There are many theories proposed by linguists and philosophers concerning the origin of language, some relate it to divine sources and some to the nature. Here are some of those theories:
The divine source
To answer the origin of language, every religion has its own explanation. In Islam, Al Ghazali, one of the greatest thinkers in Islam, said that human basically is animal that can speak. Talking about speaking, it is different than making sound, since in the process of speaking, it involves the process of thinking, and of course the only creature that can think is human. Another view said that God created Adam and "whatsoever Adam called every living creature, that was the name thereof" (Genesis 2:19). So the name of things were invented by Adam for the first time and it continually develops along with the development of mankind. In Hindu believes that, Sarasvati, Brahma's wife is the goddess of language, so all languages came from her. In addition, most all religions believe that God grands human with languages.
Moreover, some experts came to some experiments to test these theories. Psammetichus, an Egyptian pharaoh, around 600 BC believed that the original language was Phyrgian, after having an experiment to two newborn infants who lived with goats and a mute shepherd for about two years. Those children can uttered a Phyrgian word bekos, literally means bread instead of Egyptian language. Another similar experiment was also conducted by James IV of Scotland, in AD 1500, the result the children spoke Hebrew. However, some, in attempt to test these theories conducted experiments with subjects who lived in isolation and in the end the did not speak any language at all.
The natural-sound source
This theory believes that the origin of language comes from the echo of nature. The early men and women imitated the sound that they heard from nature/ the things that produce that sounds to name the things. This theory believes that language is onomatopoeic.
However, in the modern linguistics, we know the term arbitrary, it refers to a concept in which the words in language are derived, the excerpt of this concept tells us how the language items have no direct connection with the concept they represent. Just say do you know about how a thing made of wood usually has square shape and four legs which function as the pair of chair is called table. Is there any direct connection between the word constructed from letters t, a, b, l, e with the above concept it represents? Note this; all letters in various languages in this surface of the earth are symbols. Or do you know how the crime of killing another person deliberately and not in self-defense or with any other extenuating circumstance recognized by law or in a simple words, an action causes the separation of soul from the body intentionally is called murder? And how the combination of qualities that makes something pleasing and impressive to look at, listen to, touch, smell, or taste is called beauty? There is no an adequate information how those words are originated, generally, we can only say that those words have been agreed by the language users. Those are the result of the convention among the language users in a society. The above concept is the simple description of what term arbitrary means. However, did you know that not all language or just say words are derived arbitrarily.
The oral-gesture source
This theory proposes that originally a set of physical gestures was developed as a means of communication. Then a set of oral gestures, specifically involving the mouth, developed, in which the movements of the tongue, lips and so on were recognized according to patterns of movement similar to physical gestures.
The excerpt of this view said that it is human organs which make human possible to speak language, starts from brain, mouth, tongue, larynx, and so on. This human organs designed by God in such a way which ease them to speak languages.