"Where do elbows and knees come from? Biologists" Topic

… track them back to our boneless, sharklike ancestors

"Ask an older person where painful arthritis strikes and most will point to their joints—knees, hips, and fingers. That's because as people age, those joints lose the cartilage and viscous fluid, known as synovial fluid, that keeps them supple. Sharks and skates have no bones—and no arthritis—but they apparently have the same kind of joints we do. Once thought to exist only in bony vertebrates, these so-called synovial joints actually evolved in the much older ancestor of cartilaginous and bony fish, researchers reported earlier this month in a preprint on bioRxiv…"

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