Mammals, including humans, are distinguished by their upright posture, a trait that has contributed significantly to their evolutionary success. However, the earliest ancestors of mammals did not exhibit this stance; instead, they resembled reptiles with a sprawled posture. The transition from a sprawled to an upright posture marked a pivotal evolutionary shift in synapsids—a group that includes mammals and their non-mammalian ancestors—eventually leading to the therian mammals (marsupials and placentals) we see today. Despite extensive research, understanding the details of this shift has remained challenging.
In a recent study published in Science Advances, researchers Peter Bishop and Stephanie Pierce provided new insights into this transition. Using fossil data and biomechanical modeling, they revealed that the evolution from a sprawled to upright stance was a complex and nonlinear process that occurred later than previously thought. Bishop and Pierce began by studying five modern animals, each representing different postures, to understand the biomechanics of limb structure. They then extended their analysis to eight fossil species, spanning 300 million years of evolution. This allowed them to create biomechanical models that simulated how extinct animals once moved.
Through these simulations, the researchers generated a three-dimensional “feasible force space” to illustrate each limb’s potential for force generation. They discovered that posture in extinct species did not progress in a straightforward line from sprawled to upright. Instead, locomotor abilities fluctuated over millions of years, with some species exhibiting flexibility between postures, similar to modern crocodiles, while others reverted to more sprawled stances. This irregular progression suggests that fully upright posture in mammals evolved closer to the origin of therian mammals, much later than previously thought.
The study’s findings also address longstanding fossil record questions, such as why mammalian ancestors often exhibit sprawled limb traits and why early mammal fossils are frequently found in spread-eagle positions, while modern mammal fossils are usually found lying on their sides. These insights support the idea that essential mammalian traits, including upright posture, developed gradually and involved multiple evolutionary pathways.
The study not only advances understanding of mammalian evolution but also suggests that major evolutionary changes, like the shift to upright posture, were complex, potentially shaped by ecological events, including the Permian-Triassic mass extinction. This event altered synapsid evolution, pushing their ancestors into ecological niches that may have influenced their posture and locomotion.
This research demonstrates how digital modeling and advanced computing can provide new perspectives on ancient evolutionary mysteries. According to Pierce, these tools allow scientists to appreciate the diverse and adaptable ways early mammals navigated their environments, revealing that their evolution was far more intricate than previously understood.
