Educator Resources: Fossil Halls

MERYL STREEP: For hundreds of millions of years, vertebrates have been evolving... thriving... and becoming extinct. But where did all these animals come from?

When did they first appear? And how are they related to one another? These are the questions that paleontologists ask. And to answer them, they must look for ancient clues --fossils. After ancient animals died, a few became buried in sediments. Their bodies decomposed, leaving behind hard parts like bones and teeth. When the sediment hardened into rock, those parts left behind were preserved as fossils.

So fossils are glimpses of a distant past. They are the key to reconstructing the fascinating story of how life evolved on Earth. The story begins three and a half billion years ago, with the first strand of DNA -- the basis of all life.

We are now traveling on a tree of life that links all the diverse species that have ever lived on Earth. The diagram used to show these links is called a cladogram. The diversity of life on the cladogram seems overwhelming, but when we look more closely, we see that groups of animals share many features.

For example, if we move down the cladogram all organisms on the branches above this point have cells with a nucleus to house the DNA. Based on the fossil record, the first organism with a nucleus lived at least one and a half billion years ago. Every creature that has descended from this common ancestor -- including us -- is made of this kind of cell.

Further up the tree, past plants and invertebrates, about 500 million years ago, is the point where the story of vertebrates begins. For the first time, an animal with a braincase and the beginnings of a backbone appeared. Their vital features -- the braincase houses the center for coordinating sensory input and the backbone stiffens the body and provides strong support for muscles. 

Everything above this point -- fish, amphibians, reptiles, birds and mammals -- has a braincase and a backbone. At the next branching point, about 430 million years ago, the first vertebrate with jaws evolved. Some of the earliest animals with jaws were armored fish called placoderms. They were powerful carnivores dominating their environment. All animals that branch from this point inherited jaws from this common ancestor. 

Placoderms inhabited the ancient seas for about 65 million years. Then they -- and many other animals -- died out. It's possible falling sea levels put stress on their coastal habitats. But that didn't stop the variety of sharks and bony fishes from growing.

Today there are more species of bony fishes than any other kind of vertebrate. 

Next up is the branching point for tetrapods -- animals with four limbs. The earliest tetrapods, at least 380 million years ago, probably used their limbs to paddle through shallow water. Tetrapods include all amphibians, reptiles, birds and mammals. About 310 million years ago, the common ancestor of all reptiles and mammals evolved an amnion --a watertight membrane that surrounds the embryo inside the egg. 

The watertight membrane allows these amniotes to reproduce on dry land. A wide variety of animals have descended from this common ancestor. By 290 million years ago, amniotes had split into two main lineages -- one is identified by a single hole that evolved in the skull behind the eye socket, called the synapsid opening. It was mammals and their now extinct relatives -- all called synapsids -- that descended from this ancestor.

The second lineage of amniotes is recognized by a pair of holes that evolved in the palate. This was the beginning of sauropsids or reptiles. Over the next 50 to 60 million years, turtles, lizards, plesiosaurs and pterosaurs all evolved from this common ancestor. About 230 million years ago, the first dinosaur evolved in the reptile lineage. It's recognized by a hole in its hip socket. 

All dinosaurs share this feature. This specialized joint allowed the hind legs to go straight down from the hips rather than sprawling out to the sides, as in turtles and lizards. Some scientists think the joint allowed dinosaurs to run upright and with greater endurance than other reptiles. Then 65 million years ago, things changed dramatically. A huge asteroid or comet hit the earth near present-day Yucatan. And there was tremendous volcanic activity in many parts of the world. 

These events would have caused severe environmental disruption -- temperature changes and acid rain. It seems that the dinosaurs could not adapt. Most died out. It was the end of the Age of Dinosaurs. But one group of animals that had evolved from the first dinosaurs survived -- birds. They have the characteristic hole in the hip socket, inherited from the common ancestor of dinosaurs. 

So, today's birds are really living dinosaurs. Throughout the 500 million years of vertebrate history, millions of species have evolved, lived and died out. It's a process that continues to this day. Cycles of evolution in extinction have produced the rich array of vertebrates -- and all organisms -- with whom we share the earth. But the diversity of living things is under threat. 

Just one species of vertebrate – humans -- has the ability to cause extinction on a scale perhaps even greater than at the end of the Age of Dinosaurs. If we humans have the ability to understand our family tree, can we take responsibility for its preservation as well?

You have just begun your exploration of the history of vertebrates, because this cladogram is also a map of the halls on this floor. Walking along the main black path through the halls is like walking the trunk of the evolutionary tree.

Circular branching points along this path represent the evolution of new features like a backbone, jaws, and limbs. At these branching points, you can walk off the main path to explore alcoves containing groups of closely related animals. 

In the Hall of Vertebrate Origins, you'll find most of the major groups of vertebrates on display, including sharks, turtles and pterosaurs. The Hall of Saurischian Dinosaurs includes long-necked herbivorous dinosaurs, such as Apatosaurus, and meeting dinosaurs, including Tyrannosaurus Deinonychus and birds.

In the Hall of Ornithischian Dinosaurs, you'll find armored dinosaurs, such as Stegosaurus horned dinosaurs, including Triceratops and the duckbills.

The next two halls, the Halls of Mammals and their Extinct Relatives -- include fossils of the fin-backed Dimetrodon, platypus, giant sloths and their extinct relatives, carnivores, hoofed mammals and our own group -- primates. So, as you walk the halls on this floor, welcome to your own extended family history.