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Muscles in the back of a 10 week old human embryo’s hand called dorsometacarpales (the two smallest horizontal muscles highlighted at center) will be lost or fuse with other muscles during development. Source: Diogo, Siomava, Gitton.

Ancient Reptilian Hand Muscles Found in Human Embryos

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High resolution 3D microscope images have revealed truths about early human development proving human embryos have more muscles than adults!

At seven weeks of gestation a human embryo is the size of a blueberry measuring a mere 0.51 inches (1.3 centimeters), but this is double what it would have measured the previous week. He or she barely registers on the scales just yet but is developing like crazy inside the pregnant belly and its embryonic hands have about 30 muscles. However, fully grown adults only have 19 muscles and a new scientific study has answered why so many muscles are lost.

High Resolution 3D Imaging Provides Answers About Evolution

In a new paper, published on October 1 in the journal Development, researchers say muscles in the feet, legs, trunk, arms, and head all appear and disappear during development. After analyzing detailed 3-D images of human embryos and fetuses up to 13 weeks of gestation the scientists concluded that these occurrences are all “remnants of evolution”.

A human embryo, seven weeks old. (Rama / CC BY-SA 3.0)

According to the lead biologist in the new study, Rui Diogo of Howard University in Washington, D.C, his team of evolutionary biologists have successfully demonstrated that numerous atavistic limb muscles which are absent in adult humans, but present in many limbed animal groups, once formed during early human development and were lost prior to birth. And the researchers also found that these muscles vanished from our human adult ancestors more than 250 million years ago as we transitioned from synapsid reptiles to mammals.

A report on PHYS.ORG discusses a particular aspect of the paper which asked “why” 30 hand and feet muscles form at about 7 weeks of gestation, yet one third of these fuse or become absent by 13 weeks of gestation. With their new data, the researchers say the long standing myth about our evolution and prenatal development is becoming increasingly more complex, with more anatomical structures such as muscles being continuously formed by the splitting of earlier muscles, can be deconstructed.

Origins and Evolution of the Human Species

The word  atavism comes from the Latin word  atavus, which means “a great-great-great-grandfather” or an “ancestor”. In social sciences, atavism, is the tendency of reversion, like for example, people in the modern era reverting to the ways of thinking and acting of former times. In biology, however, an atavism is a modification of a biological structure in which a reappearance of an ancestral trait occurs having been lost through evolutionary change in previous generations.

Early embryos display some ancestral features, like the tail on this human embryo. These features normally disappear in later development, but it may not happen if they have an atavism. (DO11.10 / Public Domain)

Ever Since Darwin's evolutionary theory was first formulated in his 1859 book On the Origin of Species which declared the processes by which organisms change over time result from changes in heritable physical and behavioral traits, scientists have argued that the occurrence of atavistic structures supports the idea that species change over time from a common ancestor through “descent with modification” the passing of traits from parent to offspring.

Human Babies Share Lizard’s Muscular Patterns

This new study, using high-quality 3D images of human embryos and fetuses provides the first detailed analysis of the development of human arm and leg muscles, revealing the transient presence of many atavistic muscles. Dr. Diogo said what he is fascinated by in the new results is that his team observed various muscles that have never before been described in human prenatal development.

In the paper’s conclusions, while some of the atavistic muscles were found on rare occasions in adults in anatomical variations, there were no noticeable adverse effects on the adults’ health. And this, according to the researchers, reinforces ideas that muscle variations and pathologies can be related to “delayed or arrested embryonic development” which helps explain why these muscles are occasionally found in adults, providing a snapshot of “evolution at play”.

A report on Science News says other animals have kept some of those muscles, for example, both adult chimpanzees and human embryos have epitrochleoanconeus muscles in their forearms, whereas most adult humans don’t. What’s more, about 250 million years ago human mammalian ancestors lost dorsometacarpales muscles from the back of their hands at the same time mammals and reptiles split on the evolutionary tree. And while lizards and human embryos still have these muscles, they cannot be found in most adults.

Muscles of the adult human hand. (u_irwan / Adobe Stock)

Top image: Muscles in the back of a 10 week old human embryo’s hand called dorsometacarpales (the two smallest horizontal muscles highlighted at center) will be lost or fuse with other muscles during development. Source: Diogo, Siomava, Gitton.

By Ashley Cowie

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Ashley is a Scottish historian, author, and documentary filmmaker presenting original perspectives on historical problems in accessible and exciting ways.

He was raised in Wick, a small fishing village in the county of Caithness on the north east coast of... Read More

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