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A new analysis of the bones and muscles of ancient fish provides new clues about how the shoulder evolved in animals – including us.
The shoulder girdle – the configuration of bones and muscles that support the movement of the arms in humans – is a classic example of an evolutionary ‘novelty’. This is where a new anatomical feature appears with no obvious precursors; where there is no smoking gun whose characteristic has clearly led to another.
The new research, which brings together a range of evolutionary research techniques including fossils, developmental biology and comparative anatomy, suggests a new way of looking at how key anatomical features such as shoulders evolved.
The results of the study, led by Dr. Martin Brazeau of Imperial College London and researchers from the Natural History Museum, were published today in Nature.
One theory about the origin of the shoulder is that it was part of the way fins paired up on either side of the fish’s body, allowing evolution to allow fish greater control over swimming and ultimately fueling movement from water to land. The ‘gill arch’ hypothesis suggests that these fins evolved from the bony ‘loops’ supporting the gills, which also formed the shoulder. However, it has been difficult to gather any evidence for this hypothesis as its features are rarely preserved in fossils.
Another theory about how the fins formed, the ‘fin-fold’ hypothesis, suggests that the precursors of the paired fins instead evolved from a line of muscle on the fish’s flanks. This theory has generated much supporting evidence in the 150 years since both were proposed, but cannot explain how the associated shoulder girdle evolved.
By reanalyzing an ancient fossil fish skull from shortly after the appearance of the pectoral girdle, the team suggests, along with other evidence, that the truth may lie in a modified version of the gill arch hypothesis that reconciles it with the fin fold hypothesis.
The fossil the team looked at is a placoderm of the species Kolymaspis sibirica, which lived about 407 million years ago and was one of the earliest jaw-bearing fish. The fossil has a well-preserved braincase: the hard inner parts of the skull that record impressions and other features of the brain.
Dr. Brazeau realized that despite the poor or absent preservation of the branchial arches in such fossils, evidence for them might be well preserved in the braincase: the cartilaginous or bony ‘box’ that surrounds the brain and supports sensory structures such as eyes and ears. . The braincase showed a peculiar head-shoulder joint, emphasized by the configuration of muscles and blood vessels.
By comparing this feature in the jawfish fossil with the features of the braincase of their ancestors, the jawless fish, he and the team discovered new ways in which the two could be compared. They found that the unusual head-shoulder joint shares similarities with the gill arches in earlier fish, suggesting that these were retained and incorporated into shoulder formation early on.
While most jawless fish have 5 to 20 gill arches, jawed fish almost never have more than five. Combining this with the new evidence from the brain, the team suggests that the sixth branchial arch was incorporated into the shoulder and became a crucial boundary separating the head from the body. Intriguingly, the blood supply to the fins of jawless fish emerges between the sixth and seventh gill arches.
Dr. Brazeau, from Imperial’s Department of Life Sciences, said: ‘The branchial arches appear to have been involved in the early separation of the head and body via the shoulder. But we no longer have gill arches – although the shoulder on them no longer needs to be there today.
“This is consistent with previous studies showing that muscles can remain very stable while the specific bones that support them gradually take over one from the other. Branchial arches may have done their part and were replaced as the shoulder adopted a new configuration.” including supporting things like our necks.”
This finding also means that it doesn’t have to be either/or in terms of how the paired fins evolved. Dr. Brazeau added: “Our study shows the merits of both theories without accepting one or the other in general. Instead, we can rationalize the areas that overlap.”
Dr. Zerina Johnson, researcher at the Natural History Museum, added: “The team will next focus on specimens from the Natural History Museum’s fossil fish collection. This includes jawless fish that have fins but no obvious pectoral girdle.
“We are currently processing many gigabytes of data, and I can hardly wait to see what these important specimens from the collection will add to the story.”
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