Language development is thought to proceed by ordinary processes of learning in which children acquire the forms, meanings, and uses of words and utterances from the linguistic input. Children often begin reproducing the words that they are repetitively exposed to. The method in which we develop language skills is universal; however, the major debate is how the rules of syntax are acquired.There are two major approaches to syntactic development, an empiricist account by which children learn all syntactic rules from the linguistic input, and a nativist approach by which some principles of syntax are innate and are transmitted through the human genome."

Source : Wikipedia

Given the grammatical complexities of most languages, the question of how children (relatively effortlessly) acquire their first language, is as yet largely unexplained.

Since the time of Plato, various (sometimes incompatible) theories have been put forward.

In the 1960s, professor of linguistics Noam Chomsky proposed the idea that human brains might have a biological equivalent of what he called a 'Language Acquisition Device'.- as a way of explaining how humans appear to be born with the instinct or innate facility for acquiring language.

Over half a century later, however, no neuro-scientific evidence of any kind has been found to support Chomsky's idea, and several high-profile linguistic researchers have substantially dismissed it ( example ref.).

There are now (at least) six currently competing theories which seek to explain how humans manage to learn how to interpret language so efficiently and easily - and with an apparently innate ability to do so. None is currently accepted as a full explanation.

The main theories are :

• The Motor Theory, Exemplar theory
• Acoustic Landmark theory
• Fuzzy Logic model
• Speech Mode hypothesis
• Direct Realist theory.

All of the above are detailed at Wikipedia

Also see : Language evolutionplugin-autotooltip__plain plugin-autotooltip_bigLanguage evolution

"Understanding the evolution of language requires evidence regarding origins and processes that led to change. In the last 40 years, there has been an explosion of research on this problem as well as a sense that considerable progress has been made.