Evolution of Earth Movement in Early Tetrapods

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The air scene depicts the Two Late Devonian early tetrapods, Ichthyostega and Acanthostega, coming out of the water to move on land. Footprints leave a mark on the animals’ backs to show a sense of movement. Credit: Davide Bonadonna

The transition from water to land is one of the most important and inspiring key transitions in the evolution of vertebrates. And the question of how and when tetrapods The transition from water to land has long been a source of surprise and scientific controversy.

Early ideas suggested that pools of dried fish on land and being out of the water created selective pressure for the development of more organ-like attachments to return to the water. New discoveries in the 1990s suggested that the first tetrapods retained many aquatic properties, such as clay and tail fins, and that muscles developed in water before the tetrapods adapted to life on land. However, there is still uncertainty about when the transition from water to land took place and what the early land tetrapods looked like.

A document published today (November 25, 2020) Nature addresses these questions with high-resolution fossil data and shows that these early tetrapods still have adaptations that show their ability to move on land, even though they are water-dependent and have water properties. However, at least by today’s standards, it may not be very good at doing so.

Lead author Blake Dickson of Harvard University’s Department of Organic and Evolutionary Biology, ’20 and Stephanie Pierce, co-author of Thomas D. Cabot’s Department of Organic and Evolutionary Biology and curator of vertebrate paleontology at the Comparative Museum. examined three three-dimensional 40 fossil humer models (upper arm bone).

Rotating Humerus Shapes

There are three main stages in the evolution of the humerus form: from the block humerus of aquatic fish to the L-shaped humerus of tetrapods and the folded humerus of dry tetrapods. Columns (from left to right) = aquatic fish, transition tetrapod and land tetrapod. Rows = Top: extinct animal silhouettes; Medium: 3D humerus remains; Below: The places used to measure the image. Credit: With permission from Blake Dickson

“Since the fossil records of land transitions in tetrapods are very weak, we went to a fossil source that could better represent the integrity of the transition from being a completely aquatic fish to a completely dry tetrapod,” he said.

Two-thirds of the fossils are from historical collections housed at the Harvard Museum of Comparative Zoology, which originated around the world. To fill in the gaps, Pierce appealed to colleagues with key examples from Canada, Scotland and Australia. TW: eed project as an initiative designed to understand the early evolution of land use, the University of Cambridge in the UK, one of the authors, Dr. New fossils recently discovered by Tim Smithson and Professor Jennifer Clack were important for research. tetrapods.

The researchers chose the humerus not only because it is abundant and well preserved in the fossil record, but also because it is present in all sarcopteries – a group of animals that includes all tetrapods, including coelacanth fish, lung fish, and all fossil representatives. . “We expected the humerus to give a strong signal as the animals evolved from being fully functional fish to completely dry tetrapods, and we can use this to predict that the tetrapods are starting to move on land,” he said. “We’ve found that surface ability coincides with the origin of muscles, and it’s really exciting.”

Shape Change along the Humerus Evolution Tree

Evolutionary path and shape change from aquatic fish humerus to dry tetrapod humerus. Credit: With permission from Blake Dickson

The humerus binds the front leg to the body, hosts many muscles, and must withstand a lot of stress during muscle-based movement. For this reason, it contains a large amount of critical functional information related to the movement and ecology of an animal. Researchers have suggested that evolutionary changes in the shape of the humerus, short in fish and longer than squats, and in tetrapods, have significant functional effects associated with the transition to land movement. This idea has rarely been quantified – that is, so far.

When Dickson was a sophomore, he was fascinated by the application of quantitative modeling theory to understand functional evolution, a technique he led in a 2016 study co-authored by Pierce under the direction of a paleontologist team. Central paleontologist George Gaylord Simpson’s adaptive landscape concept, developed in 1944 to model quantitative features, is a solid three-dimensional surface with peaks and valleys like a mountain range. The increased altitude in this landscape represents better functional performance and adaptability, and over time it is expected that natural selection will lead populations to the peak of adaptation.

Dickson and Pierce thought they could use this approach to model the transition of a tetrapod from water to land. They assumed that as the shape of the humerus changed, so did the landscape. For example, there would be an adaptive peak where the functional performance of the fish for swimming and the functional performance of the land tetrapods for walking on land is maximized. “We can then use these views to see if the humerus shape of the previous tetrapods is better adapted for speech in water or on land,” he said.

“We’re starting to think about what functional features will be important to collect from the humerus,” Dickson said. “It wasn’t an easy task because fish fins were so different from tetrapod limbs.” Finally, as an advocate for step lengths, they narrowed their focus to six features that could be reliably measured on all fossils, including simple measurements such as the relative length of bone and more sophisticated analyzes that simulated mechanical stress in different weighted scenarios. assess the strength of the humerus.

“If you have an equal representation of all the functional features, you can determine how performance changes as you move from one adaptive peak to another,” Dickson explained. Using computational optimization, the group was able to detect a precise combination of functional features that increased performance for aquatic fish, land tetrapods, and the earliest tetrapods. Their results showed that the earliest tetrapods had a unique combination of functional features, but did not conform to their peak of adaptation.

“What we found was that the humerus of the early tetrapods was assembled at the base of the dry landscape,” Pierce said. “It shows increased performance to move on land. However, these animals only developed a number of functional features that were limited to walking on earth. ”

Researchers suggest that the ability to move on land may be limited by other characteristics, such as feeding on water, which connects early tetrapods to their ancestral aquatic environment. Once the tetrapods got rid of this restriction, the humerus was free to develop muscle-based movements and morphologies and functions that greatly increased the invasion of ecosystems on Earth.

“Our study provides the first quantitative, high-resolution understanding of the development of land movement along the water-land transition,” he said. “It also predicts when and how it will happen [the transition] occurred and what functions were important in the transition, at least in the humerus. ”

“As we move forward, we are interested in expanding our research to other parts of the tetrapod skeleton,” Pierce said. “For example, it has been suggested that the front legs are capable of on the ground before the back foot angles, and that our new methodology will be used to test this hypothesis.”

Dickson recently started as a Postdoctoral Researcher at the Animal Placement Laboratory at Duke University, but continues to collaborate with Pierce and laboratory members on additional research covering the use of these methods in other parts of skeletal and fossil records.

Reference: “Functional Adaptive Landscapes Predict Earth Capacity at the Origin of Aladdin” BV Dickson, JA Clack, TR Smithson and SE Pierce, 25 November 2020. Nature.
DOI: 10.1038 / s41586-020-2974-5

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