Evolution of Animals

Animals are multicellular heterotrophs that ingest food and are characterized by mobility and the absence of cell walls. Most animals have a diploid life cycle and reproduce sexually. Muscle and nerve tissues allow animals to actively seek food. All animals share a common ancestor, forming a single lineage that branches into vertebrates, which possess a spinal cord at some stage, and invertebrates, which do not. The colonial flagellate hypothesis suggests that animals descended from a hollow colony of flagellated cells. Choanoflagellates are the closest living relatives of animals. Developmental differences in body plans arise from shifts in HOX genes.