Humans Got Their Flexible and Sturdy Joints From Fishes, Evidence Found in 400 Million-Year-Old Jawed Fish

Evolution has provided humans with many features that have made their lives more efficient. Experts are constantly on the lookout for the trajectory associated with these features. A team recently figured out certain aspects of the trajectory associated with synovial joints, stated Science Daily. Findings regarding this discovery have been published in PLOS Biology. 

Synovial joints allow human skeletons to be both flexible and sturdy. These joints ensure that body parts have adequate mobility and stability. Such joints facilitate the sliding of bones or cartilage over each other with the aid of a lubricated cavity. Before this study, researchers speculated that these joints were inherited from land vertebrates and bony fish. The story before that, though, remained unclear.

The team studied two members of vertebrate lineages to understand where these joints possibly originated during evolution. They took into consideration sea lampreys (a jawless fish species), bamboo sharks, and little skates (cartilaginous fish species). Researchers examined how joints developed in the anatomy of these subjects. They concluded that cavitated joints were present in the cartilaginous fish species but not in the jawless one. 

The team examined fossils that were 400 million years old, indicating that these joints were present when jawed species started exhibiting articulated movements, possibly for predatory purposes, stated the University of Chicago. "This evidence suggests that these kinds of joints evolved at a time when creatures were becoming much more active predators with jaws and mobile skeletons so they could swim around and eat each other or avoid being eaten," said Neil Shubin, PhD, the Robert R. Bensley Distinguished Service Professor of Organismal Biology and Anatomy at UChicago and senior author of the study.

Experts further claimed that certain developmental processes and proteins noted in the cartilaginous fish species were also detected in other vertebrates. The genetic expression of proteins like TGF-β and Wnt, which was detected in the jawed vertebrates of this study, was also observed in zebrafish, mice, and chickens. The team also noted that skates require muscle contraction to develop properly. "This is such a conserved property that you see in animals which develop in eggs, they start twitching after they develop their muscles," said Neelima Sharma of the University of Chicago, and lead author of the study. "The embryo has to move around for them to be able to develop joints properly, so when you remove muscle activity, that genetic expression disappears."

The team also analyzed a fossil belonging to Bothriolepis, and spotted a cavitated joint, stated Science Daily. This finding made the fossil the oldest known specimen to exhibit synovial joints. The results indicate that these joints are present across all jawed vertebrates. This implies that this feature came from the ancestors of jawed vertebrates. "The origin of mobile joints in our fish ancestors enabled them to move about and feed in new ways. This study shows that the developmental processes that are responsible for these joints arose deep within the fish evolutionary tree," the authors expressed. 

Researchers believe these findings will help experts garner more insights regarding the skeletal system of all vertebrates, including humans. They are hoping to examine the joint morphology present in other fish species. The team will also conduct more comparisons between the anatomy of jawed and jawless vertebrates to figure out early joint evolution.