An easy question, with a hard answer.
First, we must define what is language?!
A language is a system of signals, such as voice sounds, gestures or written symbols that encode or decode information.
Many scientists from different fields of science, are involved in answering this question.
Christain theologians believe that at the beginning of humanity, in Babel(Which is called now Baghdad; the Iraqi capitol), there was only one language. Then some angels came down to earth and separated between humans, which led to many differences in the main language, leading then to the creation of new languages.
In arabic, the word “Babel”, came from the action the angels had (Separated between tongues) as – The book of Genesis – mentioned.
But no one yet agrees on when language was first used by humans (or their ancestors). Estimates range from about two million (2,000,000) years ago, during the time of Homo habilis, to as recently as forty thousand (40,000) years ago, during the time of Cro-Magnon man.
Other living beings, have their own languages.
Ants for example, have a chemical language.
Birds, have a sound languages.
Human, first had body language, then symbolic language, then spoken one.
So during the human evolution by natural selection, language has aslso been evolved.
But how did this happen ?!
Is there a relation between what we see as a specific shape, and its name ?!
Now think of a certain shape for the word “BOOBA”.
Most likely, the shape you imagined is like a fat balloon or circle .. A high angle shape.
Again, think of a shape for the word “KIKI”.
It’s a low angle shape, like lines linked to other .. Isn’t it?!
Almost 100% of people, have the same results.
But how this happened ?!
During the human evolution from a specie to another, the relation between human and the environment around him, was also evolving.
The human base system, is the brain. It is where all basic neural functions occur.
The human interaction with the environment, always affects these neural activities.
By these effects, new nerves systems appear, and new connections occur between them.
With more interactions with the environment, connections between the Auditory cortex, the Visual cortex, and the limbic system(emotional system in the brain), have been created.
What we see, are not only light waves that get into our retina, then transformed as electric signals to our optical system in the brain. But also a feeling, that is translated in our limbic system.
For example, when you see a circle, you see it, and feel it.
The same, is for hearing.
What we listen to, are not only sound waves that get into our ears, then transformed as electric signals to our auditory system in our brains. But also a feeling that is transferred into our limbic system.
When you listen to a specific voice, you hear it, and feel it.
That’s because of the connection between the limbic system, visual cortex, and auditory cortex.
Return to “Booba”, and “Kiki”.
The word “Booba”, is a word that you can hear. So, you also can feel it. Because it is translated into two meanings in your brain. An auditory meaning, and an emotional meaning. The same, is for the word “Kiki”.
The “Booba” shape, also have a meaning in your visual cortex, and in your limbic system. It is translated into, emotional, and visual meanings. And the same, is for the “Kiki” shape.
Why did people choose the word “Booba” for the Booba shape, and the word “Kiki” for the Kiki shape ?!
That’s because visual and emotional signals of the word, are similar to its auditory signals!
That’s why nearly all people choosed the same word for the same shape.
And that’s one of the theories of the appearance of human language; which is a connection between what we hear, see, and feel.