In the 13th century, Roger Bacon advanced the theory that all languages have the same basic grammar. In 1637, Rene Descartes wrote in his "Discourse on the Method" that language acquisition is the thing that separates humans from other animals.
In 1957, B.F. Skinner and other behaviorists theorized that language is learned primarily by reinforcement. Any phonemes, morphemes or similar language structures that are not reinforced extinct themselves.
In response to the behaviorist theory, Noam Chomsky advanced the theory that humans evolved a "language organ" within their brains that allows for language acquisition. He later cited similarities between languages that he believed supported this theory.
In response to Chomsky's innateness theory, Nathan Stemmer and others developed a competing theory. "Empiricist" means that language is learned through experience. In this theory, children learn language by watching it used in culture and by being taught by adults.
Some experts, such as Esther Thelen and Linda Smith, believe that language is acquired through a combination of the above theories. Under this theory, there seems to be an innate ability to learn language that all children share, but the specifics of language are explicitly taught by the adults and culture around them.