Your hip and knee joints were invented by fish with jaws 400 million years ago
Scientists once assumed fluid-filled joints evolved when animals crawled onto land. The evidence now points much further back, to the first jawed vertebrates of the Silurian and Devonian. Sharks and rays have true synovial joints; lampreys and hagfish, with no jaws, have none. Biting may have made them necessary.
A synovial joint, the most common and most mobile kind in the body, wraps the ends of two bones in a capsule. Its tough outer layer holds the bones together and may contain ligaments, while an inner membrane seals in lubricating fluid. The bone ends are capped with smooth hyaline cartilage that resists sticking, soaks up shock and cuts friction. Many such joints add extras: fibrocartilage pads called menisci, fat pads like the one below the kneecap, tendons, reinforcing ligaments and fluid-filled sacs named bursae around the shoulder and knee.
Curiously, the capsule is richly supplied with nerves yet contains no blood or lymph vessels. Nutrients reach it from the surrounding circulation, slowly by diffusion or much faster by convection, which exercise drives. Joints elsewhere work differently: the fused seams of the skull are fibrous, and the discs between vertebrae are cartilaginous.
The evolutionary story comes from comparing living groups and fossils. Cartilaginous fishes show clear joint cavities, cartilage lined with flattened cells, the same developmental signals seen in land animals, and a need for muscle contraction to hollow out the cavity. Jawless cyclostomes, by contrast, have joints packed with tissue. Among fossils, jawless osteostracans had fin connections riddled with canals that rule out a fluid-filled space, while early armoured placoderms, which had jaws, show matching surfaces separated by cavities. The leading suggestion is that the heavy forces of catching and chewing prey drove the innovation, which later allowed complex skeletons to evolve.
The seven types range from stable, low-mobility designs to ball-shaped joints like the hip and shoulder, which swing forward and back, out and in, and rotate the limb about its own axis, at a higher risk of injury. On X-rays, the gap between bones is a health gauge: normal is at least 2 millimetres in the hip and 3 in the knee, and narrowing signals osteoarthritis or inflammatory damage.
Source: Synovial joint
