Something fishy We probably inherited our joints from a fish Cartilaginous fish form the same style of joint that we do, while jawless fish don't. Elizabeth Rayne Mar 21, 2025 10:31 am | 1 Credit: Rizel_S Credit: Rizel_S Story textSizeSmallStandardLargeWidth *StandardWideLinksStandardOrange* Subscribers only Learn moreWhat do we have in common with fish, besides being vertebrates? The types of joints we (and most vertebrates) share most likely originated from the same common ancestor. But its not a feature that we share with all vertebrates.Humans, other land vertebrates, and jawed fish have synovial joints. The lubricated cavity within these joints makes them more mobile and stable because it allows for bones or cartilage to slide against each other without friction, which facilitates movement.The origin of these joints was uncertain. Now, biologist Neelima Sharma of the University of Chicago and her colleagues have taken a look at which fish form this type of joint. Synovial joints are known to be present in jawed but not jawless fish. This left the question of whether they are just a feature of bony skeletons in general or if they are also found in fish with cartilaginous skeletons, such as sharks and skates (there are no land animals with cartilaginous skeletons)As Sharma and her team found, cartilaginous fish with jaws, such as the skate embryos they studied, do develop these joints, while jawless fish, such as lampreys and hagfish, lack them.So what could this mean? If jawed fish have synovial joints in common with all jawed vertebrates, including us, it must have evolved in our shared ancestor.Something fishy in our pastWhile the common ancestor of vertebrates with synovial joints is still a mystery, the oldest specimen with evidence of these joints is Bothriolepis canadensis, a fish that lived about 387 to 360 million years ago during the Middle to Late Devonian period.When using CT scanning to study a Bothriolepis fossil, Sharma observed a joint cavity between the shoulder and pectoral fin. Whether the cavity was filled with synovial fluid or cartilage is impossible to tell, but either way, she thinks it appears to have functioned like a synovial joint would. Fossils of early jawless fish, in contrast, lack any signs of synovial joints.Fossils from the extinct clades along the [jawed fish] stem suggest that joints with reciprocally articulating surfaces arose in the dermal skeleton of the common ancestor of all jawed vertebrates, Sharma and her team said in a study recently published in PLOS Biology. Synovial joints in cartilaginous tissue were a subsequent gnathostome innovation. This suggests that the earliest joints arose in dermal bone, which forms beneath the skin into a plate-like structure like that found in skull bones. But it appears that synovial joints arose sometime after that, which raises the possibility that they appeared with the evolution of bones.While cartilage and bone are both connective tissue, the extracellular matrix these cells are embedded in is much thinner and softer in cartilage, while in bone, that matrix is calcified and hard. Demonstrating that cartilaginous jawed fish also have synovial joints opens up the questions of whether theirand ourcommon ancestor also had them.The way you moveTo see if cartilaginous fish share our synovial joints, Sharma studied little skates (Leucoraja erinacea) and compared their joint morphology to that of sea lampreys (Petromyozon marinus)and hagfish (Myxine glutinosa), two types of jawless fish. She found the skeletons of both lampreys and hagfish are made of cartilage segments connected by muscle, connective tissue, or more cartilage. There were no cavities in between.The skeleton of an adult little skate showed a jaw that slides much like a synovial joint and that its pectoral fin and pelvic joints are also built like synovial joints, with lubricated cavities between the bones that help them slide against each other.Embryos of little skates were then studied to see when cavitation actually happened. Their bones do not undergo the cavitation necessary for synovial joints until later stages of development. At that point, joints in the jaw and pelvis are the first to cavitate. This process is similar in species with bony skeletons.Right before their joints cavitate, little skate embryos start producing the protein aggrecan. This is crucial to the function of joint cartilage because of its hydrated gel structure, which gives cartilage more strength. Aggrecan is first made throughout the entire skeleton and then concentrates in the joints. There are several other proteins needed for joint formation in land vertebrates (notably, members of the TGF-beta family of signaling molecules), and these proteins were also active in the jaw, pelvis, and pectoral fin joints of little skate embryos as they developed.Synovial joints also cannot develop without muscle activity, which is crucial in the development of joints in chicken and mouse embryosand the synovial joints of little skates actually develop similarly. Some of the little skate embryos were purposely paralyzed right before their joints would begin to cavitate. While embryos that were not paralyzed continued with the cavitation process, cavitated joints did not form in those that were paralyzed.We havent yet unearthed a fossil of the common ancestor of all vertebrates with synovial joints. However, Sharmas team was able to demonstrate that they develop in cartilaginous jawed fish just as they do in bony fish, but theyre not found in jawless fish, meaning that this mysterious ancestor had to have also been a jawed vertebrate.Our results are consistent with such functional studies showing that [jawed cartilaginous fish], like tetrapods, rely on synovial joints for exhibiting considerable jaw and fin movements, the researchers said in the same study. But with the exception of the rest of the vertebrates, our results do not support their presence in [jawless fish].We dont look like fish, move like fish, breathe underwater like fish, or (this is probably for the best) smell like fish, but somehow, even if they are used for different types of motions, our joints are structured the same way as those of fish. Evolution has a sense of humor.PLoS Biology, 2025. DOI: 10.1371/journal.pbio.3002990Elizabeth Rayne Elizabeth Rayne is a creature who writes. Her work has appeared on SYFY WIRE, Space.com, Live Science, Grunge, Den of Geek, and Forbidden Futures. She lurks right outside New York City with her parrot, Lestat. When not writing, she is either shapeshifting, drawing, or cosplaying as a character nobody has ever heard of. Follow her on Threads and Instagram @quothravenrayne. 1 Comments