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

Illustration for article titled Human Fetuses Develop Lizard-Like Body Parts That Disappear Before Birthi/ii/i
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’re born. And some of these muscles were last seen in our adult ancestors over 250 million years ago.

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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 typically 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 entirely 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 haven’t been able to track the formation of these temporary body parts in humans with any great detail. Using advanced 3D imaging techniques, the authors say they were able to provide the clearest picture yet of our limbs’ early growth—and it’s pretty weird.

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The dorsal view of the left hand of a 10-week-old human embryo, with the dorsometacarpales highlighted.
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’re known, is called the dorsometacarpales. And though it’s 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 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,” study author Rui Diogo, an evolutionary biologist at Howard University in Washington D.C., 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 even can reappear if someone is born with a rare mutation or is exposed to something in the womb that damages their development.

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Though these muscles in particular wouldn’t likely cause much harm if you were to be born with them, the authors say their research reinforces that such variations and anomalies can be caused by the 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.”

Science writer at Gizmodo and pug aficionado elsewhere

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DISCUSSION

...the appendix is typically singled out as a vestigial organ, though a better example might be our wisdom teeth...

Neither of these is really any good as an example of a vestigial organ. The appendix probably serves as a reservoir for the gut biota, allowing it to re-establish itself after illness. And wisdom teeth were perfectly good, useful teeth in the West until about a century ago or so, when our tendency to eat soft processed food resulted in widespread environmentally-induced developmental changes in lower facial structure - our jaws just don’t have room for the wisdom teeth any more. If we went back to living on chewier foods we’d stop having trouble with our widom teeth after a generation or so. and orthodonty would become a dead art.

The coccyx also has a modern function. It serves as an origin for a couple of pelvic-floor muscles having to do with the anus. You’d miss these if they weren’t there. You’d also probably be unable to leave the house.

Vestigial organs are hard to identify - generally, if something doesn’t serve a function, natural selection will get rid of it, as it uses resources. Maybe those muscles that we use to wiggle our ears - free-riding for 40 million years. But even there, you have to be really careful. They might be left over from some vital developmental role they played early in our embryonic development.