Evolution
Muscles have evolved at least twice, once in cnidaria and once in bilateria; they seem to have evolved from contractile cells in sponge-grade organisms.
Evolutionarily, specialized forms of skeletal and cardiac muscles predated the divergence of the vertebrate/arthropod evolutionary line. This indicates that these types of muscle developed in a common ancestor sometime before 700 million years ago (mya). Vertebrate smooth muscle was found to have evolved independently from the skeletal and cardiac muscles.
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