All animals with four limbs — reptiles, birds, amphibians and even humans — descended from common ancestors that made that initial transition from fins to feet. But how and why this evolutionary leap occurred has long been a scientific puzzle.
Now, the partial remains of a 375-million-year-old fish have emerged to help fill in some of the blanks. Paleontologists have uncovered new fossils from Tiktaalik roseae, which, while still a fish, is considered a transitional fossil that also has traits common to the first four-footed animals.
This more complete picture of Tiktaalik suggests that the creature had strong, mobile hind fins. That challenges the view that such enhanced rear appendages arose much later, perhaps even after animals had made the transition to land. Known as the "front-wheel-drive" hypothesis, that view held that front fins evolved into limbs first, while back fins stayed small and unimportant.
"These are four-wheel-drive animals, not just front-wheel-drive only," said study author and paleontologist Ted Daeschler of the Academy of Natural Sciences in Philadelphia. The results were published online Monday in the Proceedings of the Natural Academy of Sciences.
Stretching up to 9 feet long, the original 2004 discovery of Tiktaalik in the Canadian Arctic included fossils from its front half only — skull, shoulders and front fins. The initial excavation was made by Daeschler and study co-authors Neil Shubin of the University of Chicago and Farish A. Jenkins Jr. of Harvard University. Jenkins died in 2012. The ancient fish caused a stir in the scientific community due to its hodgepodge of features: It had scales and gills like a fish, strong limb-like front fins, a flexible neck and a flat alligator-like head with eyes on top.
The latter characteristics are reminiscent of amphibians, or more generally, tetrapods. These are animals that have (or whose ancestors had) four legs and include all land-living vertebrates such as frogs, tigers and seals.
But Tiktaalik's incomplete remains forced scientists to only guess what its rear section was like. A few years after the original discovery, Daeschler and his team finally excavated the remaining pizza box-size blocks grain-by-grain to reveal the surprisingly robust back portion of the same animal.
Earlier evidence suggested that the rudder-like hind fins remained small while front fins evolved into strong limbs that could propel the fish along the ground; hind limbs emerged afterward. But Tiktaalik's enlarged pelvis points to the possibility that hind limbs started to evolve simultaneously and were being used for propulsion.
"The size of the pelvic girdle, the region that holds the fins, was as big as the region that holds the front fin," Daeschler said. "That was not what we have predicted."
This means the uppermost part of the pelvis, the ilium, is big enough to have contact with the vertebral column — crucial for walking on all fours. Also, the hip socket of Tiktaalik is strikingly different from that of a typical fish. Although the researchers didn't find a femur bone, the socket shape and size means Tiktaalik likely had a larger rear fin range of motion than traditional fish.
"For you and I, we have this nice ball-and-socket arrangement, but for more fin-like fishes, that is not a deep rounded socket — it's flatter," Daeschler said. "For Tiktaalik, it was a really deep, round socket."
However, some of the bones that made up the pelvis remained fish-like and primitive in a piecemeal fashion as seen in the front half of the fossil — an example of what scientists call mosaic evolution. It means that different parts of the body change at different rates through many years.
"Tiktaalik is like a poster child of an animal that has mosaic features," Daeschler said.
"It's the parts nearest the body that get modified first, both in terms of embryo development and evolution," said paleontologist Jennifer Clack of the University of Cambridge, who was not involved in the study. Extremities, such as fins and distinct digits, are last in line to develop.
Ten million years after Tiktaalik, what could truly be called four-limbed animals finally emerged in the form of clunky, salamander-like creatures. They were still tied to the water but could also handle the gravitational force and dryness of land.
The exact reasons why the move to land happened remains a mystery, but the environment of the time provides some clues. Tiktaalik and other transitional species lived in the Late Devonian period about 395 million to 362 million years ago.
A few million years earlier, plants had started growing on dry ground, not just in water, and land-based ecosystems slowly began to form, said Daseschler. Before that, he said, land was barren and provided no incentive for vertebrates to pop out of the water.
Being able to move around and push off the ground in shallow waters became an evolutionary advantage. Clack said she believes fish had a number of different motives to venture into shallow waters and, subsequently, land: to escape predators, catch land-dwelling insects or bask in the sun.