Complex Evolutionary Pathways of Mammals to Upright Posture Revealed

For over a century, the evolutionary transition of mammals from a sprawling gait to upright posture has been a subject of intense scientific debate. Recent research, spearheaded by Dr. Robert Brocklehurst of the University of Reading, challenges the traditional linear view of this transformation, suggesting instead that it was a complex, branching process shaped by various evolutionary experiments. This study, published in the journal PLOS Biology on June 26, 2025, analyzes over 200 species of tetrapods, uncovering that the path to upright walking was not straightforward but rather characterized by bursts of innovation and diverse limb functions across different mammalian ancestors.

Dr. Brocklehurst stated, "The evolution of mammals has previously been characterized as a series of steps from sprawling to semi-sprawling to upright. However, our findings reveal a more nonlinear evolutionary progression throughout mammalian history.” The research team employed a computational model that connected bone shape with function and posture, allowing for the creation of adaptive landscapes that linked bone mechanics to various walking styles, including sprawled and upright positions.

The findings indicate that early synapsids, while exhibiting sprawling limbs, did not function in the same manner as modern reptiles, as noted by Dr. Kenneth Angielczyk, a paleontologist at the University of Chicago. He emphasized that these ancestors were unique animals with distinct characteristics that diverged from today's reptiles. The researchers posited that rather than a rigid sequence of postural transitions, mammalian evolution was marked by a multitude of limb use forms.

Professor Stephanie Pierce of Harvard University echoed this sentiment, pointing out that the path to upright posture was not linear. "The ancestors of mammals weren’t steps on a ladder with modern mammals at the top," she remarked, emphasizing the significant musculoskeletal changes required for modern mammals to achieve an upright stance. These changes included adaptations in bone shape and the interaction of muscles with the skeletal framework.

The study's implications extend beyond mere anatomical changes; it offers insights into the dynamic history of life on Earth. Dr. Brocklehurst remarked, "Understanding how mammals came to walk upright isn’t just about bones. It’s about uncovering the dynamic history of life on Earth." While acknowledging the limitations in evolutionary timelines and divergence dates, the researchers believe their large dataset and mechanical modeling provide a robust framework for future studies.

The research utilized innovative techniques to compare hundreds of bones from vastly different animals, overcoming the limitations of traditional shape analysis tools. This approach, described as a “slice-based” landmarking system, allowed for precise measurement of traits across species separated by millions of years. Professor Pierce reflected on the legacy of past researchers at Harvard who explored similar questions, stating, "Now, with new tools and data, we can revisit those ideas and see the story more clearly."

The findings underscore the significance of upright posture in mammalian evolution, suggesting that this trait was not an early defining characteristic but rather a late evolutionary innovation that laid the groundwork for the ecological success of mammals. As the research team continues to model limb function in fossil species, they anticipate unlocking further details about how ancient organisms lived and navigated their environments. Dr. Brocklehurst concluded, "The origin of upright mammalian posture is a key part of their evolutionary story. People have been working on this problem for over 100 years. We knew we needed to see as many fossils as possible and truly grasp bone function and mechanics, not just shape."

In summary, this groundbreaking study illuminates the multifaceted nature of mammalian evolution, portraying it as a dynamic landscape rather than a simple, linear progression. The research opens new avenues for understanding how various forces shaped the locomotion of one of Earth's most diverse and adaptive groups of animals.