Study reveals twists and turns in mammal evolution from sprawled to upright posture
Mammals, including humans, are distinguished by their unique upright posture, a key characteristic that has fueled their remarkable evolutionary success. But the earliest known ancestors of modern mammals resembled reptiles with limbs sticking out to the sides.
The transition from the sprawling posture of lizards to the upright posture of modern mammals like humans, dogs, and horses marked a pivotal moment in evolution.
It involved a major reorganization of limb anatomy and function in monoapsids (a group that includes both mammals and their non-mammalian ancestors), eventually leading to the therians we know today. Connected to mammals (marsupials and placentals). Despite more than a century of research, the exact “how,” “why,” and “when” behind this evolutionary leap remain elusive.
Now, in a study published in Science Advances, researchers at Harvard University provide new insight into this mystery, showing that the transition from a sprawling to an upright position in mammals was anything but simple. It became clear.
Using cutting-edge techniques that fuse fossil data and advanced biomechanical modeling, researchers show that this transition is surprisingly complex, nonlinear, and occurs much later than previously thought. discovered.
Lead author Dr. Peter Bishop, a postdoctoral fellow in Harvard’s Department of Biological and Evolutionary Biology, and senior author Professor Stephanie Pearce began by examining the biomechanics of five extant species representing the full range of limb postures. Tegu lizard (sprawling), crocodile (semi-erect), greyhound (erect).
“Studying these extant species for the first time greatly increased our understanding of how an animal’s anatomy relates to how it stands and moves,” Bishop said. Ta. “You can then put that into an evolutionary context of how posture and gait actually changed from early monoapsids to modern mammals.”
The researchers extended their analysis to eight exemplary fossil species from four continents spanning 300 million years of evolution. Species ranged from the 35-gram primitive mammal Megazostrodon to the 88-kilogram Ophiacodon, and included iconic animals such as the sail-backed Dimetrodon and the saber-toothed predator Lycaenopus.
Bishop and Pearce used principles of physics and engineering to build a digital biomechanical model that shows how muscles and bones attach to each other. Using these models, they were able to generate simulations that determined how much force the hind limbs (back legs) could exert on the ground.
“The amount of force a limb can exert on the ground is an important determinant of an animal’s locomotor performance,” Bishop said. “If you can’t generate enough force in a certain direction when you need it, you won’t be able to run fast, change direction, or worse, you might fall.”
Computer simulations generated a three-dimensional “actionable force space” that captures the limb’s overall functional performance. “Calculating the space of realizable forces implicitly accounts for all possible interactions between muscles, joints, and bones throughout the limb,” Pearce said.
“This allows us to see the big picture more clearly and to take a more holistic view of limb function and movement and how it has evolved over hundreds of millions of years.”
The concept of viable force spaces (developed by biomedical engineers) has existed since the 1990s, but this study extends it to understand how extinct animals once moved. This is the first study to apply it to the fossil record.
The authors packaged their simulations into a new “fossil-friendly” computational tool that other paleontologists can use to explore their own questions. These tools could also help engineers design better biological robots that can navigate complex or unstable terrain.
The study revealed several important ‘signals’ for locomotion, including that the overall force-generating capacity of extant species is greatest around the positions that each species uses in its daily activities. . Importantly, this meant that Bishop and Pearce could be confident that the results they obtained for extinct species truly reflected the way they stood and moved when they were alive.
Analyzing extinct species, researchers found that locomotor performance did not progress simply and linearly from irregular to upright, but instead peaked and declined over millions of years. .
Some extinct species, like modern crocodiles and crocodiles, appear to be more flexible and able to switch back and forth between a sprawled and upright position. Others, on the other hand, showed a strong reversal to a more sprawled posture before mammals evolved.
Combined with other results of this study, this suggests that traits associated with upright posture in today’s mammals may have evolved much later than previously thought, closer to the common ancestor of therians. was shown to be high.
These discoveries also help resolve some unresolved questions in the fossil record. For example, we know that asymmetrical hand, foot, and limb joints persisted in the ancestors of many mammals, and that these features are commonly associated with a stretched posture among modern animals. I’m explaining.
Additionally, fossils of early mammalian ancestors are often found in a squished and spread eagle position, whereas this position is more likely to be found with arms and legs outstretched. This also explains why placentas and marsupial fossils are often found lying on their sides.
“As a scientist, it’s very gratifying when one set of results helps clarify other observations and brings us closer to a more comprehensive understanding,” Bishop said.
Dr. Pearce’s lab has been studying the evolution of mammalian body plans for nearly a decade, and notes that these findings are consistent with patterns seen in other parts of the uniarchite body, such as the vertebral column.
“A picture is emerging of a complex and long-term assembly of a complete complement of typical therian traits, achieved relatively late in the history of monoarchs,” she said. Ta.
The study suggests that, beyond mammals, some major evolutionary transitions, such as the shift to an upright posture, are often complex and may have been influenced by chance events. For example, a major reversal in the posture of monoapsids, returning to a more sprawled position, appears to coincide with the Permian-Triassic mass extinction, when 90% of life became extinct. .
This extinction event forced other groups, such as dinosaurs, to become the dominant animal group on land, relegating monoapsids into the shadows. The researchers speculate that this “ecological marginalization” may have significantly altered the evolutionary trajectory of monoapsids, changing the way they migrated.
Whether or not this hypothesis is borne out, understanding the evolution of mammalian posture has long been a complex puzzle. Pearce highlighted how advances in computing power and digital modeling have provided scientists with new perspectives to address these ancient mysteries.
“By using these new techniques on ancient fossils, we’re giving us a better perspective on how these animals evolved and that it wasn’t just a simple, linear evolutionary story. “You can get it,” she said. “It’s really complex, and these animals were probably living and moving in their environments in ways that we didn’t realize before. A lot of things happened, and today’s mammals are It’s really very special.”
Further information: Peter Bishop, “Late acquisition of upright hindlimb posture and function in therian mammalian pioneers,” Science Advances (2024). DOI: 10.1126/sciadv.adr2722. www.science.org/doi/10.1126/sciadv.adr2722
Provided by Harvard University
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