407-Million-Year-Old Fossil Fish Reveal the Origin of Shoulders in Vertebrates

The shoulder's evolution in animals, including humans, is illuminated by an analysis of bones and muscles in ancient fish. The shoulder girdle, crucial for arm movement, represents an evolutionary novelty, lacking clear precursors.

The study, led by Dr. Martin Brazeau from Imperial College London and researchers from the Natural History Museum, integrates various evolutionary investigation techniques. The findings offer a fresh perspective on the evolution of significant anatomical features.

Evolution Takes Control in Making Vertebrates Widespread

In the analysis, two predominant hypotheses concerning the origins of the bones connecting vertebrates' arms to their trunks are addressed.

One theory suggests that these connections stem from head tissues, while the other proposes an origin from lateral folds in ancient fish. This novel perspective, founded on a comprehensive investigation of 407-million-year-old fossils, allocates distinct roles to each in the evolution of paired limbs.

Shigeru Kuratani, an evolutionary morphologist at the RIKEN Center for Biosystems Dynamics Research, who was not directly involved in the study, considers this research a potentially significant step toward resolving the longstanding debate.

Approximately 500 million years ago, vertebrates existed as jawless marine creatures with tails and numerous paired gill arches but no paired fins. Fast-forwarding by 100 million years, placoderms, a fish species, inhabited the seas. These placoderms featured jaws and a primitive "shoulder" or pectoral girdle, supporting paired front fins.

Ichthyologists have postulated that the development of jaws originated from the foremost gill arch of a common ancestor. Subsequently, another gill arch evolved into the hyoid, a head bone. In the 1800s, biologists suggested that the pectoral girdle, ultimately leading to the human shoulder, had a different origin, deriving from separate gill arches.

Conversely, some argued that the paired fins and their supporting structures found their origins in lateral folds-elongated flaps of soft tissue running along the fish's body, signifying trunk tissues rather than head tissues.

Although the notion of arms and legs evolving from these folds logically aligns, the intricate nature of the girdle, connected to the head and its musculature, presented a perplexing enigma, as elucidated by Martin Brazeau, a paleontologist at Imperial College London.

Surprising Origins of Shoulders

To understand the origins of shoulders in vertebrates, the team turned their attention to Kolymaspis sibirica, a species discovered in Siberia during the 1950s. This species provided a unique opportunity as it is represented by the only known fossil preserving a portion of its skull in 3D, allowing for a detailed examination of its structure.

Their analysis revealed intriguing evidence of points on the braincase where cartilage could have once attached. These points corresponded significantly with the blood vessels and nerves responsible for serving the pectoral fin.

The researchers hypothesize that the sixth gill arch underwent gradual adaptation over time, ultimately forming a joint that separated the head from the developing shoulder girdle.

Although the precise mechanism of pectoral fin formation remains elusive, the findings suggest that it could involve the fusion of certain aspects of the head and body. This implies that elements of both the fin-fold and gill arch hypotheses might hold validity.

The team anticipates that future investigations involving fossils held in collections and the discovery of new specimens will further enrich our understanding of this evolutionary puzzle.

Dr. Zerina Johanson, a co-author and researcher at the Natural History Museum, emphasized their forthcoming focus on specimens from the museum's fossil fish collection, particularly jawless fish with fins but lacking a distinct shoulder girdle. The team eagerly awaits the data processing and the valuable contributions these specimens may make to this captivating story.

Check out more news and information on Paleontology in Science Times.