While still in the womb, humans have extra lizard-like muscles in their hands

Research involving a non-invasive scan of living human embryos shows that some muscles, thought to have been abandoned by our mammalian ancestors 250 million years ago, are still present before birth. They’re among the oldest, albeit fleeting, remnants of evolution yet seen in humans.

X-ray of translucent purple hand with muscles in white labeled dorsometacarpales.

Scan of the left hand of a 10-week-old living human embryo. The muscles are highlighted: these muscles are present in adults of many other limbed animals, while in humans they normally disappear or become fused with other muscles before birth. Image via Rui Diogo, Natalia Siomava and Yorick Gitton.

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.

Evolutionary biologist Rui Diogo of Howard University led the study. Diogo told the BBC:

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.

He added:

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.

Source: Development of human limb muscles based on whole-mount immunostaining and the links between ontogeny and evolution

Via The Company of Biologists

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