Animalia -- a category that includes nearly one million living species, ranging from jellyfish to humans -- can be broadly characterized by several traits. Animals are eukaryotic, multicellular, and heterotrophic, meaning that they feed off other life forms (as opposed to plants, which produce their own food). They also lack the rigid cell walls that characterize plants. Aside from sponges, animals possess specialized tissues and are capable of movement. Virtually all the major groups of animals came into existence around 570 million years ago, during a period termed the "Cambrian explosion." It is believed that they evolved from a single unicellular choanoflagellate ancestor. All animals also contain a set of developmental regulatory genes called Hox genes, which are important for body plan patterning. In general, the visible complexity of animals is reflected in the number and complexity of these genes. Although the traditional classification of animals and their presumed phylogeny has been based on overt anatomical features, molecular studies of Hox genes and of ribosomal RNA sequences over the past decade have forced scientists to reassess the evolutionary relationships between animals and their ancestors. According to this tree, animals are divided into two distinct groups: choanozoans, which are unicellular protists; and metazoans, the multicellular animals.