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The key to how vertebrates first emerged from the ocean and began to conquer land around 390 million years ago may lie in the arm bone, according to a study.
US and British experts have found that the development of the humerus – the bone that runs from the shoulder to the elbow – made animals better walkers than swimmers.
This development led to the emergence of tetrapods, four-legged animals that, unlike their aquatic ancestors, lived mostly on land.
It triggered the emergence of tetrapods, four-legged animals that mainly live on land. Their ancestors had been limited to water.
This discovery highlights a key process in the history of evolution – the transformation of fins into limbs – which has been poorly understood.
US and British experts have found that the development of the humerus – the bone that runs from the shoulder to the elbow – made animals better walkers than swimmers. This development led to the emergence of tetrapods (pictured) – four-legged animals that, unlike their aquatic ancestors – lived mainly on land
“The ability to walk on land has essentially laid the foundation for all biodiversity and established modern terrestrial ecosystems,” said article author and zoologist Stephanie Pierce of Harvard University.
“It represents an incredibly important period of time in the history of evolution.”
Scientists have been trying to unravel the mysteries behind the vertebrate transition to earth for more than a century.
Early tetrapods such as Acanthostega and Ichthyostega were the first vertebrates to possess fingered limbs, 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 houses key muscles that absorb much of the stress generated by quadrupedal, ie “on all fours” locomotion.
Additionally, bone is found in all tetrapods as well as in the fish they evolved from – and is quite common throughout the fossil record.
That said, the bone represents a kind of time capsule, as it can be examined through the transition from fin to limb, the researchers explained.
“We got into the idea that the humerus should be able to tell us the functional evolution of locomotion as we go from being a fish that swims only around and when we arrive on land and start walking”, he said the author of the article Blake Dickson.
In their study, the team analyzes 3D reconstructions of 40 fossilized humerus, including recently collected specimens dating back more than 350 million years, found by researchers at the University of Cambridge.
The team ran their data through a supercomputer for thousands of hours – a period of nearly four years – to assess how the bone likely changed over time and what impact it may have had on how the creatures moved.
The analysis covered the transition from aquatic fish to terrestrial tetrapods, including an intermediate type with previously unknown locomotor skills.
The researchers found that the emergence of limbs in the latter group coincided with the transition to earth, but that these pioneers were not good at walking.
They found that a first L-shaped humerus (center) derived from fish block bone (left) provided some benefits for moving around on land, but not much. The pioneer creatures still had a long way to go before they developed the ability to use their limbs with ease. Subsequently, the bone transformed into a more robust, elongated and twisted shape (right), leading to more effective gaits that helped fuel new biological diversity and expansion of ecosystems.
As these creatures left the water, their humerus bones changed shape, resulting in new functional traits that proved more beneficial to life on earth.
It made sense, Mr Dickson said. 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 earth.
The team captured these changes and were captured on a morphological map showing how the first tetrapods were related to the inhabitants of water or land.
They found that a first “L-shaped” humerus provided some advantage for moving around on earth, but not much. The pioneer creatures still had a long way to go before they developed the ability to use their limbs with ease.
The analysis involved the transition from aquatic fish (left) to terrestrial tetrapods (right), including an intermediate type (center) with previously unknown locomotor skills. The researchers found that the emergence of limbs in the latter group coincided with the transition to earth, but that these pioneers were not good at walking.
Subsequently, the bone transformed into a more robust, elongated and twisted shape, leading to more effective gaits that helped fuel the new biological diversity and expansion of ecosystems.
This “shows how much information can be obtained from such a small part of an animal skeleton that has been recorded in the fossil record,” said Professor Pierce.
Analysis of this, he added, helped unveil one of the greatest evolutionary transformations that have ever occurred. This is really cutting edge stuff. ‘
The full results of the study were published in the journal Nature.
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