Human shoulders evolved from the jawbones of ancient fish called placoderms

The shoulder is a crucial anatomical configuration in humans responsible for the movement of our arms. How the shoulder evolved in animals remains a subject of intense debate among evolutionary biologists.

Representing a classic instance of evolutionary ‘novelty’, the origin of this anatomical structure has been elusive due to the absence of clear precursors. Recent research, however, is poised to shift the paradigm of our understanding by bringing in fresh insights from ancient fish.

Theories on how shoulders evolved

Gill-arch hypothesis

Historically, the ‘gill-arch‘ hypothesis on how shoulders evolved proposed that the paired fins of fishes — precursors to limbs in terrestrial animals — evolved from the bony structures supporting their gills.

This transformation was theorized to have given fish enhanced swimming control, which eventually paved the way for the aquatic-to-terrestrial transition.

However, this theory faced challenges due to the rarity of fossil evidence. Features supporting this hypothesis were seldom preserved in ancient specimens, leaving it in contention for many years.

Fin-fold hypothesis

In contrast, the ‘fin-fold’ hypothesis on how shoulders evolved suggests that the paired fins originated from a muscle line on the fish’s flanks. While this theory has garnered substantial support over the past 150 years, it falls short in explaining the evolution of the associated shoulder girdle.

From placoderm fossils to shoulder evolution

A collaborative study by Imperial College London and the Natural History Museum offers a reconciliatory perspective. By reexamining the fossil of a placoderm, specifically Kolymaspis sibirica from roughly 407 million years ago, researchers unearthed crucial details about early shoulder evolution from the shoulder girdle.

Dr. Martin Brazeau‘s keen observation revealed that even in the absence of well-preserved gill arches, vital evidence could be retained within the brain case, which is a protective enclosure for the brain. The brain case exhibited a peculiar head-shoulder joint, informed by a distinctive arrangement of muscles and blood vessels.

When the team juxtaposed this feature from the placoderm with that of earlier jawless fish, they stumbled upon striking similarities. The head-shoulder joint bore a resemblance to the gill arches of ancestral fishes. This discovery suggests that the arches played a pivotal role in the early development and evolution of the shoulder.

The discovery that jawed fishes rarely had more than five gill arches, combined with the new evidence from the brain case, led to an intriguing proposition. The researchers propose that the sixth gill arch was integral to the shoulder’s formation. It acted as a boundary demarcating the head from the body. The vascular network supplying blood to the fins in jawless fishes notably arises between the sixth and seventh gill arches.

Future shoulder evolution research

Dr. Brazeau from Imperial College elucidated that while gill arches were foundational in differentiating the head and body through the shoulder, they may no longer exist in modern anatomy. The study underscores the dynamism of evolution, where muscles can maintain stability even as the bones supporting them undergo transformation.

He also emphasized the possibility of amalgamating insights from both established theories, hinting at the convergence of the gill-arch and fin-fold hypotheses.

Further research is underway, with Dr. Zerina Johnson from the Natural History Museum expressing excitement about the potential revelations from their fossil fish collection. With a treasure trove of data being processed, the scientific community eagerly awaits the ensuing discoveries that promise to shed more light on the evolutionary journey of the shoulder.

In summary, the study by Dr. Brazeau and his team illuminates the intricacies of the evolutionary process. They emphasize the great value gained from interdisciplinary research. By drawing from fossils, developmental biology, and comparative anatomy, researchers are piecing together the puzzle of how one of our body’s vital anatomical features came into existence.

The revelations about shoulder evolution from ancient fish fossils not only enhance our understanding of the past but also provide a foundation for future evolutionary studies.

More about placoderms

Placoderms, known for their distinctive armored plates, hold a significant place in the history of vertebrate evolution.

As mentioned previously, the evolution of animal shoulders may have begun with placoderms. These ancient fishes roamed our planet’s waters during the Devonian period, between 420 and 360 million years ago.

Physical features

Armor-Like Exoskeleton

Placoderms are most famously recognized for their tough, bony exoskeletons. These protective armor plates covered their head and thorax, giving them a formidable appearance. Behind this armor, their bodies were more flexible, with a structure similar to modern fishes.

Jaws and Teeth

One of the features of placoderms was the presence of jaws. They were among the first jawed vertebrates, paving the way for the evolution of diverse feeding strategies.

In addition, the evolution of their jaws may be the reason that humans and animals have shoulders. Unlike modern fishes, however, placoderms did not have true teeth. Instead, they had sharp bony plates that they used to grasp and crush their prey.

Habitat and distribution

Placoderms thrived in a variety of aquatic environments. From freshwater rivers and lakes to salty seas, these fishes demonstrated a wide ecological range. Fossils of placoderms have been unearthed on every continent, showcasing their global distribution during the Devonian period.

Extinction and legacy

While placoderms dominated aquatic ecosystems for millions of years, they faced a sudden and mysterious extinction at the end of the Devonian. The reasons behind their disappearance remain a topic of debate among paleontologists.

However, placoderms have left a lasting legacy. Their emergence as the first jawed vertebrates marked a significant evolutionary milestone. The study of their fossils provides insights into the early evolutionary history of jawed vertebrates, including modern fishes and ultimately, humans.

Placoderms, with their armored exteriors and pioneering jaws, offer a glimpse into the rich tapestry of life during the Devonian period. Although they no longer swim in our waters, these ancient fishes have provided valuable clues about the evolutionary journey of vertebrate life on Earth.

The full study was published in the journal Nature.

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