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We already touched on development a little bit when we talked about homology, because homologous structures are especially easy to identify at the early stages of development. This is because, among vertebrates, embryos all look kind of similar at their really early stages. If you have two features that look really different in adults (like the wings of a bird and the arm of a human), studying their development can help you to see that the bones making up wings and arms are homologous, which means they shared a common ancestor.

Sometimes, embryos also reveal rudiments, which are the remains of an ancestral feature. Rudiments are kind of like vestigial structures, except that vestigial structures show up in adults, and rudiments are only briefly visible during the embryonic period. For example, baleen whales don't have any teeth—instead, they use their baleen like a giant sieve to capture plankton to eat. Interestingly, before they are born, they show evidence of tooth formation—those little teeth are rudiments. They go away by the time the baby whale is born, but the fact that they're present in the womb tells us that the ancestors of baleen whales had teeth, and that, over evolutionary time, they lost their teeth and acquired baleen.

Many modern scientists focus on developmental pathways, or on genes that regulate development, and these, too, provide evidence for evolution. Really different organisms—jellyfish and aardvarks, for example—have strikingly similar genetic pathways for development. For example, Hox genes are a group of genes that affect the formation of body segments—and all animals, regardless of shape, have them! Studying how mutations affect Hox genes also sheds light on how so many different kinds of animal shapes exist on the planet. For example, mutations in Hox genes can cause a certain kind of body part—like a leg or a wing—to grow in the completely different place. Over evolutionary time, different animals have evolved different numbers and kinds of Hox genes, leading to a diversity of body shapes and types. Studying these genes and understanding how they differ in jellyfish, aardvarks, and other animals can help us understand how certain kinds of physical traits came to be, and makes it clearer than ever that all these different organisms are related to each other.

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