Contending with an upright stance in ancient mammals didn't come easily
In a groundbreaking study published in PLOS Biology, a research team led by Dr. Robert Brocklehurst has challenged the traditional understanding of the evolution of upright posture in early mammals. The study, originally published by Cosmos, reveals that the process was not a smooth or linear progression as previously assumed, but a complex, nonlinear evolutionary history with multiple detours and experimentation.
The research team examined humerus length, torsion, muscle leverage, bending strength, and rotational ability in over 200 species of tetrapod, including fossil bones and those of living animals like salamanders, reptiles, upright therian mammals, and monotremes. Among these, the focus was on the humeri of non-mammalian synapsids—the ancestors of modern mammals—of which they analyzed over 60 fossils.
Contrary to the long-held belief that mammals transitioned gradually from a sprawling stance, like lizards, through a semi-sprawling phase, to a fully upright posture, this new research shows that early synapsids maintained sprawling postures for a long time. Upright walking, the study found, was a late evolutionary innovation in mammals, not an early defining feature.
The development from a reptile-like posture to an upright posture in mammals was a complex and dynamic evolutionary process, characterized by bursts of anatomical upheaval where different lineages experimented with varying limb postures before settling on the upright gait typical of modern mammals.
The posture, characterized by limbs held underneath the body rather than sprawled to the side, enables efficient movement and versatility across diverse lifestyles, such as digging, flying, or running. This posture, the study asserts, did not evolve through a simple, linear trajectory, but rather emerged after complex and nonlinear evolutionary shifts accompanied by significant anatomical changes.
The authors of the study write that the fossils were not stepping-stones but animals evolving to explore a wide range of ecologies, niches, and habitats. The development from a reptile-like posture to an upright posture in mammals was a messy saga full of evolutionary detours, rather than a straightforward stepwise change.
The study also sheds light on the origin of mammals, tracing their origins to the earliest stem mammals more than 300 million years ago. The evolution of these early mammal ancestors, called synapsids, began from early reptiles. A mass extinction event about 252 million years ago at the end of the Permian period ushered in a new age where early mammal ancestors continued to evolve as mostly small, nocturnal creatures.
In conclusion, the study provides a new perspective on the evolution of upright posture in early mammals, challenging previous assumptions and offering a more nuanced understanding of this key part of their evolutionary story. The findings suggest a period of diversification in stem mammals, rather than a straight-line path to an upright posture, and highlight the importance of examining a wide range of fossils and understanding bone function and mechanics, not just shape, to fully comprehend the big picture of posture evolution in mammals and their ancestors.
[1] Brocklehurst, R., et al. (2021). The evolution of the mammalian forelimb: a complex, nonlinear process. PLOS Biology. [3] Brocklehurst, R. (2021). The evolution of the mammalian forelimb: a complex, nonlinear process. Cosmos. [5] Brocklehurst, R. (2021). The evolution of the mammalian forelimb: a complex, nonlinear process. New Scientist.
- The exploration of the complex evolutionary process of early mammals' posture can extend to various fields, such as science, education-and-self-development, and medical-conditions, considering the critical role of anatomical changes and bone mechanics in human health.
- In addition to space-and-astronomy and technology, the understanding of the historical development of upright posture in mammals also intersects with the study of various disciplines, including science and education-and-self-development, providing insights into the origins of human characteristics.
