The fish-to-tetrapod transition has been described as one of the fundamental problems in evolutionary biology. Over 350 million years ago, during the late Devonian period, fish began to inhabit shallower and shallower water, until the four-limbed vertebrates (tetrapods), of which humans belong, began to roam across land.
Many questions surround this transition, including at what point one of the most significant evolutionary changes, the development of vertebrate hand and feet, originated. However, an ancient fish fossil found in Canada has helped to uncover the mystery of how the human hand evolved from fish fins.
The remarkably complete specimen of Elpistostege, measuring in at 1.57 meters (5.15 feet) long, included the first entire arm skeleton seen in this order of fish. Using high energy CT scans, researchers were able to uncover the tetrapod-like arm features in the pectoral fin of the fossil. Some of the structures revealed in the study, published in Nature, includes the presence of a humerus (arm), radius and ulna (forearm), row of carpus (wrist) and phalanges organized in digits (fingers).
“This is the first time that we have unequivocally discovered fingers locked in a fin with fin-rays in any known fish,” John Long, Professor in Palaeontology at Flinders University, Australia, said in a statement. “The articulating digits in the fin are like the finger bones found in the hands of most animals.”
Previous fossils from around the time of the fish-to-tetrapod transition have been studied by paleontologists to help us understand the evolution from a fish fin to a tetrapod limb. Until now, most fossils of the lobe-finned fish that were around in this period (elpistostegalians), between 393 and 359 million years ago, have been found incomplete. This Elpistostege specimen is not only crowned as the most complete elpistostegalian fossil to date, but it also has now overtaken the Tiktaalik species, from Arctic Canada to claim the position of the nearest fish from the tetrapods.
“Elpistostege is not necessarily our ancestor, but it is closest we can get to a true ‘transitional fossil’, an intermediate between fishes and tetrapods,” study co-author Richard Cloutier, from Universite du Quebec a Rimouski, Canada, said.
Cloutier also explained the necessity for the fish to develop fingers when transitioning to the land. “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,” Cloutier said. “The increased number of small bones in the fin allows more planes of flexibility to spread out its weight through the fin.”
Speaking to IFLScience, John Long, one of Cloutier’s fellow co-authors, emphasized the teamwork involved in “showing precisely how fishes gave us the finger!”
“For me, the most rewarding part of the research was working with the entire team from Rimouski and at Flinders, as we slowly revealed the story of this incredible fossil, the first ancient fish to have digits present in its fins,” Long told IFLScience.