The transition from a sprawled stance, akin to that of lizards, to the upright posture we observe in mammals today—like humans, dogs, and horses—signifies a crucial chapter in our evolutionary saga. This transformation entailed a profound reconfiguration of limb structure and functionality within the synapsids, the lineage that encompasses both mammals and their predecessors. Ultimately, this evolution paved the way for the therian mammals—marsupials and placentals—who inhabit our world now. Yet, despite years of inquiry, the specifics of the “how,” “why,” and “when” surrounding this monumental evolutionary shift have remained somewhat enigmatic.
In a new study published in *Science Advances*, Harvard researchers shed some light on this complex narrative, challenging previous assumptions about the transition from sprawling to upright posture in mammals. Employing innovative techniques that fuse fossil records with sophisticated biomechanical modeling, the research team uncovered that this evolutionary shift was far from straightforward and occurred much later than initially believed.
Dr. Peter Bishop, a postdoctoral fellow, led the research alongside Professor Stephanie Pierce, an expert in the Department of Organismic and Evolutionary Biology at Harvard. They began their exploration by analyzing the biomechanics of five contemporary species, representing a wide range of limb postures, from the sprawled posture of a tegu lizard to the semi-upright stance of an alligator, and the completely upright alignment of a greyhound.
“By first examining these modern species, we significantly enhanced our understanding of how anatomy influences an animal’s posture and movement,” Bishop stated. “This perspective allowed us to trace the evolutionary trajectory of posture and locomotion from early synapsids to modern mammals.”
Expanding their investigation, the team scrutinized eight fossil species across four continents, encapsulating 300 million years of evolutionary history. Ranging in size from the 35-gram proto-mammal Megazostrodon to the robust 88-kilogram Ophiacodon, the collection included notable animals such as the sail-backed Dimetrodon and the fearsome saber-toothed predator Lycaenops. By applying principles of physics and engineering, they developed digital biomechanical models to illustrate how the muscles and bones of these creatures interacted. These models allowed for simulations that assessed the force applied by the hind limbs (back legs) on the ground.
“The force that a limb exerts on the ground plays a vital role in an animal’s locomotion,” Bishop explained. “If that force isn’t adequate in a particular direction when needed, speed decreases, turning becomes sluggish, or worse, you risk a fall.”
The computer simulations generated a three-dimensional “feasible force space” that encapsulated a limb’s overall performance. “This concept accounts for all interactions between muscles, joints, and bones throughout a limb,” Pierce added. “It provides a holistic perspective on limb functionality and the evolution of locomotion over millions of years.”
While feasible force spaces have been utilized by biomedical engineers since the 1990s, this study marks the first instance of applying this model to fossil records to comprehend the movements of extinct animals. The authors also developed “fossil-friendly” computational tools that promise to aid other paleontologists in addressing their own research queries. Additionally, these tools could inspire engineers to create better bio-inspired robots capable of navigating complex terrains.
The research revealed several key insights into locomotion, highlighting that modern species exhibited peak force-generating abilities that correlated closely with their everyday postures. This finding bolstered the authors’ confidence that their results for extinct species accurately reflected how these creatures stood and moved when they roamed the Earth.
When the researchers turned their attention to extinct species, they discovered that locomotor capabilities fluctuated over millions of years, rather than following a linear progression from sprawling to upright. Some fossil species demonstrated a remarkable adaptability, oscillating between sprawled and upright postures, reminiscent of today’s alligators and crocodiles. Conversely, others exhibited a reversion back to a sprawled stance prior to the emergence of mammals. This complex evidence suggests that traits associated with upright posture evolved far later than initially believed, likely around the time of the common ancestor of therians.
These discoveries also help resolve lingering questions in the fossil record. For instance, they shed light on the asymmetric arrangement of hands, feet, and limb joints found in many mammal ancestors—characteristics typically linked to sprawling postures. Moreover, it explains why early mammalian fossils are often unearthed in spread-eagle positions—suggestive of sprawled limbs—while remnants of modern placental and marsupial ancestors are usually found lying on their sides.
“It’s incredibly rewarding as a scientist to see one set of findings clarify other observations, moving us closer to a comprehensive understanding,” remarked Bishop.
Pierce, who has dedicated her lab’s research to studying the evolution of mammalian body plans for nearly a decade, noted that these revelations align with trends observed in other elements of the synapsid body plan, such as the vertebral column. “A clearer picture is emerging, illustrating that the full array of distinctly therian traits evolved over an intricate and prolonged timeframe, with that complete suite materializing relatively late in synapsid history,” she stated.
Moreover, this study implies that significant evolutionary transitions—like the move toward an upright posture—are often multifaceted and potentially shaped by chance occurrences. Notably, the pronounced reversal in synapsid posture back toward sprawled positions seems to coincide with the catastrophic Permian-Triassic mass extinction, which eradicated an estimated 90% of life. This ecological upheaval allowed groups like the dinosaurs to dominate terrestrial environments, effectively sidelining synapsids. The researchers hypothesize that this “ecological marginalization” significantly altered the synapsids’ evolutionary path, changing how they moved.
Regardless of whether this hypothesis gains further support, delving into the evolution of mammal posture remains a convoluted puzzle. Pierce highlighted how advancements in computational technology and digital modeling have offered scientists fresh perspectives to tackle these ancient enigmas. “Utilizing these new techniques alongside fossil data helps us to gauge how these animals evolved, revealing that this wasn’t merely a linear process,” she emphasized. “It’s a complex story reflecting ways of life and movement that we have not yet fully appreciated. There’s so much more to be discovered, and modern mammals truly are remarkable.”
Priyam Gera