Evolving Ideas: How Do We Know Evolution Happens?

Resource for Grades 6-12

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Evolving Ideas: How Do We Know Evolution Happens?

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Running Time: 7m 04s
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Source: Evolution: "Evolving Ideas: How Do We Know Evolution Happens?"

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This video from Evolution for high-school students describes two lines of evidence, fossil and molecular, which contribute to our picture of evolution. It focuses on whales, which provide an excellent opportunity to examine the transition between species because so many intermediate fossils have been found.

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Background Essay

Mammals evolved on land more than 200 million years ago. So how did the world's largest mammals -- whales -- end up back in the water? That's the question Dr. Philip Gingerich, a paleontologist at the University of Michigan, is trying to answer. Gingerich became intrigued when he found what looked like the fossil skull of an early wolf in Pakistan in 1978. But closer examination of this wolflike skull revealed it contained an ear structure resembling that of modern-day whales! This was the first fossil ever found that supported one of Darwin's most controversial ideas -- that whales had descended from land mammals.

Whales are so different anatomically from any other mammals that they make up a separate branch of mammal evolution. Had Gingerich discovered the beginning of that branch? The skull he unearthed was located among land mammal fossils, not in a marine layer of rock. Gingerich named the creature Pakicetus, meaning "whale from Pakistan." Was Pakicetus the land mammal whose descendants became modern whales?

Gingerich wanted to go back to Pakistan to find the animal's legs. However, war in the area kept him from returning. Instead, he went to a place called Zeuglodon Valley, or Valley of the Whales, in Egypt. Here, in the middle of the Sahara, hundreds of whale skeletons lie buried in sandstone. Gingerich's excitement turned to disappointment when he found that most of the skeletons were of Basilosaurus, a known aquatic whale ancestor. But Gingerich kept on digging. A few days later he made a new discovery -- Basilosaurus had legs. Even though Basilosaurus was fully aquatic, it still had vestiges of its terrestrial past. Ten million years of whale evolution had passed between Pakicetus and Basilosaurus, and yet whales still had hind legs and feet. Now the challenge for Gingerich and his colleagues was to fill in the gaps of whale evolutionary history.

Since Gingerich's early discovery in Pakistan, evolutionary biologists have found a series of what they call transitional fossils. These fossils link earlier groups of organisms with more recent groups, often combining traits of each. Some such fossils include 55-million-year-old land-dwelling mesonychids, walking whales called Ambulocetus that could also swim, and Rodhocetus, which were mostly aquatic animals but could probably walk a little on land.

In this way, Gingerich and other evolutionary biologists are slowly piecing together the whale family tree. Unfortunately, because only a small proportion of organisms ever become fossils, it is unlikely that every transition in the evolution of whales or any other group of animals will be recovered. Nonetheless, scientists have uncovered enough fossils to illustrate many important evolutionary transitions, including those between fish and amphibians, reptiles and mammals, and dinosaurs and birds.

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Discussion Questions

Gingerich discusses specifically only the whale fossils found in Egypt's "valley of the whales," but he points out that this now desert area was once a sea. Discuss the other kinds of fossils, and the types of rocks that must have been present for him to make this conclusion.

Examine the cladogram of whales and their ancestors presented in this video. Note that this diagram does NOT show modern whales evolving from any specific fossil form, but form the common ancestors of known fossil species and modern animals. Discuss the important difference between this view of evolutionary history, and the old view -- which often attempted to identify specific fossils as THE ancestors of a living form.

Discuss the similarities and differences between whale flippers and flukes and fish fins, using the scientific terms "homologous structures" and "analogous structures" in the discussion.

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Transcript

NARRATOR: How do we know evolution happens?

JOHN BANISTER-MARX, Teacher: Fossils provide the most interesting evidence for evolution, and certainly the most powerful for most people.

PHIL GINGERICH, Paleontologist: I want you to look behind us…

NARRATOR: There is an abundance of hard evidence at the molecular, chemical and anatomical level that evolution happens. But perhaps it's the clues hardened in stone that tell evolution's story most vividly.

