Have you ever wondered why do dogs bark?
In terms of evolution, there has to be a reason for barking, or it probably wouldn't be there. Is it to warn of predators? But dogs bark when there are no predators around. Is it play? Some dogs play without barking. In fact, people who study canine behavior find there isn't much of a pattern to barking--it seems to be an all-purpose noise.
Now here's another fascinating thing: adult wolves don't bark. They growl, and they whine, but no barking. And dogs evolved from wolves. So where did the bark come in?
Mark Feinstein and Ray Coppinger of Hampshire College in Massachusetts have a theory. These biologists have noted that while adult wolves don't bark, adolescents do. Wolf pup barking seems to be an intermediate noise you grow out of.
Now, a few tens of thousands of years ago, wolves and people started becoming friendly species. You can imagine a person tossing a bone to a friendly wolf from time to time. But "friendly" is the key. Nobody ever gets close to a hostile wolf. So over many thousands of years we would have been gradually selecting for certain behavioral traits--playfulness, low hostility, the ability to bond. Very puppy-like behaviors!
See what's happening? A species that resembles a wolf puppy that never grows up may have slowly evolved in this way. That species would retain its puppy-like behaviors, such as barking, but never grow into growling, howling adult wolves. We would now call that species "dogs."