Part 1: Animals With Backbones

Evolution

We can tell from fossils that living things have been on Earth for over three billion (3,000,000,000) years. Those living things were not like you and me. They were very small and made of only one cell. A cell is the smallest part of any living thing, and a human being has trillions of cells, so you can see how small those living things were. Even though they were tiny, they were alive.

Those tiny things were the ancestors of all living things today. We say that all life has evolved from them. Evolution is a word meaning change in living things over long periods of time, producing new kinds of living things. Scientists can tell the age of fossils by the age of the rocks where they are found. They can tell it took a long time for complicated living things to evolve.

700 million (700,000,000) years ago, the first animal did not look like a dog or a cat. It was a sponge, a thing without a brain or nerves or a backbone. It sat on the bottom of the ocean and ate bits of food in the water, but it was an animal. Maybe that does not sound like an animal to you, but it does to a zoologist (a zoologist is a person who studies animals). To a scientist, an animal is a living thing, made of many cells, that can move (at least part of its life) and must eat other living things. Animals are one of the five kingdoms of living things.

Classification of Animals

Think of a dog. Did you think of a retriever, a chihuahua, a golden retriever, or a beagle? All of these are dogs. They are very closely related, even though they look different. A retriever and a chihuahua can have puppies together, though a retriever and a cat cannot. That is because all dogs are the same species. A species is a group of living things that is closely related and can have young together.

As you learned in the last chapter, organizing things into groups is called classification, and it is an important science skill. Scientists classify living things according to how they are related, using many different clues. If they see that kangaroos and elephants both have hair and feed milk to their young, they group both kangaroos and elephants as mammals. However, because kangaroos raise their young in pouches, they know they belong to a different group of mammals from elephants.

Just because things have similar traits does not always mean they belong to the same group. For instance, birds and bats can both fly, but they are not classified in the same group because bats are mammals.

Every year, new fossils are discovered and new species are found, and scientists work hard to classify them. Sometimes, new discoveries mean they must change the way they classify things.

Classification of Vertebrates

The animals are classified into two large main groups. 98 out of 100 types of animals in the world today are invertebrates like the sponge because they do not have backbones. Animals with backbones are called vertebrates, and they have only been around for 500 million years.

Invertebrates like insects are the
most common type of animal on Earth

Why are people so interested in vertebrates, when there are so few of them? Vertebrate animals are a big part of human life. Humans have dogs and cats as pets, they ride horses, they feed squirrels, and they raise cows for meat. Vertebrates can be much bigger than invertebrates, so they are easier to see.

Also, human beings are vertebrates, related to all the other vertebrates. If you feel your back, you can feel the bones running down the center. That is your backbone. The history of vertebrate animals is the history of you and me.

The vertebrates are divided into five main classes: fish, amphibians, reptiles, birds, and mammals. All of these are animals because they move around, are made of many cells, and eat other living or once-living things. All of these are vertebrates because they have backbones. Yet each class is different in important ways.

In the diagram below, you can see how vertebrates are classified.

You will be reading about these different vertebrates in this unit.