The evolutionary chain regained a missing link this week thanks to the discovery of a 375-million-year-old fossil with characteristics of both aquatic and land-dwelling organisms. The finding illustrates one of the intermediate steps taken by our ancient forebears on their slow movement out of the oceans and onto land.
"This branch of tree of life is part of our branch of the tree of life," said Neil Shubin, a professor of organismal biology and anatomy at the University of Chicago and an author of a paper describing the newly discovered species in the April 6th issue of Nature. "We're talking about the branch of the tree of life that gave rise to everything that has limbs: from amphibians, to mammals, to birds, to reptiles, to us, of course."
Shubin and his team, co-led by Ted Daeschler of The Academy of Natural Sciences in Philadelphia, had been searching for a fossil of this importance on Ellesmere Island, in Canada's frosty north, since 1999. Geological studies had revealed a large quantity of rocks on the island from the late Devonian period, the time of the first land crawlers, about 385 to 359 million years ago. The mass of land the researchers were combing had been a swampy delta spanning the equator during that epoch.
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Up until 2004, Shubin's team had barely uncovered enough material to keep their interest piqued. Then they happened upon a set of fossils, piled on top of each other in what Shubin knew to be the bed of a 375-million-year-old stream.
The new species, named Tiktaalik roseae, from the Inuit name for a shallow-water fish, exhibit many fish-like attributes, especially on its exterior: The specimen had scales on its back, fins and fin webbing.
But when scientists examined Tiktaalik's skeleton, an entirely different story emerged.
"What we saw when we looked at this fossil was a real mosaic between characteristics of fish and characteristics that were previously thought to be only in land-living animals,"said Shubin. "It's showing us how creatures were assembled over evolutionary time to live on land."
Unlike any fish before or after it, Tiktaalik had jointed bones, making up a shoulder, elbow and a wrist. The prehistoric creature would have been capable of performing a sort of push-up, Shubin said, with its finned hand flat on the ground and its elbow flexing. Also, Tiktaalik's head more closely resembled that of a reptile than a fish.
"You look at the head of this fish, it was very crocodile-like: a flat head with eyes on top," Shubin said. "You look at how the head is attached to the body—it had, likely, a mobile neck, which fish don't have."
Shubin and his team conjectured that Tiktaalik would have lived in shallow water with occasional spells on land, since it had a sturdy enough skeleton to withstand the stronger pull of gravity out of the water. With its sharp teeth, they suspect it was a predator.
The discovery of Tiktaalik provides an unparalleled insight into the progression from fish to land animal, said Hans Sues via e-mail, a paleontologist at the Smithsonian Institution, who called it a groundbreaking discovery of the kind that all paleontologists wish for.
"Tiktaalik represents a critical link between fishes and land-dwelling vertebrates (tetrapods) in many of its skeletal features," Sues said.