Mammals tried different ways to move before walking upright

For over a century, scientists have debated how mammals shifted from sprawling like reptiles to walking upright. This transition marked one of the most significant events in vertebrate evolution. It shaped how mammals, from bats to humans, move through their world.

While earlier views suggested a simple step-by-step progression, new research suggests a very different path.

A study led by Dr. Robert Brocklehurst of the University of Reading shows that upright posture evolved through a complex branching process.

Evolution of upright posture in mammals

“The evolution of mammals has previously been characterized as a series of steps from sprawling, to semi-sprawling, to upright,” said Dr. Brocklehurst. “However, what we discovered was a more nonlinear evolutionary progression throughout mammalian history.”

The researchers analyzed over 200 species of tetrapods. They compared humerus bones across fossilized synapsids and modern animals like salamanders, reptiles, monotremes, and upright mammals.

The team examined traits such as bone length, torsion, bending strength, and muscle leverage.

Rather than showing a clear ladder of progress, the data revealed bursts of change. Different mammalian ancestors experimented with varying forelimb functions. These experiments shaped the eventual rise of upright walking.

Mapping the mechanics of movement

The researchers used a computational model to connect bone shape with function and posture. They created adaptive landscapes that linked bone mechanics to different walking styles – sprawled or upright.

“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,” said Dr. Brocklehurst.

“We expected to see a neat progression – from sprawling pelycosaurs to a bit more upright therapsids, then cynodonts, then fully upright mammals. Instead, we found bursts of innovation.”

The team discovered that while early synapsids did have sprawling limbs, they didn’t function like 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,” said said Dr. Kenneth Angielczyk of the University of Chicago.

Mammals tried many movement styles

This research challenges the idea of direct, staged transitions. Instead of rigid postural categories, the ancestors of mammals explored many forms of limb use.

“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,” the researchers wrote.

“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.”

Professor Stephanie Pierce of Harvard University noted that the path to upright posture wasn’t a straight line. “The ancestors of mammals weren’t steps on a ladder with modern mammals at the top.”

Muscles behind mammalian motion

The upright posture of modern mammals required deep musculoskeletal changes. This included shifts in bone shape and how muscles interacted with the skeleton.

The study showed that parasagittal posture – the fully upright limb arrangement – appeared relatively late in mammal history.

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

“Understanding how mammals came to walk upright isn’t just about bones. It’s about uncovering the dynamic history of life on Earth,” said Dr. Brocklehurst.

The researchers acknowledge some limitations in their study. These include uncertainties in evolutionary timelines and divergence dates. However, the large dataset and mechanical modeling provide a strong framework for future research.

Tracing upright posture in mammals

To compare hundreds of bones from vastly different animals, the team had to innovate. Traditional shape analysis tools failed. So, they retooled an R software package into a “slice-based” landmarking system.

This approach allowed for precise measurement of traits even across species separated by millions of years. Former undergraduate Magdalen Mercado helped assemble the dataset, contributing to the massive scope of the project.

Professor Pierce reflected on the legacy of Harvard researchers who tackled similar questions decades ago. “Now, with new tools and data, we can revisit those ideas and see the story more clearly.”

“This delayed acquisition [of upright posture] ultimately laid the foundation for the extraordinary ecological success of mammals,” said Pierce.

New chapter in mammalian evolution

This study is the first of its kind to use large-scale biomechanics to trace mammalian posture. It reveals that upright walking didn’t come early, but evolved after numerous experiments in movement.

As the team continues modeling limb function in fossil species, they aim to unlock more details about how ancient creatures lived and moved.

“The origin of upright mammalian posture is a key part of their evolutionary story,” said Dr. Brocklehurst.

“People have been working on this problem for over 100 years. We knew we needed to see as many fossils as possible, and really get to grips with bone function and mechanics, not just shape.”

With this broader view, scientists now see evolution not as a tidy path, but as a dynamic landscape full of surprises.

The study is published in the journal PLOS Biology.

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