GINGERICH: There are the backbones. You can see how big these vertebrae are. The history of life is all a record in fossils. Millions of them from strata layer by layer by layer back through time are all interconnected. Now, partly, this layering and succession is how we tell time in geology. But it's also the principal evidence that life has changed through time; that life has evolved through time; that the things living today can all be connected back through time to common ancestors.

NARRATOR: These now lifeless dry rocks are gold to paleontologists. They are the puzzle pieces to the origin of one of the most powerful and majestic animals on Earth—the whale.

NEIL SHUBIN, Paleontologist: The evolution of whales has always been sort of a mystery. They're like us in that they're very intelligent. They're mammals; they're warm-blooded. They nurse young and feed them milk. But they're very, very different. I mean, they live in water. They can hear in water and they swim around like fish.

So the question is this: We know that mammals originally evolved on land, yet whales live in the water. How did that shift, how did that evolution happen from life on land to life in the water?

NARRATOR: That question consumes paleontologist Phil Gingerich. The evolution of whales has been his passion ever since 1978, when his team unearthed a rock in Pakistan that appeared to have a skull embedded in it.

GINGERICH: When we found it, all you could see was the braincase. I thought it was a... a wolf-like animal with a very small brain. When it was cleaned, the ears were uncovered, and I could see that it had a very characteristic whale ear. And that shows us that what we have is a whale—not a wolf, not a deer, not some other kind of mammal, but a fossil whale.

NARRATOR: Gingerich named the whale ancestor Pakicetus. With only its skull as evidence, scientists do not know for sure what the rest of Pakicetus looked like, but some scientists hypothesize that it resembled a modern seal.

GINGERICH: Okay, look at this, Bill.

NARRATOR: Paleontologists look for fossils that have transitional features from one way of life to another. They use these related forms to create a comprehensive picture of evolutionary change.

When fossils of related species are located, scientists organize them by their physical similarities. Sometimes these similarities don't evolve all at once, like the whale ear of Pakicetus, which lacks many other whale-like features in its backbone and limbs.

SHUBIN: Whales' bodies contain inside them, connections, or clues, to their past. I mean, their mammalian traits are signs of their recent history, but there are also signs of a very ancient history. Take their flippers. On the outside, their flippers look like fish fins, but on the inside, the bones of that flipper are very similar to those in a monkey's arm, in the wing of a bird and in the limb of a frog.

NARRATOR: Egypt today seems an unlikely place for Phil Gingerich to find intermediate whale fossils.

GINGERICH: This is Whales' Valley in Egypt. You can see behind me there are many hills of sandstone standing above shale. What's important, it's called Valley of Whales because there are about 400 skeletons of whales weathering out here.

This was a wide-open, blue sea with the full range of sea life living in it—a lot of invertebrate shellfish, sharks, about five or six kinds of whales living here.

NARRATOR: Gingerich searches for intermediates with hind limbs and feet, to trace back until those feet and hind limbs are as big and fully functional as any four-footed animal that lives on land.

GINGERICH: It is a Basilosaurus—look at this. Here's the skull, incisors up front. The rest of the teeth come back to here. And then the whole length of the skull is over a meter.

NARRATOR: His find, Basilosaurus, had tiny hind legs with a mobile knee and several toes and lived 35 to 41 million years ago.

Here's where scientists place this transitional form on the fossil record. The discovery of another fossil has led scientists to conclude that whales evolved from a land mammal much like this one, called Sinonyx.

Perhaps its descendants found the water a source of abundant food and a haven from competition. Over millions of years, through variation and selection, front legs became fins, rear legs disappeared, bodies lost fur and took on their familiar streamlined shape.

SHUBIN: Whales show us the sheer creative power of evolution. I mean, from a wolflike creature to a whale in ten or so million years is... is amazing.

NARRATOR: Each dig and discovery adds more stone, more bone, more hard evidence for evolution, for the origin of life, for its history here on Earth and for the evolutionary relationships among all forms of life.