Evolution: The upper arm may be the key to how vertebrates rose from the seas 390 million years ago

A study has concluded that the key to how vertebrates first appeared outside the ocean and began to invade Earth about 390 million years ago may lie in the arm bone.

Experts from the United States and the United Kingdom found that the growth of the humerus – the bone that extends from the shoulder to the elbow – made animals walk better than swimmers.

This development gave rise to tetrapods – four-legged animals that live mostly on land, unlike their aquatic ancestors.

It gave birth to tetrapods – quadrupedal animals that live mostly on land. Their ancestors were trapped in water.

This finding highlights a key process in the history of evolution – the transformation of fins into limbs – that has not been well understood.

“Being able to roam the earth is essentially the pathway for all biodiversity and the creation of modern terrestrial ecosystems,” said paper author and zoologist Stephanie Pearce of Harvard University.

“It represents a very important time period in evolutionary history.”

Scientists are trying to unravel the mysteries behind the vertebrate’s move to Earth for more than a century.

Early tetrapods such as Acanthostega and Ichthyostega were the first vertebrates to have fingertip limbs, rather than fish-like fins.

Their offspring include extinct and living amphibians, reptiles and mammals – including humans.

The humerus is invaluable for movement because it hosts the major muscles that absorb a lot of the pressure from the quadriceps movement – that is, the quadriceps movement.

Moreover, bones are found in all tetrapods as well as the fish that evolved from them – and are very common throughout the fossil record.

In light of this, the bone represents a kind of time capsule, as it can be examined by moving from the fins to the extremities, the researchers explain.

Author of the paper, Blake Dickson, said: “ We got into the idea that the humerus should be able to tell us about the functional development of movement as you go from a fish that just swims as you reach the ground and start walking.

In their study, the team analyzed 3D reconstructions of 40 fossilized humerus bones – including recently collected specimens, dating back more than 350 million years, that were found by researchers from 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 bones were likely to change over time and what effect that might have had on how the creatures moved.

The analysis covered the transition from aquatic fish to terrestrial tetrapods – including an intermediate species with previously unknown motility capabilities.

The researchers found that the emergence of limbs in this latter group coincided with the transition to Earth – but these pioneers were not good at walking.

They found that the early L-shaped (middle) humerus derived from a fractured fish bone (left) provided some benefits for moving on the ground – 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 morphed into a stronger, elongated, and twisted shape (right) – resulting in a more active gait that helped nourish the new biodiversity and expand ecosystems

As these organisms left the water, the shape of their humerus bones changed – resulting in new functional features that proved most beneficial to life on Earth.

Dickson said it makes sense. He added, “You cannot be good at everything. You have to give up something to go from fish to being four legged on the ground.”

The team captured these changes on a morphological map showing how early tetrapods stood in relation to water or Earth’s population.

And they found that the L-shaped humerus offered some benefits for moving on the ground – 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 fish (left) to terrestrial tetrapods (right) – including an intermediate (middle) species with previously unknown motility abilities. The researchers found that the limb emergence in this latter group coincided with the transition to land – but these pioneers were not very good at walking.

Later on, the bone morphed into a stronger, elongated, and twisted shape – resulting in a more active gait that helped fuel new biodiversity and expand ecosystems.

Professor Pearce said this “shows how much information you can get from such a small part of an animal’s skeleton that has been recorded in the fossil record.”

She added that an analysis like this has helped “unravel the biggest evolutionary shifts that have ever happened. These are really advanced things.”

The full results of the study have been published in the journal nature.