Head over heels: How mammals stood up and took over the world

For more than a century, scientists have puzzled over a fundamental mystery in our evolutionary history: how did mammals go from sprawling like lizards to striding like cats and dogs? This transition to an upright posture marked a pivotal moment in mammal evolution. The earliest non-mammalian synapsids, ancestors of living mammals, had a sprawling posture, but when and how mammals adopted an upright posture remained unresolved.

A new study published in PLOS Biology, led by Robert Brocklehurst, a former postdoctoral fellow in the Department of Organismic and Evolutionary Biology (OEB), offers a surprising answer: the path to upright posture wasn’t linear, but full of unexpected detours, evolutionary experimentation, and dramatic anatomical upheaval.

All mammals—from bats and whales to moles and humans — share a distinctive way of moving, with their limbs underneath their bodies. This posture enables more efficient movement and is tied to mammals’ ability to adapt to diverse lifestyles, from digging to flying. This dramatic transition was also accompanied by big changes in limb bone shape and mechanics.

To assess these changes, the researchers analyzed the humerus (upper arm bone) from more than 60 non-mammalian synapsid fossils and 140 living animals — including mammals, reptiles and amphibians—that span hundreds of millions of years and reflect various stages of evolution.

Using a novel analytical technique pioneered by senior author professor of organismic evolutionary biology Stephanie Pierce’s lab (OEB), the team digitally mapped each bone’s surface to measure length, mass distribution, muscle leverage, and torsion. These traits are linked to specific modes of limb function and allowed the researchers to reconstruct posture and locomotion in the fossils.

The researchers placed the fossil synapsids on a functional adaptive landscape, similar to a topographic map, with peaks and valleys that related to high and low performance of different locomotor postures.

“We expected to see a neat progression — from sprawling early synapsids to increasingly upright  forms,” said Brocklehurst, who is currently a postdoc at University of Massachusetts Lowell. “Instead, we found bursts of innovation.”

The findings suggest that mammal evolution involved a series of adaptive radiations, with each major ancestral group exploring a range of forelimb functions and postures—some of which were closer to modern mammals, others not.

“The path to upright posture wasn’t a straight line,” said Pierce, Alexander Agassiz Professor of Zoology in the Museum of Comparative Zoology. “The ancestors of mammals weren’t steps on a ladder leading to modern mammals. Mammals have been evolving and radiating into many different niches and habitats throughout their history, and their postures reflect that variation.”

One fossil, a close relative of today’s marsupials and placentals, showed bone features consistent with a modern upright gait, suggesting that fully parasagittal limb postures (when legs extend downward) evolved relatively later in mammalian history than previously thought. The study also supports recent work from the same lab on the backbone and hindlimb evolution.

“Our work challenges the idea that posture changed gradually and early on,” said Pierce. “It shows that upright posture and locomotion were a late evolutionary innovation, not an early defining trait of the mammalian lineage.”

The researchers also challenged the assumption that the earliest ancestors sprawled in a similar way to living lizards or crocodiles. “Our study showed that most synapsid limbs functioned differently from modern reptiles,” said co-author Kenneth Angielczyk of Chicago’s Field Museum. “They weren’t reptile copies, but unique animals different from anything that’s alive today,” 

Traditional shape-analysis methods didn’t work for analyzing such a wide range of bones. So, the team re-engineered existing software into a novel “slice-based” landmarking tool tailored for this study. Co-author Magdalen Mercado ’22, who is now working toward at doctorate at University of Chicago, helped gather the extensive dataset as part of her senior thesis.

Pierce, who is also Curator of Vertebrate Paleontology in MCZ, noted that researchers and former MCZ curators such as Alfred Sherwood Romer and Farish Jenkins Jr., grappled with these same questions a century ago. “This is the first large-scale evolutionary analysis of mammalian posture using quantitative biomechanics,” she said. “With new tools and data, we can revisit those ideas and see the story more clearly.”

Partial funding for this research was provided by the National Science Foundation.