Animal evolution and evolution of all living things on earth

Some scientists believe that living things developed from other simpler living things, which have slowly become more complex over millions of years.
There are enough fossils to show clearly how some kinds of animals have changed over time.
Studying fossils has helped scientists to reach the conclusion that mammals have branched off and followed reptiles.
Other fossils show what the ancestors of some animals, such as the camel, looked like.
Camels branched off a line of small animals the size of a fox. Camels then developed as larger animals with hooves, a long neck, and large teeth.

The animals that became better suited to their surroundings are the ones who have survived.
The giraffe's long neck, for example, enables it to reach high up into trees and eat the leaves there.
Scientists believe that animals have survived because their ancestors were the ones that could make best use of their surroundings.
Some animals, such as the dinosaurs, were not able to survive. 
No one knows exactly why they died out. Other species have also disappeared completely.
Some were hunted until they became extinct.
Others were destroyed by disease or by changes in climate.

 Over millions of years, bigger and heavier elephants appeared.
The mammoth was about 15 feet (4.5 meters) tall.
There are now only two kinds of elephant— the African and the Indian elephant. The African elephant is about 11.5 feet (3.5 meters) tall.