New, high-res 3D images of human embryos show a variety of muscles that were "present in our ancestors but normally absent from the adult human." For example, there are hand muscles that temporarily appear in human embryos but usually disappear before birth. According to the researchers, our human ancestors lost those "muscles from the back of the hand about 250 million years ago as mammals and reptiles split on the evolutionary tree." From Science News:
These appearing and disappearing, or atavistic, muscles are remnants of evolution, says biologist Rui Diogo of Howard University in Washington, D.C. Such atavistic muscles are built as a base from which to start paring down to the final set of muscles that people are born with, he says. "Losing and specializing, that's what happens in human evolution."
Other animals have kept some of those muscles. Adult chimpanzees and human embryos have epitrochleoanconeus muscles in their forearms, but most adult humans don't. Human's mammalian ancestors also lost dorsometacarpales muscles from the back of the hand about 250 million years ago as mammals and reptiles split on the evolutionary tree. Lizards still have those muscles, and they appear in human embryos, but then are lost or fuse with other muscles during development and aren't found in most adults.