Fossils Reveal the Jaw-Dropping Origins of Human Mobility

The ability of our skeletons to move with both flexibility and stability can be traced back to ancient jawed fish.

Scientists studied modern cartilaginous fish, such as sharks and skates, and compared them to their jawless counterparts, like lampreys. They discovered that synovial joints — lubricated, cavitated structures found in most vertebrates — first appeared in jawed fish ancestors. Fossil evidence further supports this timeline, with the earliest known synovial joint identified in an ancient fish called Bothriolepis.

The Ancient Origins of Our Flexible Joints

The flexible yet sturdy design of our joints can be traced back to our earliest jawed fish ancestors, according to a study published today (February 25th) in the open-access journal PLOS Biology by Neelima Sharma of the University of Chicago and colleagues.

Synovial joints, found in most vertebrates, allow for smooth movement and increased stability by enabling bones or cartilage to glide past each other within a lubricated cavity. These joints exist in both land vertebrates and bony fish, suggesting they originated in their common ancestors. However, the exact point in vertebrate evolution when synovial joints first appeared has remained uncertain.

Tracing Joint Evolution in Early Vertebrates

To explore this, Sharma and her team studied joint anatomy and development in two early vertebrate lineages: one species of jawless fish, the sea lamprey, and two species of cartilaginous fish, the bamboo shark and little skate. Their analysis found that cartilaginous fish had cavitated joints — fluid-filled spaces that enhance mobility — while lampreys did not. The researchers also discovered that cartilaginous fish share key proteins and developmental processes with synovial joints in other vertebrates. Using CT scans, they identified a similar cavitated joint in the fossilized fish Bothriolepis, the oldest known example of a synovial joint.

Fossil Evidence Confirms Ancient Joint Origins

Altogether, these results show that synovial joints are shared across jawed fish, but apparently absent in jawless fish, indicating that these joints first evolved in the ancestors of jawed vertebrates. This study provides critical information for research into the origins of the skeletal architecture of vertebrates, including ourselves. The authors suggest that future steps might include analysis of joint morphology in other fossil fish lineages and further comparisons between joints of jawed and jawless vertebrates to uncover more details about early joint evolution.

The Evolutionary Impact of Mobile Joints

The authors add, “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.”

Reference: “Synovial joints were present in the common ancestor of jawed fish but lacking in jawless fish” by Neelima Sharma, Yara Haridy and Neil Shubin, 25 February 2025, PLOS Biology.
DOI: 10.1371/journal.pbio.3002990

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