Mammal evolution: The long journey from sprawling to upright posture

Humans stand tall and walk with a sense of achievement, but this ability took millions of years to develop. Our ancient ancestors crawled like reptiles before evolving into our upright posture. What caused this posture evolution?

Dr. Peter Bishop and Professor Stephanie Pierce from Harvard are using computer simulations and fossils to share their findings.

Evolution to upright posture

It would be a simple and straightforward tale if our ancestors just one day decided to stand up straight and never look back. But evolution, the researchers discovered, is far from a linear journey.

After scrutinizing the biomechanics of various modern species and eight exemplary fossil species spanning 300 million years, they found dynamics that are more thrilling than a sci-fi movie.

The journey to modern posture

In order to make sense of the evolutionary changes in posture, our researchers took a closer look at some modern analogs, including the sprawled tegu lizard, semi-upright alligator, and upright greyhound.

In their own words, understanding how current creatures move and stand offers a fascinating glimpse into how posture evolved over time.

Moving from the present to the past, the researchers cast their eyes on eight fossil species from four continents, each representing a different epoch.

From the tiny proto-mammal Megazostrodon to the mammoth Ophiacodon, they used advanced digital biomechanical models to unravel the secrets of locomotion.

Locomotion of species

“By first studying these modern species, we greatly improved our understanding of how an animal’s anatomy relates to the way it stands and moves,” said Bishop.

“We could then put it into an evolutionary context of how posture and gait actually changed from early synapsids through to modern mammals.”

Dr. Bishop noted that the amount of force that a limb can apply to the ground is a critical determinant of locomotor performance in animals.

“If you cannot produce sufficient force in a given direction when it’s needed, you won’t be able to run as fast, turn as quickly, or worse still, you could well fall over.”

To better understand the factors behind mammals posture evolution, the researchers utilized a three-dimensional “feasible force space” that encapsulates the overall performance of a limb.

Transition to upright posture

Armed with this biomechanical data, Dr. Bishop and Professor Pierce from Harvard’s Department of Organismic and Evolutionary Biology discovered that the evolutionary transition to an upright posture was a rollercoaster ride.

Instead of a simple progression from sprawling to upright, they found that locomotor performance peaked and dipped over millions of years.

Some ancient species showed flexibility. They were able to switch between more sprawled or upright postures, like today’s versatile alligators and crocodiles.

These findings help to clarify some peculiarities seen in fossils. For example, they may explain why many early mammal ancestors had asymmetric hands, feet, and limb joints, traits more associated with sprawling postures.

The experts noted that, unlike modern placentals and marsupials found lying on their side, early mammal ancestors were often found in a splayed pose.

Evolution in reverse

“The picture is emerging that the full complement of quintessentially therian traits was assembled over a complex and prolonged period, with the full suite attained relatively late in synapsid history,” said Professor Pierce.

Furthermore, the study hints at evolution’s susceptibility to chance events and mass extinctions.

For example, the Permian-Triassic mass extinction – a catastrophe that eliminated 90% of life – seemingly influenced the evolutionary path of synapsids and forced a postural shift back towards sprawled poses.

Tracing the footprints of ancient creatures

The research examines how the footprints of ancient creatures provide compelling evidence of posture evolution.

Footprint fossils, with their preserved stride lengths and toe arrangements, act as historical snapshots of prehistoric locomotion.

By analyzing these prints, Dr. Bishop and Professor Pierce could draw conclusions about the gait and stance of extinct species. They discovered that some primordial footprints resembled those of both semi-upright and upright creatures, hinting at a transitional phase.

This intersection between skeletal analysis and ichnology – the study of trace fossils – offers a richer, more nuanced view of the evolutionary journey to upright posture, revealing how our forebears literally left their mark on the planet.

Future studies of posture evolution

Understanding the nuances of postural evolution contributes significantly to modern scientific and medical research.

Insights gained from these evolutionary studies can inform the development of advanced prosthetics and robotics, as the principles of biomechanical efficiency and adaptability seen in nature can inspire innovative design solutions.

Furthermore, exploring the evolutionary trajectory of posture provides critical lessons concerning the adaptability of life in the face of climate change and environmental upheaval.

As we confront modern challenges, these studies serve as a reminder of the resilience embedded in life’s history, offering guidance on how species might continue to adapt in an ever-changing world.

New perspective on posture evolution

Unraveling the intricacies of how mammals came to stand upright has been a massive challenge. Thanks to advances in computing power and digital modeling, scientists like Dr. Bishop and Professor Pierce are offering fresh insights into this ancient mystery.

As the researchers assert, the evolution of posture in mammals was far from a linear journey.

Their findings hint at a complex and fascinating history of evolutionary adaptation – and remind us just why we humans are quite extraordinary creatures.

The study is published in the journal Science Advances.

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