A team of evolutionary biologists has demonstrated that numerous atavistic limb muscles – remnants of anatomy that evolution never completely discarded – are still formed during early human development and then lost prior to birth. The new study was published October 1, 2019, in the journal Development. It shows that these muscles – studied via non-invasive scans of living human embryos – revealed ancient reptilian “hand” muscles that have been gone from our adult ancestors for more than 250 million years. These muscles are among the oldest, albeit fleeting, remnants of evolution yet seen in humans.
Some of these muscles, such as the dorsometacarpales – the muscles between the metacarpal bones of the left hand – shown in the image above, are thought to be a relic from when reptiles transitioned to mammals. The scientists aren’t sure why the human body makes and then deletes them before birth.
Why are they there? Probably, we cannot just say in evolution, ‘Look, I will delete from scratch, from day zero, the muscle going to digits two, three, four, five and I will just keep the one going to the thumb.’
According to a statement from the researchers about the study:
Remarkably, in both the hand and the foot, of the 30 muscles formed at about 7 weeks of gestation one third will become fused or completely absent by about 13 weeks of gestation. This dramatic decrease parallels what happened in evolution and deconstructs the myth that in both our evolution and prenatal development we tend to become more complex, with more anatomical structures such as muscles being continuously formed by the splitting of earlier muscles.
For the study, the team scanned the tissues of more than a dozen living embryos and young fetuses – while still in the womb – in high-resolution 3D over a number of weeks. They found tiny muscles in the hands and feet in 7-week-olds that were no longer visible by week 13. They said it was the “unprecedented” resolution offered by the 3D images that revealed the transient presence of several of such very primitive muscles, Diogo said in a statement.
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.
Interestingly, some of the atavistic muscles are found on rare occasions in adults, either as anatomical variations without any noticeable effect for the healthy individual, or as the result of congenital malformations. This reinforces the idea that both muscle variations and pathologies can be related to delayed or arrested embryonic development
Bottom line: Ancient reptilian “hand” muscles have been found in human embryos.
Eleanor Imster has helped write and edit EarthSky since 1995. She was an integral part of the award-winning EarthSky radio series almost since it began until it ended in 2013. Today, as an EarthSky.org Editor, she helps present the science and nature stories and photos you enjoy. She and her husband live in Tennessee, where they enjoy guitar playing and singing. They have 2 grown sons.