Carl Zimmer in The New York Times:
Dr. Pinker has focused much of his research on language on a seemingly innocuous fluke: irregular verbs. While we can generate most verb tenses according to a few rules, we also hold onto a few arbitrary ones. Instead of simply turning “speak” into “speaked,” for example, we say “spoke.” As a young professor at the Massachusetts Institute of Technology, he pored over transcripts of children’s speech, looking for telling patterns in the mistakes they made as they mastered verbs. Out of this research, he proposed that our brains contain two separate systems that contribute to language. One combines elements of language to build up meaning; the other is like a mental dictionary we keep in our memory.
This research helped to convince Dr. Pinker that language has deep biological roots. Some linguists argued that language simply emerged as a byproduct of an increasingly sophisticated brain, but he rejected that idea. “Language is so woven into what makes humans human,” he said, “that it struck me as inconceivable that it was just an accident.” Instead, he concluded that language was an adaptation produced by natural selection. Language evolved like the eye or the hand, thanks to the way it improved reproductive success. In 1990 he published a paper called “Natural Language and Natural Selection,” with his student Paul Bloom, now at Yale. The paper was hugely influential. It also became the seed of his breakthrough book, “The Language Instinct,” which quickly became a best seller and later won a place on a list in the journal American Scientist of the top 100 science books of the 20th century.