Create a list of articles to read later. You will be able to access your list from any article in Discover.
You don't have any saved articles.
400-million-year-old fish fossil reveals how we got our arms
A pair of gills may have allowed vertebrates to go head and shoulders above other animals.
While our shoulders might support our arms today, new research suggests that they may have originally helped our aquatic ancestors to breathe.
Where did our limbs come from? It’s a question that has puzzled scientists for over 150 years.
While arms and legs have been vital for the success of humans and our ancestors, it’s been hard to find evidence of where and when they first appeared. Now, new scans of a 407-million-year-old fish known as Kolymaspis sibirica suggest that their origins are ultimately linked to the development of the head.
The fossil shows signs that one pair of gill arches, structures which support the gills in fish, gradually transitioned from helping fish to breathe into a ‘hinge’ at the back of the head. Over millions of years, this structure eventually became the shoulder. As the head and body separated, further adaptation would have allowed parts of the skeleton to become the basis of early fins.
Dr Martin Brazeau, the study’s lead author, says, ‘The gill arches seem to have been involved in the early separation of the head and body via the shoulder. But we no longer have gill arches – though the shoulder was templated on them, they don’t need to still be around today.’
‘This is consistent with some earlier studies that showed muscles can remain highly stable, while the specific bones that support them gradually take over one from the other.’
The findings of the study were published in the journal Nature.
Lampreys are part of an ancient group of vertebrates which evolved before jaws had developed. Image © Gena Melendrez/Shutterstock.
Why did vertebrates become so widespread?
Wind back the clock 500 million years, and our vertebrate ancestors would have been very different from how we are now. They were primarily limbless, and perhaps similar to hagfish and lampreys. These are two groups of eel-like jawless fishes that split off from the rest of the vertebrates very early in their evolution.
These early vertebrates would ultimately give rise to a variety of animals including birds, mammals, reptiles and fish.
Key to this success was a variety of adaptations which allowed vertebrates to take advantage of their environment. The evolution of the jaw, for instance, meant that these animals could eat a wide range of different foods, allowing them to survive in a range of environments..
Paired appendages were also a key part of this early evolution. Nearly all living vertebrates have at least one pair of fins, wings, arms or legs.
In the oceans, these allowed early vertebrates to better control their swimming, and take advantage of underwater currents. They also helped to ventilate the gills, allowing the animals to breathe more efficiently.
Eventually, these paired fins also allowed the vertebrates to take their first steps onto land around 390 million years ago before evolving into arms and legs. Since then, limbs have allowed animals to climb trees, take to the skies, and manipulate food and tools.
Osteostracans like Cephalaspis lyelli have a pair of front fins, but don't have back fins - posing an issue for the fin-fold hypothesis. Image © The Trustees of the Natural History Museum, London (All rights reserved).
Taking up arms
Despite how common these appendages are in nature, it’s surprisingly difficult to know where they came from. Part of the issue is that early vertebrates tended to have internal skeletons made of cartilage, rather than bone, which doesn't fossilise very well.
From what has been found, scientists have come up with two main theories about where these appendages came from.
The first is known as the fin-fold hypothesis, where muscular ridges running along either side of the body split into two during the early evolution of vertebrates. The front part became the pectoral fins (which would go on to form arms), while the rear limbs developed from the back half (which would become legs).
This hypothesis emerged as the front-runner in the race to explain where limbs came from, as these ridges have been found in a variety of early jawed fishes. Studies of the genetics and development of modern fish also seemed to support this idea.
However, the fin-fold hypothesis didn’t fit all the evidence. One issue was that front fins appear earlier in the fossil record than back fins, when the hypothesis predicts they should have evolved at about the same time.
This isn’t an issue with the other theory, known as the gill arch hypothesis. In this scenario, the pectoral girdle (in humans, the shoulder blade and collarbone) formed out of the gill arch. This would have freed another supporting structure known as the gill ray to become the basis of the first fins.
While this competing hypothesis could explain more of the available evidence, there was one main problem. Gill arches are made of cartilage, so they are extremely rare as fossils.
To help solve this problem, the researchers turned to a group of animals known as placoderms, which were a variety of different heavily armoured, predatory fishes. While their gill arches haven’t survived, their braincases do, meaning that researchers can see where the arches and associated soft tissue once connected to their head.
While the gill arches no longer survive, Kolymaspis sibirica preserves the points they attached to the braincase. Image © M. Brazeau & M. Castiello.
Getting a head
To investigate whether these connections could finally give researchers the insight they were looking for, the team examined Kolymaspis sibirica, a species found in Siberia in the 1950s. The only known fossil preserves part of the skull of this placoderm in 3D, allowing the team to look at its structure.
They found evidence of points on the braincase where cartilage could have attached. Importantly, these line up with the blood vessels and nerves that serve the pectoral fin. The researchers think that the sixth gill arch gradually adapted over time to become a joint that separated the head from the new shoulder girdle.
While the researchers aren’t still exactly certain how the pectoral fins formed, they think that parts of the head and body fused together. This might mean that parts of the fin-fold and gill arch hypotheses are both right.
Moving forward, the team hope that new investigations of fossils held in collections and the discovery of new specimens could give them the leg-up on evolution they’ve been waiting for.
Dr Zerina Johanson, a co-author and research at the Natural History Museum, adds, ‘The team will next focus on specimens from the Natural History Museum’s fossil fish collection. This will include jawless fish that have fins but lack a distinct shoulder girdle.’
‘We are currently processing many gigabytes worth of data, and I can hardly wait to see what these important specimens from the collection will add to the story.’
