Muscles Not Present for More than 250 Million Years Found in Embryos

"It used to be that we had more understanding of the early development of fishes, frogs, chicken, and mice than in our own species, but these new techniques allow us to see human development in much greater detail," wrote Diogo in a press release highlighting the study results. "What is fascinating is that we observed various muscles that have never been described in human prenatal development, and that some of these atavistic muscles were seen even in 11.5-weeks old fetuses, which is strikingly late for developmental atavisms." 

Findings suggest humans become more simple in the womb

The muscles that fused are atavistic limb muscles, formed during the early development and disappear as the embryo prepares to enter the world. It flies in the face of evolution in which scientists have long argued that humans become more complex. The new imagery shows some of the anatomical features actually become more simple in the womb. 

The findings, which were published in journal Development, provide a new look at how human arms and legs evolved from our ancestors as well as provides deeper insights into human variations and pathologies. Some of the atavistic muscles found in the developing embryos were also found in adults, although it's rare. 

"This reinforces the idea that both muscle variations and pathologies can be related to delayed or arrested embryonic development, in this case perhaps a delay or decrease of muscle apoptosis, and helps to explain why these muscles are occasionally found in adult people. It provides a fascinating, powerful example of evolution at play," said Diogo.