Head over heels
New study reveals the surprising and twisty path our ancestors took to develop an upright stance
Harvard University, Department of Organismic and Evolutionary Biology
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The adaptive landscape of posture evolution in mammals and their ancestors. Living and fossil species with sprawling vs upright postures occupy different adaptive peaks.
view moreCredit: Credit: Magdalen Mercado
For over 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—from a sprawled stance (like a lizard) to an upright (parasagittal) posture—marked a pivotal moment in mammal evolution. While the earliest non-mammalian synapsids, the ancestors of living mammals, had a sprawling posture, researchers debated when and how the upright postures of modern mammals evolved.
Now, a groundbreaking study in PLOS Biology led by Dr. Robert Brocklehurst, a former postdoctoral fellow in the Department of Organismic and Evolutionary Biology (OEB) at Harvard University, offers a surprising answer: the path to upright posture wasn’t linear, but full of unexpected detours, evolutionary experimentation, and dramatic anatomical upheaval.
“The evolution of mammals has previously been characterized as a series of steps from sprawling, to semi-sprawling, to upright,” said Brocklehurst. “However, what we discovered was a more nonlinear evolutionary progression throughout mammalian history.”
All mammals—from bats and whales to moles and humans—share a distinctive way of moving: they hold their limbs underneath their bodies, unlike the sprawling posture with limbs out to the side. 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 team analyzed the humerus (upper arm bone) of over 60 non-mammalian synapsid fossils and 140 living animals, including mammals, reptiles and amphibians.
Using a novel analytical technique, pioneered in senior author Professor Stephanie Pierce’s lab (also in OEB), researchers mapped each bone’s surface to measure traits like length, mass distribution, muscle leverage, and torsion (the degree to which the bone twists along its length). These traits correlate with specific modes of locomotion and allowed the researchers to reconstruct posture and locomotion in the fossils.
“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,” Brocklehurst said. The researchers achieved this by mapping the fossil non-mammalian synapsids onto a functional adaptive landscape, similar to a topographic map, with peaks and valleys that relate to high and low performance of different locomotor postures.
“We expected to see a neat progression—from sprawling pelycosaurs to a bit more upright therapsids, then cynodonts, then fully upright mammals,” said Brocklehurst. “Instead, we found bursts of innovation.”
The findings suggest that mammal evolution involved a series of adaptive radiations, with each major ancestral groups 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,” says Pierce, “the ancestors of mammals weren’t steps on a ladder with modern mammals at the top. 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 postures evolved relatively late in mammalian history as opposed to previously held hypotheses. This result supports recent work from the same lab on the backbone and hindlimb.
“Our work challenges the idea that posture changed gradually and early on,” said Pierce, “instead, it shows that upright posture and locomotion were a late evolutionary innovation, not an early defining trait of the mammalian lineage.”
The researchers also challenge the long-held idea that the earliest non-mammalian synapsids sprawled in a similar way to living lizards or crocodiles. “Our study showed that most synapsid limbs functioned differently than those of modern reptiles. They’re not just copies of reptiles, but distinctive animals in their own right that are a little different from anything that’s alive today,” says co-author Kenneth Angielczyk of Chicago's Field Museum.
To compare such a wide range of bones—spanning hundreds of species, including those hundreds of millions of years apart in age and wildly different in shape—the team had to overcome major technical hurdles. Traditional methods that describe shape in similar structures didn’t work. So, the team re-engineered an existing R software package designed for a different task, transforming it into a novel “slice-based” landmarking tool tailored for this study. Co-author Magdalen Mercado, former undergraduate student in the Integrative Biology program at Harvard, helped gather the extensive dataset as part of her senior thesis and research in the Pierce lab.
The study builds on a rich scientific legacy—both at Harvard and in paleontology. Pierce, who is also Curator of Vertebrate Paleontology in the Museum of Comparative Zoology (MCZ), noted that, “Researchers and former MCZ curators, like Alfred Sherwood Romer and Farish Jenkins, Jr., were grappling with these same questions a century ago. Now, with new tools and data, we can revisit those ideas and see the story more clearly.”
This study marks the first large-scale evolutionary analysis of mammalian posture using quantitative biomechanics. But that’s just the beginning. The team is now building detailed models of forelimbs in select fossil species to understand how joints and muscles functioned in ancient animals, offering even deeper insights into the evolution of mammalian motion.
As Brocklehurst put it: “Understanding how mammals came to walk upright isn’t just about bones, it’s about uncovering the dynamic history of life on Earth.”
The sprawling-upright transition across mammal evolution, showing changes in posture and limb bone shape.
Credit
Credit: Magdalen Mercado
The sprawling-upright transition; an early forerunner of mammals, Ophiacodon, and a modern mammal, a dog
Credit
Credit: Kenneth Angielczyk
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Funding was provided by the US National Science Foundation (DEB1754459 and DEB1754502), by the Harvard Museum of Comparative Zoology and the Wetmore Colles Fund.
Robert Brocklehurst is currently a postdoctoral research associate in the Department of Biological Sciences at the University of Massachusetts, Lowell. Magdalen Mercado is currently a graduate student in the Committee on Evolutionary Biology at the University of Chicago.
