A team of evolutionary biologists, led by Dr. Rui Diogo at Howard University, USA, and writing in the journal Development, have demonstrated that numerous atavistic limb muscles - known to be present in many limbed animals but usually absent in adult humans - are actually formed during early human development and then lost prior to birth. Strikingly, some of these muscles, such as the dorsometacarpales shown in the picture, disappeared from our adult ancestors more than 250 million years ago, during the transition from synapsid reptiles to mammals.
Also 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.
Dorsal view of the left hand of a 10-week old human embryo. The dorsometacarpales are highlighted: these muscles (like others described in this study) are present in adults of many other limbed animals, while in humans they normally disappear or become fused with other muscles before birth.
Credit: Rui Diogo, Natalia Siomava and Yorick Gitton
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.
These findings offer new insights into how our arms and legs evolved from our ancestors', and also about human variations and pathologies, as atavistic muscles are often found either as rare variations in the common human population or as anomalies found in humans born with congenital malformations.
Since Darwin proposed his evolutionary theory, scientists have argued that the occurrence of atavistic structures (anatomical structures lost in the evolution of a certain group of organisms that can be present in their embryos or reappear in adults as variations or anomalies) strongly supports the idea that species change over time from a common ancestor through "descent with modification".
For example, ostriches and other flightless birds have vestigial wings, while whales, dolphins and porpoises lack hind limbs but their embryos initiate and then abort hind limb development. Similarly, temporary small tail-like structures are found in human embryos and the remnant of the lost ancestral tail is retained as our coccyx. Researchers have also suggested that atavistic muscles and bones can also be seen in human embryos, but it has been difficult to visualize these structures clearly, and the images that appear in modern textbooks are mainly based on decades old analyses.
This is changing with development of new technology that provides high-quality 3D images of human embryos and fetuses. In the new study published in the journal Development the authors have used these images to produce the first detailed analysis of the development of human arm and leg muscles.
The unprecedented resolution offered by the 3D images reveals the transient presence of several of such atavistic muscles. Dr. Diogo said: "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.
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 ".
He further 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, in this case perhaps a delay or decrease of muscle apoptosis, and helps to explain why these muscles are occasionally found in adult people. It provides a fascinating, powerful example of evolution at play."
If reporting this story, please mention the journal Development as the source and, if reporting online, please carry a link to:
REFERENCE: Diogo, R., Siomava, N. and Gitton, Y. (2019) Development of human limb muscles based on whole-mount immunostaining and the links between ontogeny and evolution (Development, in press)
Rui Diogo | EurekAlert!
A new 'cool' blue
17.01.2020 | American Chemical Society
Neuromuscular organoid: It’s contracting!
17.01.2020 | Max-Delbrück-Centrum für Molekulare Medizin in der Helmholtz-Gemeinschaft
Styrofoam or copper - both materials have very different properties with regard to their ability to conduct heat. Scientists at the Max Planck Institute for Polymer Research (MPI-P) in Mainz and the University of Bayreuth have now jointly developed and characterized a novel, extremely thin and transparent material that has different thermal conduction properties depending on the direction. While it can conduct heat extremely well in one direction, it shows good thermal insulation in the other direction.
Thermal insulation and thermal conduction play a crucial role in our everyday lives - from computer processors, where it is important to dissipate heat as...
In order to advance the transfer of research developments from the field of quantum sensor technology into industrial applications, an application laboratory is being established at Fraunhofer IAF. This will enable interested companies and especially regional SMEs and start-ups to evaluate the innovation potential of quantum sensors for their specific requirements. Both the state of Baden-Württemberg and the Fraunhofer-Gesellschaft are supporting the four-year project with one million euros each.
The application laboratory is being set up as part of the Fraunhofer lighthouse project »QMag«, short for quantum magnetometry. In this project, researchers...
Microtubules, filamentous structures within the cell, are required for many important processes, including cell division and intracellular transport. A...
Researchers from the University Hospital Zurich, ETH Zurich, Wyss Zurich and the University of Zurich have developed a machine that repairs injured human livers and keep them alive outside the body for one week. This breakthrough may increase the number of available organs for transplantation saving many lives of patients with severe liver diseases or cancer.
Until now, livers could be stored safely outside the body for only a few hours. With the novel perfusion technology, livers - and even injured livers - can now...
A balloon-borne scientific instrument designed to study the origin of cosmic rays is taking its second turn high above the continent of Antarctica three and a half weeks after its launch.
SuperTIGER (Super Trans-Iron Galactic Element Recorder) is designed to measure the rare, heavy elements in cosmic rays that hold clues about their origins...
16.01.2020 | Event News
15.01.2020 | Event News
07.01.2020 | Event News
17.01.2020 | Life Sciences
17.01.2020 | Power and Electrical Engineering
17.01.2020 | Life Sciences