How Have Animals Evolved and Adapted?

How Have Animals Evolved and Adapted? — page 1 — WHAT ARE EVOLUTION AND ADAPTATION The Arctic fox may have evolved from ancient foxes that lived in the mountains of Tibet Have you ever noticed how many different plants and animals live on Earth They come in all shapes sizes and colors They live in all kinds of places How did so many different living things come to be Scientists believe that plants and animals and other forms of life went through a process called evolution Evolution is the idea that all living things that exist today developed from earlier types 4

How Have Animals Evolved and Adapted? — page 2 — Traits are qualities that make one living thing different from another VOCABULARY Animals such as walrus hippopotamuses and ducks developed traits to adapt to their environments Evolution takes place over thousands or millions of years Evolution happens because habitats or the places where living things usually dwell change over long periods of time The environment or surroundings may become warmer or colder or it may change from a swampy area to a desert Living things must adapt to survive as their surroundings change They adapt by developing new traits For example an animal may have better eyesight or move faster than another animal of its kind These traits may help it to ﬁnd food more efﬁciently and to avoid its enemies Such useful traits give the animal an advantage over others of its kind 5

How Have Animals Evolved and Adapted? — page 3 — FROM SINGLE CELLS TO VERTEBRATES Ancient whales had characteristics similar to land-dwelling animals Most scientists believe that the ﬁrst life forms on Earth were single-celled organisms They lived more than 3 5 billion years ago in warm salty pools of water Over time these organisms developed into more complex forms Over many millions of years some animals developed bones At some point some grew hollow jointed backbones They became known as vertebrates or animals with backbones Animals that do not have backbones are called invertebrates Earth’s environment continued to change over time The vertebrates had to adapt to thrive The animals that lived in the salty ocean water moved to freshwater Then as the streams dried up or became shallow they had 6

How Have Animals Evolved and Adapted? — page 4 — to learn to crawl on land and to breathe air They developed lung-like structures and ﬁns that could help them crawl In time some grew legs and began to walk on land Vertebrates that live both in water and on land are called amphibians Early amphibians were the ancestors of all reptiles birds and mammals Some living amphibians include frogs toads salamanders and wormlike creatures called caecilians The ﬁrst animals to live completely on land were reptiles Dinosaurs are well-known reptiles that died out millions of years ago Today there are thousands of species or types of living reptiles They include snakes lizards turtles and tortoises alligators and crocodiles Amphibians and reptiles share some traits For example they are all vertebrates But what traits would an amphibian need that a reptile would not COMPARE AND CONTRAST Reptiles all have scales on their bodies 7

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