Mammals, including humans, are characterized by their upright posture, which has mainly prompted their evolutionary success. However, the earliest mammalian ancestors resembled reptiles, with limbs splayed outward in a sprawled posture more similar to modern-day lizards.
The transition from this sprawled stance to the upright posture seen in modern mammals, such as humans, dogs, and horses, was a significant evolutionary shift. Despite over a century of research, the precise details of how, why, and when this change occurred remain unclear.
A new study sheds light on mammals’ complex and nonlinear transition. This shift, believed to have occurred earlier, took place much later than previously thought.
The researchers examined the biomechanics of five modern species with varying limb postures: a tegu lizard (sprawled), an alligator (semi-upright), and a greyhound (upright). They used fossil data with advanced biomechanical modeling to reveal the intricacies of this evolutionary transition.
Evolutionary origins of animal biodiversity
Researchers studied modern species to understand better how an animal’s anatomy influences its posture and movement. They then applied this knowledge to an evolutionary context, tracing how posture and gait evolved.
By examining species with varying limb postures, they could better understand the gradual shifts from sprawled to more upright stances in mammals, providing new insights into the complexities of this evolutionary transition.
The study includes eight fossil species from four continents, covering 300 million years of evolution. These species ranged from the tiny 1-ounce proto-mammal ‘Megazostrodon’ to the massive ‘Ophiacodon’, as well as other notable animals like the sail-backed ‘Dimetrodon’ and the saber-toothed predator ‘Lycaenops’.
By applying principles from physics and engineering, researchers created digital biomechanical models of how muscles and bones were connected in these species. These models enabled them to run simulations that measured how much force the hindlimbs could exert on the ground, providing key insights into the evolution of posture and locomotion.
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Lead author and postdoctoral fellow Peter Bishop in the Department of Organismic and Evolutionary Biology said, “The amount of force that a limb can apply to the ground is a critical determinant of locomotor performance in animals. If you cannot produce sufficient force in a given direction when needed, you won’t be able to run as fast, turn as quickly, or worse still; you could fall over.”
The computer simulations generated a 3D “feasible force space” representing a limb’s overall functional performance. Computing feasible force spaces implicitly accounts for all the interactions between muscles, joints, and bones throughout a limb.
This approach provides a more transparent, more holistic understanding of limb function and locomotion, revealing how these evolved over hundreds of millions of years.
The researchers developed new “fossil-friendly” computational tools from their simulations, which can assist other paleontologists in exploring their questions. These tools could also help engineers design bio-inspired robots capable of navigating complex or unstable terrain.
The study uncovered several key insights into locomotion, including the fact that the force-generating ability of modern species was most significant around the postures they used for daily activities. Researchers believe this finding validates that their results for extinct species accurately reflect how those species stood and moved during their lifetime.
After analyzing the extinct species, the researchers found that locomotor performance did not follow a linear progression from sprawling to upright. Instead, it peaked and dipped over millions of years.
Some extinct species appeared more flexible, shifting between sprawled and upright postures, similar to modern alligators and crocodiles. Others showed a reversal back to more sprawled postures before mammals evolved. These findings suggest that the traits associated with the upright posture of modern mammals likely evolved much later than previously believed, likely near the common ancestor of therian mammals.
The study also helps explain several unresolved issues in the fossil record. For instance, it clarifies why many mammal ancestors have asymmetric hands, feet, and limb joints, traits commonly linked to sprawling postures in modern animals.
Additionally, it sheds light on why early mammal ancestor fossils are often found in a squashed, spread-eagle pose, which is more characteristic of sprawled limbs. In contrast, fossils of modern placental and marsupial mammals are typically found on their sides.
Bishop said, “It is gratifying as a scientist when one set of results can help illuminate other observations, moving us closer to a more comprehensive understanding.”
Journal Reference:
- Peter Bishop and Stephanie Pierce. Late acquisition of erect hindlimb posture and function in the forerunners of therian mammals. Science advances. DOI: 10.1126/sciadv.adr2722
