How Fossils Show Mammals Began Walking Upright Later in Evolution

For more than a century, the evolution of mammals and their transition from a sprawling stance akin to that of lizards to a more upright posture reminiscent of modern cats and dogs has baffled scientists. This shift from a sprawling form to an upright (or parasagittal) posture is a crucial milestone in mammalian history. Early non-mammalian synapsids, the ancestors of contemporary mammals, exhibited a sprawling posture, leading researchers to question not only when but also how modern mammals achieved their upright stance.

In a groundbreaking paper published in PLOS Biology, Dr. Robert Brocklehurst, a former postdoctoral fellow from Harvard University’s Department of Organismic and Evolutionary Biology, reveals that the evolution towards upright posture was anything but straightforward. Instead of a direct trajectory, this process involved numerous evolutionary deviations, adaptations, and significant anatomical changes.

“The evolution of mammals has often been depicted as a linear progression from sprawling to semi-upright to fully upright,” Brocklehurst remarked. “However, our research indicates a much more complex and nonlinear evolutionary history.”

This unique mode of locomotion is shared among all mammals—from bats to whales and humans. They possess limbs positioned directly beneath their bodies, in contrast to the lateral limb positioning seen in sprawling species. This adaptation allows for more efficient movement and is closely linked to the diverse lifestyles that mammals have developed, including activities such as digging and flying. This notable transition also involved significant changes in the shape and mechanics of limb bones. To investigate these transformations, the research team examined the humerus (upper arm bone) from over 60 non-mammalian synapsid fossils as well as 140 modern species, including mammals, reptiles, and amphibians.

Using a groundbreaking analytical method developed in the lab of Professor Stephanie Pierce, the senior author, the team was able to create detailed maps of each bone’s surface, measuring characteristics such as length, mass distribution, muscle leverage, and torsion. These metrics correlate closely with specific locomotion styles and helped researchers reconstruct the postures and movements of the fossilized specimens.

By examining the relationship between bone structure and limb mechanics, the researchers could assess how well the fossil bones facilitated particular behaviors, such as upright versus sprawling movement. Surprisingly, rather than a tidy evolutionary progression from ancient forms to modern mammals, the researchers found numerous bursts of evolutionary innovation and experimentation.

The study suggests that mammal evolution was characterized by various adaptive radiations, with major ancestral groups exploring a spectrum of forelimb functions and stances—some closer to those of modern mammals than others. “The path to upright posture was not a straightforward journey,” Pierce noted. The earliest ancestors of mammals cannot simply be viewed as steps toward modern forms but rather as part of a diverse evolutionary landscape that illustrates the varied adaptations of mammals throughout their history.

One significant finding included a fossil that was closely related to today’s marsupials and placentals, which exhibited bone characteristics consistent with a modern upright walking gait. This discovery proposes that fully upright postures emerged relatively late in mammalian evolution, challenging previous theories that attributed this change to earlier periods. Additionally, the research contests long-standing beliefs suggesting early synapsids sprawled in a manner similar to that of extant reptiles. Instead, the study indicates that these early creatures had limb functionalities distinctly different from those of modern reptiles.

To manage the extensive array of bones, which span hundreds of species with considerable variation in both age and form, the team surmounted considerable technical challenges. Traditional methodologies for analyzing shape were inadequate, prompting the development of a new, customized software tool to facilitate the analysis.

This landmark study not only sheds light on mammalian evolution but also builds on a rich scientific tradition at Harvard. The extensive analysis significantly advances understanding of mammalian posture using quantitative biomechanics, with further models and research already in the pipeline to reveal even more about ancient locomotion.

As Brocklehurst succinctly summarized, comprehending the evolution of upright walking among mammals is not merely a question of bone structure; it’s about revealing the dynamic history of life on Earth.