Human embryos have extra hand muscles found in lizards but not most adults

Human embryos are more muscle-bound than adult humans, new microscope images cataloging early development show.

For instance, at seven weeks of gestation, embryonic hands have about 30 muscles. Adults have about 19. Many of the muscles are lost, and some fuse with others, adopting the adult arrangement by 13 weeks of gestation, researchers report October 1 in Development.

Muscles in the feet, legs, trunk, arms and head also appear and disappear during development, researchers discovered after analyzing detailed 3-D images of human embryos and fetuses up to 13 weeks of gestation.

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.

Sometimes, people retain some of the usually lost muscles, resulting in harmless anatomical variations. For example, about 13 percent of people in one study had epitrochleoanconeus muscles in their forearms.