Tiktaalik: Bridging the Gap Between Water and Land
Tiktaalik is a transition fossil that bridges the gap between water and land in our evolutionary history. Its discovery sparked both excitement and controversy.
Photo by Mathias Reding on Unsplash
It takes a lot of guts to take that first step—especially if you’re a fish stepping onto dry land for the first time.
But the benefits for that brave fish are huge. There are all sorts of tasty insects and millipedes on land that don’t have natural predators yet. There are also scary predators in the sea that try to eat you. So yeah, it might be a good idea to get away for a bit, even if that dry land is a bit intimidating.
Perhaps that’s what was going through Tiktaalik’s mind when this adventurous fish first decided to take a step onto land. More likely, though, it was acting on an urge to get food or to stop from becoming food. But in doing so, it unwittingly changed the course of evolution, leading to reptiles, amphibians, mammals, and finally, us.
[When Tiktaalik] decided to take a step onto land…it unwittingly changed the course of evolution, leading to reptiles, amphibians, mammals, and finally, us.
What’s so special about Tiktaalik?
Darwin knew there would be critics of his theory of evolution. The problem wasn’t the theory itself—rather, it was (and still is) the fossil record. The fossil record is a book with lots of pages missing. Entire species are lost to memory as rocks erode and change. Darwin believed that some of these gaps might never be filled—the fossil record is just too incomplete. One of these “gaps” was understanding how limbed animals moved from the oceans, where it was plentiful and diverse, to the land. At the beginning of the 21st century, there were still no animals that seemed to fill in this gap.
Scientists surmised that this transition occurred about 375 million years ago. Ted Daeschler, a professor at the Academy of Natural Sciences in Philadelphia, and colleagues headed to a remote area in the Canadian Arctic, a mere 500 miles from the North Pole. It was in this frigid location that rocks of the right age are exposed.
It was an adventure. “It just feels incredible,” Daeshler told me. “Taking a helicopter across barren, ice-covered areas, and then dropping into valleys that are tundra and green and wildflowers, and seeing places where…[only] a handful of people… have been.” They searched for years. Many times they were close to giving up. Money was running low. Time in the wilderness meant time away from family and friends.
“We have this framework for the history of life. We have ideas about the types of animals that are closely related along the lineage that’s leading toward limbed animals. Tiktaalik roseae just sort of fulfilled expectations.”
But one day, the scientists spied a triangular-looking jaw fossil sticking out of the rock. They knew immediately it was special. The fossil belonged to a yet undiscovered type of fish. They dubbed it Tiktaalik roseae. (The name Tiktaalik is Inuit for “large, freshwater fish”.)
An artist’s recreation of Tiktaalik roseae. By Zina Deretsky, National Science Foundation (Courtesy: National Science Foundation) [Public domain], via Wikimedia Commons
Like most fish, it had scales and fins. But the head was unique. Instead of having its eyes on the sides, the eye sockets were on the top of a rather flat head, similar to modern alligators or crocodiles. One could almost imagine that Tiktaalik could gaze out of the water, looking for prey. On top of the head were spiracles, small openings that led to a respiratory system. These fish not only had gills, but a primitive lung as well.
Its fins, however, show why Tiktaalik is really special. The bones are very different than what you would find in a fish’s fin. Some bones resemble a shoulder, elbow, and wrist. These fins were strong enough to bear the weight of Tiktaalik out of the water. Similar bones can be traced through amphibians, reptiles, mammals, and eventually, us.
Later, Daeschler and his team found more fossils of Tiktaalik. These indicated that its back half was also very powerful, and could propel the animal out of water. Another thing that was incredible about the find—it wasn’t that surprising. “It’s not unexpected,” Daeschler told me, discussing the fossil. “We have this framework for the history of life. We have ideas about the types of animals that are closely related along the lineage that’s leading toward limbed animals. Tiktaalik roseae just sort of fulfilled expectations.”
[Tiktaalik’s] bones are very different than what you would find in a fish’s fin. Some bones resemble a shoulder, elbow, and wrist… Similar bones can be traced through amphibians, reptiles, mammals, and eventually, us.
In other words, they had an idea of what a transitional fossil from finned to limbed organisms would look like. It might have a flat head with eyes on top. It could have structures that would serve the purpose of propelling and supporting weight. Tiktaalik lined up with a lot of what they expected. “We could not have predicted the details of the combination of features. But we certainly had predictions that were true,” he continued.
It looked like Tiktaalik was the fossil that scientists had been searching for—a transitional life form, in between fish and those first land-dwelling animals.
Transitional fossils, evolution, and controversy
Back in the early 2000s, a controversy was brewing in many school districts. One came to trial in 2005 in the court case Kitzmiller v. Dover Area School District. Eleven parents challenged the school district, demanding students be presented with multiple views of the origin of life in science classes—including both Intelligent Design and evolutionary theory.
As a textbook explaining Intelligent Design, the parents suggested the book Of Pandas and People. One of the central arguments against Darwinian evolution in the book was that there were no transitional fossils. This, the book said, showed that there were literal “holes” in evolutionary biology.
As this case drew worldwide attention, Daeschler and his colleagues were writing up their findings on Tiktaalik. The fossils of Tiktaalik are an example of transitional fossils. These remains bridge the gap between two different forms of life over time. There are many other examples. Archaeopteryx is a fossil that could be the transition between dinosaurs and birds. Pakicetus could bridge the gap between land animals and those mammals that returned to the sea—whales, dolphins, and porpoises.
Tiktaalik in the Field Museum, Chicago. Eduard Sola, CC BY-SA 3.0
The fossils of Tiktaalik are an example of transitional fossils. These remains bridge the gap between two different forms of life over time.
Lessons from fossils
Even though Tiktaalik couldn’t be used as evidence in the court, the teaching of Intelligent Design in the classroom was struck down based on the First Amendment—separation of church and state. Intelligent Design was aligned with religion, said the court. Most importantly, it was not science.
The trial showed the public what science truly is—a body of ideas that stand up to evidence, rigorous testing, research, and questioning. With this definition, Intelligent Design is not a science. Instead of being built on its own foundations of evidence and questioning, it was structured upon the supposed “holes” in evolutionary theory. “Unless you assume that science and religion are trying to teach us the same thing in the same way,” says Daeschler, “these things aren’t mutually exclusive.”
Tiktaalik, like many other fossils, can show us a part of where we came from…We are part of a family that extends throughout all of life on Earth. We are connected…We are all in this together.
The reaction of the scientific community to the fossil find was very positive. And it even entered mainstream public knowledge. “I did the Colbert Report,” Daeschler recalls. He really got a kick out of seeing his work in Ranger Rick, a magazine he grew up reading. “Tiktaalik roseae in Ranger Rick!” he laughed. “That was such an important magazine for me getting interested in nature.”
Besides helping us to understand just how life began to walk on land, there is more that Tiktaalik can teach us. Tiktaalik, like many other fossils, can show us a part of where we came from. Looking at the limbs of Tiktaalik, we can see bones that look like the bones in the limbs of alligators, cats, horses, and us. We are part of a family that extends throughout all of life on Earth. We are connected. Whether we are looking at climate change and other environmental crises or just trying to understand how we fit into this world, we were made as part of a larger family. We are all in this together.
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About the author
Elizabeth Fernandez
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