Scientists Might Finally Know Where Shoulders Come From

We spend so much time looking at our own bodies that it's easy to forget the impressive and complex evolutionary engineering that went into making them. Every joint, bone and muscle comes together to produce an extensive catalog of compound movements. But exactly where they all came from is still something of a mystery.

Roughly 500 million years ago, our ancestors were fish-like marine creatures with no jaws or fins. And yet, in the years since, we have evolved a complex array of appendages that allow us to walk on land, climb trees, drive cars and everything else we can do that fish can't. One of the biggest questions in this evolutionary journey has been how our fishy forefathers were able to grow shoulders.

For decades, researchers have debated this question: did our paired limbs grow from the tissues at the base of the head or the muscular ridges along the fish-like creature's body? But in a recent study, published in the journal Nature, researchers have taken a major step towards answering this long-standing question and reconciling these two hypotheses.

To study these ancient origins, the team from Imperial College London, the London Natural History Museum, St. Petersburg State University in Russia and the University of Michigan analyzed the fossilized brain case of a 407-million-year-old fish from Siberia called Kolymaspis sibirica, one of the earliest jaw-bearing species of fish. The researchers were particularly excited by one pair of gill arches at the back of the fish's head, bony structures that support the gills in fish.

The sixth gill arch in the Kolymaspis sibirica fossil showed signs of gradually transitioning from its role in supporting the animal's breathing to acting as a hinge at the back of the head. This hinge would have eventually developed into our modern-day shoulder.

"The gill arches seem to have been involved in the early separation of the head and body via the shoulder," lead author Martin Brazeau said in a statement.

While jawless fish can have anywhere between five and 20 gill arches, jawed fish rarely have more than five, supporting the theory that the sixth gill arch became incorporated into what is now the shoulder joint.

In other words, both hypotheses might be right: the sixth gill arch at the base of the head may have fused with the fish's body, becoming incorporated into the shoulder.

"Our study shows how there is merit to both theories without accepting one or the other wholesale," Brazeau said. "Instead, we can rationalize the areas that overlap."

The team will continue to analyze fossils in the museum's collection in the hopes that more specimens will provide new insights into the evolution of our own anatomy.