Fossil Fish Fingers Shed New Light on the Evolution of the Human Hand

Ruth Schuster
Ruth Schuster
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An illustration of the Tiktaalik fish.
Missing link outed: An illustration of Tiktaalik, one of the elpistostegalian fish Credit: NobuTamura
Ruth Schuster
Ruth Schuster

A fossil fish that lived almost 400 million years ago in a shallow primordial sea had both fin rays and finger bones in its pectoral fins, an international team of paleontologists revealed on Wednesday.

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These strange fish, discovered in Miguasha, Quebec, in 2010 and analyzed in the new paper in Nature, may be the missing link between fish and tetrapods – early four-legged animals that ventured beyond the seas onto the land and became the ancestors of all legged and winged vertebrates.

The Quebecois Elpistostege watsoni specimen wasn’t only complete, it was huge: 1.57 meters (5 feet, 2 inches) long, which is helpful to analyzing its morphology. Scanning the long-deceased predatory piscine with high-energy computed tomography, Richard Cloutier and the team found the skeleton of the pectoral fin included not only fishy fin rays but also bones homologous to our upper arm, forearm, wrist and fingers.

It was the most tetrapod-like arrangement of bones found in a pectoral fin to date, say the paleontologists from Flinders University in Australia and  the University of Quebec at Rimouski.

Ancient fossil reveals evolutionary origin of the human handCredit: Flinders University, Universite du Quebec a Rimouski

The pectoral fins are the ones nearest the fish’s head.

Until now, fossil evidence of the elpistostegalians has been sparse and fragmented: a famous one in piscine paleontological circles is Tiktaalik, found in today’s Arctic Canada, which is known only from incomplete specimens. The new complete fossil provides evidence that the digits of four-legged animals evolved in fish before they left the water – which is earlier than previously thought.

In other words, the hands and feet of land-dwellers apparently evolved from the skeletal pattern “buried” within the fairly typical aquatic pectoral fin of elpistostegalians, the team says.

The Miguasha specimen “reveals extraordinary new information about the evolution of the vertebrate hand,” said John Long, strategic professor in paleontology at Flinders University in Adelaide. “This is the first time that we have unequivocally discovered fingers locked in a fin with fin-rays in any known fish. The articulating digits in the fin are like the finger bones found in the hands of most animals.”

The extraordinarily complete skeleton of Elpistostege watsoni, enabling us to see proto-finger and arm bones in its pectoral finCredit: Placoderm2

Elpistostege, are you my mother?

Elpistostegalian and tetrapod fossils are known from the Late Devonian period, around 390 to 360 million years ago. But fossil trackways indicate that early tetrapods left the waters earlier, probably looking for food in shallow water and on land, the team explains.

Evolving to survive out of the water requires not only feet homologues but the ability to breathe air, and adaptation of auditory and feeding structures as well. As for the feet: “The origin of digits relates to developing the capability for the fish to support its weight in shallow water or for short trips out on land,” the scientists explain. The more small bones in the fin it evolved, the more flexibility it had to spread out its weight through the fins.

One thought. This was a monster fish with impressive fangs and seems to have been the alpha predator in the shallow sea habitat that was Quebec around 380 million years ago. Yet the transition from sea to land is presumed to have been evolutionarily driven by a search for new sources of food. Elpistostege watsoni was clearly just one member of a long line of distinguished tetrapod-like fish.

Nor is there any particular reason to think it was our mother, ancestral to the terrestrial vertebrate line; some other transitional tetrafish may have been. But even if Elpistostege is not necessarily our mother, it’s the closest we know to the missing link fishes and four-legged beings that led to us.

Illustration of the simplified phylogeny of the bony vertebrates.Credit: Dr. Brian Choo
Evolution of prehistoic fish.Credit: Maija Karala
Fragmented ancient remains of the Tiktaalik, found in today’s Arctic Canada.Credit: Eduard Sol

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