For decades, in the history of evolution, the presence of fingers was considered the dividing line between fish and land animals. Now a group of researchers from Flinders University in Australia and L’Université du Québec à Rimouski in Canada identified this feature in the fossil of an ancient fish dating back hundreds of millions of years, shedding light on how the gap between aquatic and terrestrial creatures was bridged and on the evolutionary origin of the human hand.
As explained in an article published Wednesday in the journal Nature, the paleontologists were able to identify a complete arm (pectoral fin) skeleton in a fish for the first time, studying a 1.57-meter-long Elpistostege fish fossil found in Miguasha, Canada.
“Basically, we have discovered that the first evolutionary pattern for the first version of a human hand appeared in fish before they left the water, and this is a big finding,” co-author of the study and Flinders University Professor John Long told The Jerusalem Post.
“There were previous papers published around 10 years ago that claimed to have found digits in fish, but they were just very irregularly shaped bones,” the professor explained, highlighting that by using high energy CT-scans, the researchers were able to determine that in this case the skeleton of the pectoral fin showed the presence of a humerus (arm), radius and ulna (forearm), rows of carpus (wrist) and phalanges organized in digits (fingers), “all the bones present in an arm.”
The evolution of fish into tetrapods – four-legged vertebrates – was one of the most meaningful steps in the history of life.
In order for vertebrates to leave the aquatic environment and live on land, the development of limbs represented a crucial transition.
Although some early amphibians with seven or eight fingers existed, most likely living an aquatic lifestyle, the first creatures fully adapted to walk on land had five fingers and five toes and appeared around 330 million years ago.
The new discovery has added a piece to the puzzle.
“Our fish is the most perfect intermediate between advanced fish living in the water and early land animals like amphibians,” the professor said.
As explained in the paper, Elpistostege was the largest predator living in a shallow marine to estuarine habitat of Quebec about 380 million years ago. It had powerful sharp fangs in its mouth and so could have fed upon several of the larger extinct lobe-finned fishes found fossilised in the same deposits.
Long told the Post that the first partial fossils of this species were uncovered in the 1930s, when a British paleontologist working in Canada acquired a small fossil that was part of a skull.
“Because of the enlarged shape of the skull, he thought it was an amphibian,” the professor noted, adding that more fossils were found in 1985, when researchers realized that the creature must have been a fish because it did not present the kind of skull bone pattern typical of early amphibians. Also, in this case, the fossils were only fragments.
“We have only studied one part of this amazing specimen. Our next step will be to examine the skull, the braincase and the structures of the head to see what other advantageous features to invade land it might have had. We have a lot more to do,” he concluded.