New Insights Reveal Late Evolution of Upright Walking in Mammals

A groundbreaking study published on June 25, 2025, in PLOS Biology has unveiled new insights into the evolution of upright walking in mammals, challenging long-held beliefs about the timeline and processes involved. Conducted by a team at Harvard University's Department of Organismic and Evolutionary Biology, the research led by Dr. Robert Brocklehurst indicates that the transition from sprawling gaits to upright walking was not a straightforward progression, but rather a complex journey marked by numerous detours and evolutionary experimentation.

For over a century, scientists have sought to understand how mammals evolved from the sprawling postures of their lizard-like ancestors to the efficient, upright stances seen in modern mammals. Previously, it was believed that this transition occurred gradually over time. However, the findings from Brocklehurst and his colleagues, including co-author Dr. Stephanie Pierce, challenge this notion, suggesting that early mammalian ancestors explored a variety of postures for millions of years before fully adopting the upright gait characteristic of contemporary mammals.

The research utilized advanced bone-mapping technology to analyze the humerus, the upper arm bone, of over 60 non-mammalian synapsid fossils and 140 living species. This comparative analysis revealed significant insights into the mechanics and evolution of limb posture. "By correlating bone shape and limb biomechanics with posture, we could test how well the fossil bones were optimized for specific functional tasks, like upright walking versus sprawled walking," Dr. Brocklehurst stated.

The study’s results indicate that the path to upright posture was characterized by bursts of innovation rather than a linear progression. According to Dr. Pierce, "The ancestors of mammals weren't steps on a ladder with modern mammals at the top; instead, mammals have evolved and radiated into many different niches and habitats throughout their history."

One notable fossil examined in the study, a close relative of current marsupials and placentals, displayed bone features consistent with a modern upright gait, suggesting that fully parasagittal postures emerged later than previously hypothesized. This revelation aligns with recent findings concerning the evolution of the mammalian backbone and hindlimbs.

The implications of this study extend beyond mere anatomical changes; they also inform our understanding of how the evolutionary pressures faced by early mammals shaped their development and adaptability. The research indicates that mammalian limbs functioned differently from those of modern reptiles, emphasizing their unique evolutionary traits.

The study marks a significant advance in evolutionary biology, representing the first large-scale analysis of mammalian posture through quantitative biomechanics. Moving forward, the research team plans to develop detailed models of forelimbs in select fossil species to gain deeper insights into how ancient animals moved.

Funding for this research was provided by the US National Science Foundation and the Harvard Museum of Comparative Zoology. The findings not only enhance our understanding of mammalian evolution but also underscore the dynamic history of life on Earth. As Dr. Brocklehurst aptly concluded, "Understanding how mammals came to walk upright isn't just about bones; it's about uncovering the dynamic history of life on Earth."