Journal
PLOS Biology
Article Title
Adaptive landscapes unveil the complex evolutionary path from sprawling to upright forelimb function and posture in mammals
Article Publication Date
24-Jun-2025
The evolution from reptile-like to upright posture in mammals was highly dynamic and complex
Ancestral four-footed animals’ locomotion was distinct from modern reptiles
image:
The sprawling-upright transition across mammal evolution, showing changes in posture and limb bone shape.
view moreCredit: Magdalen Mercado, from Brocklehurst RJ, et al., 2025, PLOS Biology, CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)
The transition from sprawling (reptile-like) to more upright (parasagittal) posture and locomotion was a transformative event in mammalian evolution. A study published June 24th in the open-access journal PLOS Biology by Dr. Robert Brocklehurst and Professor Stephanie Pierce at Harvard University, USA and colleagues suggests that parasagittal posture evolved via an indirect, dynamic, and radiating process.
Non-mammalian synapsids (tetrapod vertebrates), the ancestors of extant mammals, underwent major musculoskeletal reorganization, including modifications to the forelimbs. However, when and how these anatomical transformations translated into mammal-like limb posture and upright locomotion is unknown. Prior research based on qualitative observations of the fossil record had hypothesized the evolutionary transition from sprawling to upright as a direct progression with discrete postural stages.
In order to better understand the origins and evolutionary pathway of mammalian posture, researchers analyzed humerus bone from over 200 species of tetrapods. They compared fossilized synapsid humeri to an array of extant salamander, reptile, monotreme and upright therian mammal humeri, examining humeral length, torsion, muscle leverage, bending strength, and radius of gyration. The researchers then computationally modeled the relationship between bone shape, function, and posture to visualize different evolutionary scenarios for how therian mammal upright posture evolved.
The researchers found that ancestral synapsids had a sprawling posture, but their anatomy and movement were distinct from extant sprawlers. The upright posture of modern-day mammals involved a fundamental reorganization of the musculoskeletal system and expansion of differences in forelimb function, suggesting that mammals started walking upright later than previously thought. The study had several limitations, such as uncertainty around phylogenic branch lengths and estimated dates of taxa divergence. Future studies are needed to develop greater specificity and certainty around limb posture in different taxa.
According to the authors, “Previous hypotheses posit the synapsid ‘sprawling-parasagittal’ transition as a series of discrete postural shifts; our study supports the view of synapsid evolution as a series of successive radiations, with major clades exhibiting considerable functional (and postural) variation. Our data on humerus morphology and functional trait evolution suggest that parasagittal posture evolved late, within stem therians.”
“The origin of upright mammalian posture is a key part of their evolutionary story”, adds Dr Robert Brocklehurst, lead author of the study. “However, there’s been a lot of uncertainty as to when and how the upright postures of modern mammals evolved. People have been working on this problem for over 100 years, but historically the focus has been on bone shape in a few exceptionally preserved fossils. We knew if we wanted to understand the big picture of posture evolution in mammals and their ancestors we would need to see as many fossils as possible, and really get to grips with bone function and mechanics, not just shape”.
“By assembling an unprecedented dataset spanning the full breadth of synapsid evolution, we achieved the resolution necessary to disentangle the transformation from our sprawling synapsid ancestors to the upright-limbed mammals of today,” says Professor Stephanie Pierce, senior author of the paper. “Our results show that the hallmark forelimb posture and function of modern mammals emerged surprisingly late in synapsid evolution—this delayed acquisition ultimately laid the foundation for the extraordinary ecological success of mammals.”
“The ancient synapsid forerunners of mammals are often compared with modern reptiles because they had sprawling limbs that look something like those of lizards or crocodiles. However, our study showed that most synapsid limbs functioned differently than those of modern reptiles,” says Dr. Kenneth Angielczyk, a co-author of the study. “They’re not just copies of reptiles, but distinctive animals in their own right that are a little different from anything that’s alive today.”
The authors add, “While the ancestors of mammals did generally get more upright as time went on, there was a lot of variation in each major group of mammalian ancestors. These fossils weren’t stepping-stones, they were animals evolving to explore a wide range of ecologies, niches and habitats.”
In your coverage, please use this URL to provide access to the freely available paper in PLOS Biology: https://plos.io/4khp9mP
Citation: Brocklehurst RJ, Mercado M, Angielczyk KD, Pierce SE (2025) Adaptive landscapes unveil the complex evolutionary path from sprawling to upright forelimb function and posture in mammals. PLoS Biol 23(6): e3003188. https://doi.org/10.1371/journal.pbio.3003188
Author countries: United States
Funding: Funding was provided by the US National Science Foundation (grant DEB1754459 to SEP and grant DEB1754502 to KDA), and by the Harvard Museum of Comparative Zoology (Grant-In-Aid of Undergraduate Research to MM). Funding for publication costs provided by a grant from the Wetmore Colles Fund. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Journal
PLOS Biology
Method of Research
Observational study
Subject of Research
Animals
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