How mammals achieved their upright posture 

A new study reveals the twists and turns and complexities in the evolution of mammals, from a spread body structure (legs tending to the sides like in reptiles) to an upright body structure where the feet are stable on the ground

Harvard University, Department of Organismic and Evolutionary Biology 

Mammals, including humans, are characterized by their upright posture, which is a key feature that has led to their impressive evolutionary success. However, the earliest ancestors of modern mammals were more reptilian, with their limbs splayed out to the sides in a spread-out position. 

The change from a splayed posture, like that of lizards, to the upright posture of modern mammals, like humans, dogs, and horses, marked an important turning point in evolution. This change included a reorganization of the structure and function of the limbs in the synapsids - the group of animals that includes the mammals and their non-mammalian ancestors - which finally led to the mammals (chairs and placental mammals) that we know today. Despite more than a century of research, the "how", "why" and "when" questions regarding this evolutionary transition remain enigmatic. 

Now, in a new study published in Science Advances, Harvard researchers provide new insights into the mystery, revealing that the transition from a splayed to an upright position in mammals was anything but simple. Using innovative methods that combine fossil data with advanced biomechanical models, the researchers discovered that the transition was complex and non-linear, and occurred later than previously thought. 

Dr Peter Bishop, post-doctoral fellow, and Professor Stephanie Pearce, principal investigator, both from the Department of Organismic and Evolutionary Biology at Harvard, began studying the biomechanics of five modern species representing a range of limb structures, including the tegu lizard (spread), crocodile (semi-erect ) and Greyhound (upright). 

"By studying these modern species first, we significantly improved our understanding of how an animal's anatomical structure relates to how it stands and moves," Bishop said. "We could then put this into an evolutionary context of how structure and movement changed from ancient synapsids to modern mammals." 

The researchers extended their analysis to eight fossil species from four continents, spread over 300 million years of evolution. Species ranged from the 35-gram proto-mammal Megastrodon to the 88-kilogram Opiacodon, and included iconic animals such as the dorsal-sail Dimetrodon and the deadly fang-bearing predator Lycanops. Using principles from physics and engineering, Bishop and Pierce built digital biomechanical models that simulated the interaction between muscles and bones. These models allowed them to run simulations that determined the quant The force the hind legs could exert on the ground. 

"The amount of force a limb can exert on the ground is a critical factor in animal locomotion performance," Bishop said. "If you can't generate enough force in a given direction when it's needed, you won't be able to run fast, turn sharply, or worse, you'll fall." 

The computer simulations created a 3D 'applicable force space' that captures the functional performance of the limb. "Calculating the applicable force fields takes into account all the possible interactions between muscles, joints and bones along the limb," Pearce said. "This gives us a broader picture of the function of the limb and the development of movement over hundreds of millions of years." 

While the idea of ​​an applicable force space has been around since the 90s, this is the first time it has been applied to the fossil record to understand how extinct animals moved. The authors have packaged the simulations into new fossil-friendly computational tools that can help other paleontologists research their questions. These tools may also help engineers design biomimetic robots capable of navigating complex or unstable terrain conditions. 

The research revealed a number of important 'motion signatures', including the overall capacity for force production in modern species that was maximal in the body structures used by them in their daily behavior. This meant that Bishop and Pearce could be sure that the finds for extinct species truly reflected their posture and movement when they were alive. 

After analyzing the extinct species, the researchers discovered that locomotion performance reached peaks and troughs over millions of years, and did not simply progress in a straight line from a spread to an upright position. Some extinct species also displayed great flexibility - they could switch between more spread out or more upright positions, like modern crocodiles. Others have shown a strong reversion to more spread-out postures before mammalian evolution. Together with other findings, this indicated that the features associated with the upright posture of modern mammals evolved much later than previously thought, most likely near the common ancestor of sight. 

These findings also help resolve unresolved issues in the fossil record. For example, they explain the persistence of asymmetric arms and legs and limb joints in mammalian ancestors, traits usually associated with spread-out postures in modern animals. They also explain why fossils of early mammalian ancestors are often found in a spread-out position, while fossils of modern placental and cetacean mammals are usually found lying on their sides. 

"It's very satisfying as a scientist when certain results can illuminate other observations, bringing us closer to a more comprehensive understanding," Bishop said. 

Pierce, who has been studying the evolution of the mammalian body plan for nearly a decade, notes that the findings are consistent with patterns seen in other regions of the synapsed body, such as the spine. "A picture is taking shape that the distinct features of mammals were assembled over a long and complex period, and full perfection was achieved relatively late in the history of synapsids," she said. 

Beyond mammals, the research suggests that some key evolutionary transitions, such as the transition to standing upright, were often complex and may have been influenced by chance events. For example, the strong reversion of synapsids toward more spread out postures coincides with the Perm-Triassic extinction, in which 90% of life was wiped out. This extinction event led to other groups such as the dinosaurs becoming the dominant animal groups on land, pushing the synapsids into the shadows.

for the scientific article

More of the topic in Hayadan:

• The first man to walk upright: remains of individuals from an ancient lineage between apes and humans have been discovered in Ethiopia
• A position of strength, a position of weakness
• A dinosaur was discovered that petrified in its sleep
• Not only cats fall on their feet