In chapter 11 of Pinker's The Language Instinct, the author analyzes the evolution of human language. He starts by comparing an elephant's trunk to speech is. Each is wholly unique, only existing in a single species throughout the entire kingdom of animals. However, according to Pinker, biologists don't spend time marveling at the elephant's trunk to the same degree they marvel at humankind's capacity for speech. He sums up the beliefs of scientists who study this field thusly:
A uniquely human language instinct seems to be incompatible with the modern Darwinian theory of evolution, in which complex biological systems arise by the gradual accumulation over generations of random genetic mutations that enhance reproductive success.
Put more simply, these scientists believe that either there is no language instinct or that it must have evolved by other means. Pinker doesn't agree with that assessment. He believes not only that there is a language instinct, but that it must have evolved through Darwin's theory of natural selection—just as all other instincts do.
The crux of the chapter revolves around Pinker examining and carefully dismantling some of the prevailing beliefs that may conflict with his conclusion. For example, he talks about the apes Washoe and Koko who were said to have learned American Sign Language. However, the author argues that these experiments weren't very scientific and that the scientists who ran them often refused to share their raw data.
After examining each of these claims, Pinker leaves us with the conclusion that mankind's capacity for language must be a product of evolution by natural selection. His argument seems to be that the only reason we don't buy this is because as humans, we feel compelled to marvel at the parts of ourselves that are unique in the animal kingdom. However, according to Pinker, our language abilities are no more unique than coral building islands, earthworms shaping the landscape, or an elephant's trunk.