A new study from the University of Chicago has uncovered the surprising origin of the flexible joints that allow us to move our shoulders, knees, and elbows smoothly.
These joints, called synovial joints, are filled with fluid and help our bones glide against each other without friction.
While scientists knew these joints existed in land animals and some bony fishes, they didn’t know exactly when they first evolved.
Now, thanks to new research published in PLOS Biology, scientists have traced the origin of these joints back nearly 400 million years—to the time when the first jawed fishes began to roam the oceans.
Researchers studied both modern and ancient fish to understand how these joints first appeared. They looked at jawed cartilaginous fishes, such as skates and bamboo sharks, and jawless fishes, like lampreys and hagfish.
They discovered that only the jawed fish had clear synovial joints, while jawless fish had solid joints filled with tissue instead of fluid.
Lead researcher Dr. Neelima Sharma, along with paleontologist Prof. Neil Shubin, found that these jawed fishes had joints in their jaws and pelvis that looked just like modern synovial joints—with cartilage lining and a cavity for fluid.
Sharma was first inspired to explore this after noticing how skates “walk” along the ocean floor using their fins. She thought such movement must require flexible joints—and she was right.
The team also studied how these joints form during development. They discovered that muscle movement is necessary for synovial joints to develop properly.
In one experiment, they paralyzed skate embryos during development. Without muscle activity, the joints didn’t form correctly and were fused instead of being flexible. This showed that movement plays a key role in shaping joints.
To explore how long ago synovial joints evolved, the researchers examined fossils of early jawed fishes, like Bothriolepis canadensis, a placoderm fish that lived about 380 million years ago.
CT scans showed clear joint surfaces and cavities in its skeleton, similar to modern synovial joints. While it’s unclear if fluid was present, the structure closely matched what we see in living animals today.
This research shows that synovial joints likely evolved alongside jaws, as fish began moving more actively to hunt or escape predators. It was a major evolutionary step that led to the amazing variety of jawed animals we see today—including us.
“These joints gave animals both mobility and stability—two things that are crucial for walking, swimming, and surviving,” said Prof. Shubin. “It’s one of nature’s most powerful inventions.”
