Researchers have unveiled new findings about the intricate evolutionary journey that led to the unique upright stance of today’s placental and marsupial mammals. This shift turned out to be far more complicated and nonlinear than previously thought and occurred significantly later in history.
Mammals, including humans, are distinguished by their characteristic upright posture, a crucial trait behind their remarkable evolutionary achievements. However, the earliest known relatives of current mammals resembled reptiles more closely, with their limbs extending sideways in a sprawled position.
The evolution from a sprawled stance akin to that of lizards to the upright posture observed in contemporary mammals, including humans, dogs, and horses, represented a crucial evolutionary milestone. This transformation required a significant reorganization of limb anatomy and functionality in synapsids—the group that includes mammals and their non-mammalian predecessors—eventually leading to the emergence of therian mammals (marsupials and placentals). Despite more than a hundred years of research, the precise mechanics of “how,” “why,” and “when” this evolutionary progression occurred has remained unclear.
In a recent study featured in Science Advances, scientists from Harvard University provided fresh insights into this question, indicating that the transition from sprawled to upright posture in mammals was notably complex. By employing advanced techniques that combine fossil evidence with sophisticated biomechanical modeling, the research team discovered that this transition unfolded in a nonlinear manner and occurred much later than earlier assumptions suggested.
Lead author Dr. Peter Bishop, a postdoctoral fellow, along with senior author Professor Stephanie Pierce from Harvard’s Department of Organismic and Evolutionary Biology, began their investigation by analyzing the biomechanics of five contemporary species, illustrating a full range of limb postures, including a tegu lizard (sprawled), an alligator (semi-upright), and a greyhound (upright).
“By studying these modern species first, we enhanced our understanding of the relationship between an animal’s anatomy and its posture and movement,” Bishop explained. “This allowed us to contextualize how posture and gait evolved from early synapsids to present-day mammals.”
The researchers expanded their analysis to include eight fossil species from four continents, representing 300 million years of evolution. These species ranged from the lightweight proto-mammal Megazostrodon weighing 35 grams to the much heavier Ophiacodon at 88 kilograms, including well-known fossils like the sail-backed Dimetrodon and the saber-toothed predator Lycaenops. By applying principles from physics and engineering, Bishop and Pierce created digital biomechanical models demonstrating the relationships between muscles and bones in these species. These models facilitated simulations to evaluate how much force their hindlimbs (back legs) could exert against the ground.
“The force a limb can apply to the ground is a key indicator of locomotor efficiency in animals,” stated Bishop. “Without the ability to generate sufficient force in a necessary direction, an animal might struggle to run quickly, change direction rapidly, or risk falling.”
The computer simulations produced a three-dimensional “feasible force space” representing the overall functional performance of a limb. “Feasible force spaces account for all potential interactions between muscles, joints, and bones in a limb,” Pierce elaborated. “This offers a clearer, comprehensive perspective on limb functionality, locomotion, and their evolutionary trajectory over millions of years.”
Although the concept of feasible force space (developed by biomedical engineers) emerged in the 1990s, this study marks the first application of this theory to fossil records to comprehend how extinct species moved. The research team developed new “fossil-friendly” computational tools from their simulations to assist other paleontologists in tackling their inquiries. These tools might also aid engineers in crafting superior bio-inspired robots for maneuvering difficult terrains.
The research highlighted several key insights regarding locomotion, showing that the ability to generate force in modern species was highest around their habitual postures. This finding provided confidence that the behaviors observed in extinct species genuinely reflected their living conditions.
Upon examining the extinct species, the researchers found that locomotor capabilities fluctuated over millions of years, instead of showing a straightforward shift from sprawling to upright. Some ancient species demonstrated greater adaptability, able to transition between sprawled and upright postures, similar to contemporary alligators and crocodiles. Conversely, other species displayed a marked regression towards more sprawled positions before the advent of mammals. This, along with the other findings, suggested that the traits linked to upright posture in present-day mammals evolved much later than previously assumed, likely around the time of the common ancestor of therians.
These discoveries also assist in addressing several unanswered questions in the fossil record. For instance, it clarifies why many mammal ancestors maintained asymmetric limbs and joints, features typically associated with sprawled postures in modern animals. Additionally, it accounts for the frequent discovery of early mammal ancestors lying in a spread-eagle position—a stance more aligned with sprawled limbs—while fossils of modern placentals and marsupials typically rest on their sides.
“As a scientist, it is incredibly rewarding when one set of findings sheds light on other observations, bringing us closer to a more complete understanding,” Bishop remarked.
Pierce, whose lab has examined the evolution of the mammalian body plan for nearly a decade, noted that these results align with observable patterns in other areas of the synapsid structure, such as the vertebral column. “It is becoming clear that the full range of traits distinctive to therians was developed over a complex, extended period, with the complete suite emerging relatively late in synapsid evolution,” she indicated.
Beyond mammals, this study implies that significant evolutionary transitions, such as the move to an upright stance, were often intricate and possibly influenced by random events. For example, the notable regression in synapsid posture back towards sprawled forms appears to coincide with the Permian-Triassic mass extinction—a period that wiped out 90% of life. This extinction opened the door for groups like dinosaurs to dominate terrestrial ecosystems, which pushed synapsids into more marginal ecological positions. The researchers suggest that due to this “ecological marginalization,” the evolutionary path of synapsids may have been altered, affecting their modes of movement.
Whether or not this hypothesis gains support, the study of mammal posture evolution has consistently presented a complex puzzle. Pierce highlighted how advancements in computational power and digital modeling offer new insights into these ancient enigmas. “Using these modern techniques alongside ancient fossils gives us a better understanding of how these animals evolved, shunning the simplistic, linear narrative we once believed to be true,” she stated. “The reality is intricate, and these animals likely engaged and moved in their surroundings in ways we have yet to fully grasp. There’s so much more to the story, and today’s mammals are genuinely remarkable.”
