Some people use dogs for hunting. Many blind people use dogs as guides.
The police use dogs to help them search. But most people keep dogs as pets.
There is an old saying that "a dog is a man's best friend."
Of all animals, dogs perhaps have the best relationship with people, and this friendship between people and dogs has deep roots.
In fact, dogs and humans have been companions for about 14,000 years.
Most scientists believe that today's dogs evolved from wolves.
"Evolved" means that they started as wolves and then, generation by generation, they slowly changed into dogs.
According to scientists, dogs today are very different from wolves because of thousands of years of change.
While most dogs are friendly to humans, wolves are fierce predators, one of the most dangerous animals around.
So how might wolves have evolved into friendly dogs?
About 14,000 years ago, humans did not live in cities or towns.
People didn't even have farms to grow food.
Instead humans hunted animals for food and moved from place to place, looking for the best hunting grounds.
The animals that humans hunted were the same animals that wolves hunted, so wherever the people went, the wolves would go as well.
Humans and wolves were certainly not friends, but they weren't enemies either.
In the evenings humans would light fires to cook their meals.
Some wolves would smell the delicious odor of cooking meat and try to sneak close the fire to get food.
If a wolf did not seem threatening, people would give it some of their leftovers.
Humans especially liked the wolf cubs because they seemed cute and friendly.
Other wolves would come to garbage piles and eat the food thrown away by the people.
As long as the wolf wasn't threatening, humans were glad to let it clean up the garbage.
So, after many generations, humans and wolves began to develop a good relationship.
Some wolves began to live near human camps.
Humans liked these camp wolves because they had a much better sense of smell and hearing than the humans.
When the wolves smelled or heard an unfriendly animal or unfriendly group of humans coming, they would bark.
This would warn the people in the camp.
Today protecting people from unfriendly animals and strangers is the job of a guard dog.
Camp wolves were the first guard dogs.
But not all wolves became camp wolves; some were too wild and fierce.
Only the friendly wolves were accepted by the people in the camp.
People eventually began to take care of the friendly wolves.
They would give them food and a dry place to sleep.
Scientists think that the shape of these animals changed over time.
Since the wolves no longer had to hunt for food, their heads and teeth probably became much smaller.
Dogs that were kept in northern areas probably began growing long, thick fur to keep them warm.
Meanwhile, dogs living in the south lost hair to help them stay cool.
The friendship between humans and dogs strengthened quickly.
Dogs began to help people on their farms. They also helped people hunt.
Soon humans began to raise dogs simply for companionship.
Today there are hundreds of different kinds of dogs.
Some dogs, like the Doberman pinscher, are large and used as guard dogs.
Other dogs, like the poodle and cocker spaniel, are small, usually harmless, and kept simply as pets.
All dogs are similar in that they are tamable, which means they will act friendly to humans.
They are trainable, which means they will do what people tell them to do.
Also unlike wolves, dogs often wag their tails in a friendly way.
But what is most amazing is that, unlike any other animal, dogs may exist only because they are useful and friendly to humans.
Maybe if there were no humans in the world, there would be no dogs either.
Most scientists believe that thousands of years of living with people led dogs to evolve from fierce, wild wolves into tamable, trainable,
and tail-wagging friends.