The cat in the hat can be merged with a verb phrase to create a new object, a sentence: The cat in the hat came back. And that sentence can be merged into bigger sentences: You think the cat in the hat came back. And so on.
Why would this be of any use? No one else had Merge. Whom did Prometheus talk to? Nobody, at least not using Merge. (Humans may already have been using cries and gestures, as many animals do.) But Merge-enabled, hierarchically structured language, according to Mr Chomsky, did not evolve for talking at all. Rather, it let Prometheus take simple concepts and combine them in sentence-like ways in his own head. The resulting complex thoughts gave him a survival advantage. If he then passed the mutant Merge gene on to several surviving children, who thrived and passed on the Merge gene to their children, Messrs Chomsky and Berwick believe that they must have then come to dominate the population of humans in Africa. Only later, as Merge came to work with the vocal and hearing organs, did human language emerge.
Many scholars find this to be somewhere between insufficient, improbable and preposterous. The emergence of a single mutation that gives such a big advantage is derided by biologists as a “hopeful monster” theory; most evolution is gradual, operating on many genes, not one. Some ability like Merge may exist, but this does not explain why some words may merge and others don't, much less why the world's languages merge so differently. (Not a single non-English example appears in “Why Only Us”, nor a single foreign language in its index.)
Mr Chomsky says those who disagree with his ever-more contentious ideas are either blind or hucksters. Critics refer to a “cult” of “acolytes” around a “Great Leader”, unwilling to challenge him or engage seriously with the work of non-Chomskyan scholars. (One critic has said “to be savaged by Chomsky is a badge of honour.”) Linguistics is now divided into a Chomskyan camp, a large number of critics and many more still for whom the founder of the modern discipline is simply irrelevant. He is unlikely to end up like Freud, a marginal figure in modern psychology whose lasting influence has been on the humanities. Mr Chomsky's career is more likely to end up like Einstein's—at least in the sense that his best and most influential work came early on.