Figure 12.16: This bizarre animal with five eyes lived during the Cambrian. Fossils of it
were discovered by Charles Walcott. ( 12 )
Evaluating Prior Knowledge
Be sure that you can answer the following questions before you begin this lesson.
What is a fossil?
How is geologic time divided?
How do organisms depend on their environment to live?
Earth’s Diversity
There are over 1 million species of plants and animals known to be currently alive on Earth
(Figure12.17). Scientists believe there are millions more that have not been discovered yet.
Look around you and you notice that the organisms on this planet have incrediblevariation.
One of the most remarkable features of Earth’s organisms is their ability to survive in their
specific environments. For example, polar bears have thick fur coats that help them stay
warm in the icy waters that they hunt in (Figure12.18). Reindeer have sponge-like hoofs
that help them walk on snowy ground without slipping and falling. Plants that live in dry
desert environments have special stems and leaves that help them conserve water.
Other organisms have special features that help them hunt for food or avoid being the food
of another organism. For example, when zebras in a herd run away from lions, the zebras’
dark stripes confuse the lions and make it hard for them to focus on just one zebra during
the chase. Hummingbirds have long thin beaks that help them drink nectar from flowers.
Some plants have poisonous or foul-tasting substances in them that keep animals from eating
them.