A team of evolutionary biologists, led by Dr. Rui Diogo at Howard University, and writing in the journal Development, have demonstrated that numerous atavistic limb muscles—known to be present in many limbed animals but usually absent in adult humans—are actually formed during early human development and then lost prior to birth. Strikingly, some of these muscles, such as the dorsometacarpales shown in the picture, disappeared from our adult ancestors more than 250 million years ago, during the transition from synapsid reptiles to mammals.
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Source: Phys.org