Atavisms, or throwbacks, are further proof that evolution has taken place. While homologies show that organisms have common ancestors, atavisms show that the genes of the ancestors are often still present in the living creature. Whales with tiny hind legs protruding from their pelvic bones; horses with two toes: such oddities are accidental reminders of the past. Sometimes, these can be created artificially, with embryology. For instance, some experiments showed that birds still have the genes to make teeth, even though modern birds do not have teeth. Others showed that by preventing signals from flowing at a crucial time during development, the bone structure of a chick’s leg could turn out to look much more like a reptile’s than a bird’s. Such experiments show that the genes are still there ­ they just don’t usually behave as they once did.
