Up close with a silky shark. (CREDIT: Pelayo Salinas de León)

CHICAGO — Every time you bend your knee, rotate your shoulder, or nod your head, you’re using a remarkable piece of biological engineering called a synovial joint. These specialized connections between bones let us move smoothly while still supporting our weight. But where did these sophisticated structures come from? When did they first appear in our evolutionary history?

A new study published in PLOS Biology has the answer, and it takes us much further back in time than previously thought. Researchers from the University of Chicago have discovered that these specialized joints first appeared in the common ancestor of all jawed vertebrates but were absent in jawless fish. This pushes the origin of our flexible joints back millions of years.

“The origin of mobile joints in our fish ancestors enabled them to move about and feed in new ways,” study authors write, in a statement. “This study shows that the developmental processes that are responsible for these joints arose deep within the fish evolutionary tree.”

What are synovial joints and how do they work?

What makes a joint “synovial” is straightforward. There’s a fluid-filled space between the connecting bones, shaped surfaces that fit together, and lubrication to keep everything moving smoothly. Think of your knee or elbow; these are classic synovial joints that allow for free movement while maintaining stability.

The research team studied joint structure and development across several key animal groups. They analyzed little skates and bamboo sharks (cartilaginous fish), sea lampreys and hagfish (jawless fish), and even examined fossils of ancient armored fish. Using advanced scanning techniques, they were able to look inside these joints without destroying the specimens.

Sharks, rays, and skates had joints that looked remarkably similar to our own, with fluid-filled cavities, matching surfaces, and specialized cells that produce lubricating substances. When they manipulated a stained skate specimen, they could see the bones sliding against each other, just like in human joints.

Jawless vs. jawed fish

These synovial joints provide cartilaginous fish with remarkable mobility. Anyone who has watched sharks or rays swim can appreciate their fluid, precise movements. Skates, for example, can elegantly maneuver their wing-like pectoral fins for both powerful swimming and delicate adjustments. Their jaws also have impressive mobility, allowing for varied feeding techniques.

Lampreys and hagfish, on the other hand, showed no signs of these specialized joints. Their cartilage pieces were either continuously connected or joined by simple tissue bridges without the hallmarks of true synovial joints. This more primitive arrangement limits their movement capabilities. Lampreys, for instance, swim with undulating whole-body movements rather than the more nuanced fin-based propulsion seen in jawed fish.

Since lampreys and hagfish represent the most primitive living vertebrates, their joint structure likely reflects the ancestral condition of all vertebrates. The presence of synovial joints in all jawed vertebrates, from sharks to humans, suggests this innovation arose in their common ancestor after the evolutionary split from jawless fish.

How joints develop

The researchers dug deeper, looking at how these joints develop in embryos. They found that skate joints form through a process remarkably similar to what happens in human development. Starting as a continuous piece of cartilage, a specific area called the “interzone” eventually hollows out to create the joint cavity. The team even identified the same key molecular signals guiding this process in skates as those known from human joint development.

For decades, scientists have studied joint formation in mice, chickens, and other laboratory animals. Finding the same fundamental process in a cartilaginous fish suggests that the basic molecular recipe for building synovial joints has remained largely unchanged for hundreds of millions of years.

The proteins involved in this process serve multiple functions. Growth differentiation factor-5 (Gdf5) helps establish the interzone where the joint will form. β-catenin, part of the Wnt signaling pathway, plays a role in determining cell fate in the developing joint. Both were found in developing skate joints, just as they are in mammalian joints. This molecular conservation across such evolutionarily distant animals reinforces the idea that synovial joints evolved just once in vertebrate history.

In a clever experiment, the researchers paralyzed skate embryos using a mild anesthetic to prevent muscle movement during development. Without the physical forces from muscle contractions, the joints failed to develop properly and remained fused. This shows that movement itself helps shape our joints during development, a principle that holds true from fish to humans.

The interplay between mechanical forces and molecular signals is not a new feature of land animals, but rather a fundamental aspect of joint development that predates the origin of tetrapods by millions of years. This integrated system of mechanical feedback was already in place in early jawed vertebrates.

The oldest known synovial joints

The team also examined fossils from an ancient armored fish called Bothriolepis, which lived about 380 million years ago. Using CT scans, they found evidence of what appears to be the oldest known synovial joint. This suggests that the capacity for sophisticated joint movement first appeared with the origin of jawed vertebrates, a key innovation that changed how animals could interact with their environment.

Bothriolepis belonged to a group called antiarchs, one of many extinct lineages of armored fish that dominated aquatic ecosystems in the Devonian period (359-419 million years ago). Finding synovial joint morphology in this ancient fish provides a minimum age for this innovation and confirms that it was present in some of the earliest jawed vertebrates.

Why synovial joints matter for evolution

The authors say that we owe much of our mobility and agility to synovial joints. These joints allow for an extensive range of motion and heightened stability compared to simpler joints that just bend.

This evolutionary innovation likely contributed to the remarkable success of jawed vertebrates, which today make up 99% of all vertebrate species. The ability to move more precisely and powerfully opened up new ways of feeding, escaping predators, and interacting with the environment.

For us humans, these findings create a direct line of connection to our distant aquatic ancestors. The same basic joint design that allows a shark to maneuver through coral reefs enables us to throw a ball, play piano, or swing a golf club. Who would hae thought our most fluid movements rely on engineering principles established hundreds of millions of years ago in the sea?