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Human Fetuses Develop Lizard-Like Body Parts That Disappear Before Birth



Photo: Government of Alberta (Flickr (CC BY 1.0))

New research this week seems to show that human fetuses develop several muscles in their legs and arms that disappear by the time they are born. And some of these muscles were last seen in our adult ancestors over 250 million years ago.

The evolutionary journey of any species is littered with detours and dead-ends. Humans, for instance, have vestigial body parts that once served a function but are effectively useless nowadays (the appendix is ​​usually singled out as a vestigial organ, though a better example might be our wisdom teeth). Many animals also form body parts early in development that largely or completely fade away before birth, such as the tailbone in humans.

But according to the authors of this new study, published in the journal Development, we were not able to track the formation of these temporary human body parts in any great detail. Using advanced 3D imaging techniques, the authors say they were able to provide the clearest picture of our limbs' early growth — and they are pretty weird.

The dorsal view of the left hand of a 10-week-old human embryo, with the dorsometacarpales highlighted.
Image: Rui Diogo, Natalia Siomava and Yorick Gitton (Development)

In the hand and foot of a seven-week-old fetus, for instance, they were able to find 30 individual muscles. But by week 13 of gestation, a third of the muscles had vanished or fused together. A pair of these atavistic muscles, as they are known, are called the dorsometacarpales. And though still found in many limbed animals today, including lizards and salamanders, it seems to have stopped appearing in our adult ancestors 250 million years ago.

"What is fascinating is that we have 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-week-old fetuses, which is strikingly late for developmental atavisms," study author Rui Diogo , an evolutionary biologist at Howard University in Washington DC, said in a release by the study's publishers.

These remnant organs and parts are a nifty illustration of how evolution works over a long period of time. While we may not need a tail anymore, our genomes still contain the blueprint for it. And they can even reappear if someone is born with a rare mutation or is exposed to something in the womb that damages their development.

Although these muscles in particular would likely cause much harm if you were born with them, the authors say their research reinforces that such variations and abnormalities can be caused by delayed or arrested development of a fetus in the womb. And perhaps more than anything, Diogo said, the findings provide "a fascinating, powerful example of evolution at play."


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