by budding vegetatively, like plants. A new baby polyp grows on the side of a freshwater Hydra, eventually breaking off as a separate individual: a clone of the parent. Many marine relatives of Hydra do something similar, but the clone doesn’t break off and assume a separate existence. It stays attached and becomes a branch, as in a plant. These ‘colonial hydrozoans’ branch and branch again, making it easy for us to understand why they were thought to be plants. Sometimes more than one kind of polyp grows on the same polyp tree, specialised for different roles, such as feeding, defence, or reproduction. You can think of them as a colony of polyps, but there is a sense in which they are all parts of one individual, for the tree is a clone: all the polyps have the same genes. Food caught by one polyp may be used by others, since their gastric cavities are all continuous. The branches of the tree and its main trunk are hollow tubes that you can think of as a shared stomach—or maybe as a kind of circulatory system playing the role that in us is played by blood vessels. Some of the polyps bud off tiny medusae, which swim away like miniature jellyfish to reproduce sexually and disperse the genes of the parent polyp tree to distant places.