Fish with Fingers

Resource for Grades 6-12

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Video

Running Time: 3m 48s
Size: 6.3 MB

Source: Evolution: "Great Transformations"

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Fish with Fingers (Audio Description) (Video)

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Background Essay

One of the most important milestones in the evolution of life began some 400 million years ago, when the first animals made their way from water onto land. At that time, known as the Devonian period, the world was changing dramatically: complex plant ecosystems formed on land, the first woody plants appeared, and the water's edge was becoming a new kind of environment.

The move to land was a very gradual process, and the evolution of limbs wasn't a simple adaptation resulting from animals crawling onto the shore and never looking back. In fact, the new picture of this transition shows that most of the changes needed for life on dry land happened in creatures that were still living in the water. Some fishlike vertebrates had already begun to evolve limbs by around 400 million years ago: They were called "lobe-fins," with fins that looked like fleshy paddles, and they had lungs as well as gills.

The transition from these lobe-fins to the earliest tetrapods -- four-legged animals that walked on land -- has long been of intense interest to biologists. Many of the most telling fossils have been dug up in Greenland in the latter part of the 20th century, particularly in the past 15 years.

One of the first vertebrates that may have ventured onto land, whose remains date from about 364 million years ago, is called Ichthyostega. Although fishlike in many ways, it had robust bony legs, arms, and digits. Ichthyostega clearly spent some time out of water.

In 1987, fossils of another related form, named Acanthostega, were discovered in Greenland. This creature had stumpy legs and a long tail, which were probably used for propulsion in water. In fact, Acanthostega, more so than Ichthyostega, was basically an inhabitant of the water; its limbs were too floppy and its backbone too weak to support itself on land. Not only that, but despite the presence of lungs, Acanthostega had very fishlike gills.

From these finds, it now appears that the four legs common to land animals today really evolved for another purpose: navigating swampy wetlands, not as a means of moving to land. But once on land, the animals found their limbs a survival advantage there, too. Evolution frequently produces adaptations that come to be useful in the future for a different purpose.

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Discussion Questions

Discuss the ways in which our current understanding of early tetrapod evolution differs from the traditional view, and explain why this is important.

Discuss the combination of insight, luck, and persistence that led Dr. Clack to her unexpected discovery.

What specific combination of characteristics of Acanthostega puzzled Dr. Clack and led to her radical re-thinking of the relationship between the first tetrapod limbs and life on land.

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Transcript

NARRATOR: The first creatures to leave the water really started something. Their descendants eventually evolved into today's reptiles...birds...and mammals.

And these creatures' common origins are still visible in their bodies. Just like us, they all have bodies with four limbs—they're all tetrapods.

JENNY CLACK, Cambridge University: The old idea was that the fish came on shore first and then developed the legs. And what we now think is that the tetrapods developed the fingers first and then left the water.

NARRATOR: Jenny Clack of Cambridge University suspected that the theory taught in many textbooks was wrong.

CLACK: The story that you will find in many of the old textbooks and the pictures that you will see in children's books and museum galleries is a picture of a fish stranded in a drying pool, trying to support itself out of water. And it looks really odd if you look at it objectively.

NARRATOR: Clack thought there had to be a better explanation, but where to look? Only a handful of early tetrapod fossils had ever been found—most of those in a remote part of Greenland at the turn of the century. All she had to guide her was a note scribbled in a journal from a scouting trip to Greenland years earlier.

It referred to tetrapod fossils on an unnamed mountain. Clack flew to Greenland to search for those bones.

CLACK: Eventually we found the spot, 800 meters up on a hillside.

NARRATOR: Clack returned with four tons of rock...and spent the next four years drilling. At the end, she had the most complete early tetrapod skeleton ever found; and it forever changed the textbooks.

CLACK: One of the first things that we found was this forelimb.

NARRATOR: At the end of the animal's limb was an unmistakable array of bones. This was a hand.

CLACK: This is a life reconstruction. The artist is using imagination on the color scheme and on the eyes, but we think this is as accurate as you can get.

NARRATOR: The creature, named Acanthostega, was clearly a water-dweller: It had a fish-like tail and gills for breathing in the water. But the ends of its arms were paddle-shaped...possibly the first hands on Earth.

CLACK: This was a swimming creature. We don't know whether it could ever have come out on land, but it certainly wouldn't have walked in the conventional sense.

Basically, it's...a fish with fingers.

NARRATOR: Clack's find was a scientific breakthrough. It proved that some fish had arms and legs in the water. So tetrapods didn't need to grow limbs after they got onto land. The limbs had already evolved and helped them survive out of the water.