Immunostaining depicts collagen-2 (cyan) and nucleus (magenta) at the articulation in the little skate embryo’s pelvic joint. (Credit: Neelima Sharma, University of Chicago)
A new study published in PLOS Biology uncovers the evolutionary roots of synovial joints, the key structures that make our skeletons both flexible and strong. According to research led by Neelima Sharma of the University of Chicago and colleagues, these efficient joints first appeared in ancient jawed fish, laying the foundation for mobility in modern vertebrates.
Synovial joints, which feature a lubricated cavity allowing bones or cartilage to glide smoothly, are crucial for stability and movement. They are found in both land vertebrates and bony fish, indicating a shared evolutionary origin. However, the precise moment these joints emerged in early vertebrate history remains a subject of ongoing scientific exploration.
This discovery sheds new light on how skeletal architecture evolved to enhance movement and durability, offering valuable insights into the evolutionary adaptations that shaped modern vertebrates, including humans.
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In this study, Sharma and colleagues examined the anatomy and development of joints in members of two early-branching vertebrate lineages: one species of jawless fish, sea lampreys, and two species of cartilaginous fish, bamboo sharks and little skates. Analysis revealed cavitated joints in the cartilaginous fish, but not in lampreys. Additionally, the cartilaginous fish exhibited certain proteins and developmental processes that are shared with synovial joints of other vertebrates. Furthermore, the researchers employed CT-scans to identify a similar cavitated joint in the fossil fish Bothriolepis, the most ancient known synovial joint.
Collectively, the data reveal that synovial joints are a conserved feature among jawed fishes, but are conspicuously absent in jawless lineages. This pattern implies the evolutionary emergence of synovial joints within the ancestral lineage of jawed vertebrates. Consequently, this research contributes fundamental knowledge to the study of vertebrate skeletal architecture origins. Future investigations, including the examination of joint morphology in additional fossil fish taxa and detailed comparative analyses of joint structures between jawed and jawless vertebrates, are recommended to elucidate the finer details of early joint evolution.
“The origin of mobile joints in our fish ancestors enabled them to move about and feed in new ways,” said the authors. “This study shows that the developmental processes that are responsible for these joints arose deep within the fish evolutionary tree.”
