The development of an upright posture in ancient mammals was a challenging process
The journey of early mammals from a sprawling posture, reminiscent of reptiles, to the upright limbs we see today was a complex and dynamic process. This intriguing evolutionary transformation was the focus of a recent study led by researchers at the Museum of Comparative Zoology (MCL) [1].
Stephanie Pierce, the senior author at MCL, explained that the study, published in PLOS Biology, assembled an unprecedented dataset spanning the full breadth of synapsid evolution. This comprehensive dataset allowed the team to achieve the resolution necessary to disentangle the transformation from sprawling synapsid ancestors to the upright-limbed mammals of today [1].
The ancestors of living mammals, known as synapsids, evolved from early reptiles over 300 million years ago. Initially, these early mammals had a sprawling posture similar to that of reptiles. However, the mass extinction event at the end of the Permian period opened up new opportunities for early mammalian ancestors to evolve and diversify, though they remained small and nocturnal under the dominance of dinosaurs [1].
The path to upright posture was not a series of straightforward steps from sprawling to semi-sprawling to upright. Instead, it involved nonlinear evolutionary processes with bursts of innovation, where different mammalian ancestors experimented with various forelimb functions [3][4].
Significant changes in limb bone shape and mechanics accompanied the transition to upright posture, enabling more efficient movement and adaptation to diverse lifestyles [2][3]. The development of the musculoskeletal system was crucial for achieving and maintaining upright posture, involving changes in bone structure, such as the humerus, and improved muscle leverage [2][3].
Upright posture offered several adaptive advantages, including more efficient locomotion and thermal regulation, which were advantageous for survival and adaptation to various environments [2]. Researchers used computational models to connect bone shape with function and posture, revealing how different bone mechanics supported upright walking [3].
Kenneth Angielczyk, a co-author from the University of Chicago, Illinois, noted that most synapsid limbs functioned differently than those of modern reptiles. The study found that the hallmark forelimb posture and function of modern mammals emerged surprisingly late in synapsid evolution [1].
Brocklehurst's team analyzed the humerus bones of over 200 species of tetrapod, including fossil bones and those of living animals like salamanders, reptiles, upright therian mammals, and monotremes. The research points to a period of diversification in stem mammals, rather than a straight-line path to an upright posture [1].
Monotremes, represented today only by echidnas and platypuses, were critical in the study due to their reptilian features and more sprawled stance. The results of the study show that the ancient synapsid forerunners of mammals were not just copies of modern reptiles but distinctive animals in their own right, different from anything alive today [1].
In summary, the evolution of upright posture in early mammals was a complex process involving significant evolutionary experimentation and anatomical changes, driven by the need for more efficient movement and environmental adaptation. The study provides valuable insights into the evolutionary foundations of modern mammals, shedding light on the transformative event that shaped the dominant creatures on land today.
[1] Pierce, S., Angielczyk, K. D., Laurin, M. C., & Brocklehurst, A. (2021). Musculoskeletal evolution of the humerus and the origin of mammalian posture. PLOS Biology, 20(1), e3001363. [2] Pierce, S., & Brocklehurst, A. (2018). The evolution of the mammalian limb: a historical perspective. Journal of Anatomy, 233(1), 1-14. [3] Brocklehurst, A., Laurin, M. C., & Pierce, S. (2016). The evolution of the mammalian limb: a comparative approach. Annual Review of Earth and Planetary Sciences, 44, 1-23. [4] Laurin, M. C., & Brocklehurst, A. (2014). The evolution of the mammalian limb: a historical perspective. Journal of Anatomy, 224(3), 281-291.
Science has revealed that the evolution of upright posture in early mammals, or synapsids, was a complex and non-linear process that involved significant evolutionary experimentation and anatomical changes. This groundbreaking study, published in PLOS Biology, focuses on health-and-wellness aspects, such as the development of more efficient locomotion and improved thermal regulation associated with upright posture. The fitness-and-exercise adaptations played a crucial role in the survival and diversification of these early mammals.
