Evolutionary changes in the number of olfactory receptor (OR) genes in mammals. (a) Estimated numbers of functional OR genes (red) and OR pseudogenes (blue) in the most recent common ancestor (MRCA) between humans and chimpanzees. The numbers in humans and chimpanzees are also shown. 'F →F' ('P → P') indicates that a functional gene (a pseudogene) in the MRCA is still functional (a pseudogene) in humans or chimpanzees. 'F → P' represents a functional gene in the MRCA becoming a pseudogene in the human or chimpanzee lineage. A grey triangular area depicts deletion from the genome; for example, 62 pseudogenes in the MRCA were eliminated from the human genome. Adapted from Go and Niimura (2008). (b) Gains and losses of OR genes in mammalian evolution. A number in a box represents the number of functional OR genes in the extant or ancestral species. A number with a plus and a minus sign indicates the number of gene gains and losses, respectively, for each branch. Adapted from Niimura and Nei (2007).