science

Evolution: Upper arm may hold the key to how vertebrates rose from the seas 390 million years ago


The key to how vertebrates first rose up out of the ocean and began to conquer the land some 390 million years ago may lie in the upper arm bone, a study has found. 

US and UK experts found that the development of the humerus — the bone that runs from the shoulder to the elbow — made the animals better walkers than swimmers.

This developed led to the emergence of the tetrapods — four-legged animals that, unlike their aquatic forebears — mostly lived on the land.

It triggered the emergence of tetrapods — four-legged animals that mostly live on land. Their ancestors had been confined to water.

This finding shines a light on a key process in the history of evolution — the transformation of fins into limbs — which has been poorly understood.

US and UK experts found that the development of the humerus — the bone that runs from the shoulder to the elbow — made the animals better walkers than swimmers. This developed led to the emergence of the tetrapods (pictured) — four-legged animals that, unlike their aquatic forebears — mostly lived on the land

US and UK experts found that the development of the humerus — the bone that runs from the shoulder to the elbow — made the animals better walkers than swimmers. This developed led to the emergence of the tetrapods (pictured) — four-legged animals that, unlike their aquatic forebears — mostly lived on the land

‘Being able to walk around on land essentially set the stage for all biodiversity and established modern terrestrial ecosystems,’ said paper author and zoologist Stephanie Pierce of Harvard University.

‘It represents an incredibly important period of time in evolutionary history.’

Scientists have been trying to unravel the mysteries underlying the transition of vertebrates onto land for more than a century.

Early tetrapods like Acanthostega and Ichthyostega were the first vertebrates to possess limbs with digits, instead of fish-like fins.

Their descendants include extinct and living amphibians, reptiles and mammals — including us humans.

The humerus is invaluable for movement because it hosts key muscles that absorb much of the stress generated by quadrupedal — that is, ‘four-legged’ — locomotion.

Moreover, the bone is found in all tetrapods as well as the fishes that they evolved from — and is pretty common throughout the fossil record.

Given this, the bone represents a sort-of time capsule, since it can be examined across the fin-to-limb transition, the researchers explained.

‘We went in with the idea the humerus should be able to tell us about the functional evolution of locomotion as you go from being a fish that’s just swimming around and as you come onto land and start walking,’ said paper author Blake Dickson.

In their study, the team analyses 3D reconstructions of 40 fossilised humeri — including recently-collected specimens, dating back more than 350 million years, found be researchers from the University of Cambridge.

The team ran their data through a supercomputer for thousands of hours — a period of almost four years — to assess how the bone likely changed over time and what impact such may have had on how the creatures moved.

The analysis covered the transition from aquatic fishes to terrestrial tetrapods — including an intermediate type with previously unknown locomotor capabilities.

The researchers found that the emergence of limbs in this latter group coincided with the transition onto land — but that these pioneers were not great at walking.

They found that an early 'L-shaped' humerus (middle) derived from the blocky bone of fish (left) provided some benefit for moving on land — but not much. The pioneering creatures had a long way to go before they developed the ability to use their limbs with ease. Later, the bone transformed into a more robust, elongated and twisted form (right) — leading to more effective gaits that helped to fuel new biological diversity and the expansion of ecosystems

They found that an early ‘L-shaped’ humerus (middle) derived from the blocky bone of fish (left) provided some benefit for moving on land — but not much. The pioneering creatures had a long way to go before they developed the ability to use their limbs with ease. Later, the bone transformed into a more robust, elongated and twisted form (right) — leading to more effective gaits that helped to fuel new biological diversity and the expansion of ecosystems

As these creatures left the water, their humerus bones changed shape — resulting in new functional traits that proved more advantageous for life on land.

That made sense, said Mr Dickson. He added: ‘You can’t be good at everything. You have to give up something to go from being a fish to being a tetrapod on land.’

The team captured these changes were captured on a morphological map showing how early tetrapods stood in relation to water- or land-dwellers.

They found that an early ‘L-shaped’ humerus provided some benefit for moving on land — but not much. The pioneering creatures had a long way to go before they developed the ability to use their limbs with ease. 

The analysis covered the transition from aquatic fishes (left) to terrestrial tetrapods (right) — including an intermediate type (middle) with previously unknown locomotor capabilities. The researchers found that the emergence of limbs in this latter group coincided with the transition onto land — but that these pioneers were not great at walking

The analysis covered the transition from aquatic fishes (left) to terrestrial tetrapods (right) — including an intermediate type (middle) with previously unknown locomotor capabilities. The researchers found that the emergence of limbs in this latter group coincided with the transition onto land — but that these pioneers were not great at walking

Later, the bone transformed into a more robust, elongated and twisted form — leading to more effective gaits that helped to fuel new biological diversity and the expansion of ecosystems.

This ‘demonstrates how much information you can get from such a small part of an animal’s skeleton that’s been recorded in the fossil record,’ said Professor Pierce.

Analysis of such, she added, has helped ‘unravel one of the biggest evolutionary transformations that has ever occurred. This is really cutting-edge stuff.’

The full findings of the study were published in the journal Nature.



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