Consider the following explanation from evolutionary philosopher Stephen Pinker regarding the origin of human speech (from Pinker, S., (1994). 'The Big Bang', Chapter 11 of The Language Instinct, pp. 332-369. New York, NY: William Morrow and Company):
These arguments might strike us as peculiar, but every one of them has been made by scientists of a different species about a complex organ that that species alone possesses, language. As we shall see in this chapter, Chomsky and some of his fiercest opponents agree and some of his fiercest opponents agree on one thing: that 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. Either there is no language instinct, or it must have evolved by other means. Since I have been trying to convince you that there is a language instinct but would certainly forgive you if you would rather believe Darwin than believe me, I would also like to convince you that you need not make that choice. Though we know few details about how the language instinct evolved, there is no reason to doubt that the principal explanation is the same as for any other complex instinct or organ, Darwin's theory of natural selection.
Language is obviously as different from other animals' communication systems as the elephant's trunk is different from other animals' nostrils. Non-human communication systems are based on one of three designs: a finite repertory of calls (one for warnings of predators, one for claims to territory, and so on), a continuous analog signal that registers the magnitude of some state (the livelier the the dance of the bee, the richer the food source that it is telling its hivematcs about), or a series of random variations on a theme (a birdsong repeated with a new twist each time: Charlie Parker with feathers). As we have seen, human language has a very different design. The discrete combinatorial system called "grammar" makes human language infinite (there is no limit to the number of complex \vords or scntcnces in a language), digital (this infinity is achieved by rearranging discrete elements in particular orders and combinations, not by varying some signal along a continuum like the mercury In a thermometer), and compositional (each of the infiniate combinations has a different meaning predictable from the meanings of its parts and the rules and principles arranging them)."
“Humans are quite different because they possess language, which underlies every major intellectual achievement of humanity. This discontinuity theory is implausible because evolution cannot proceed by inspired jumps, only by accretion of beneficial variants of what went before” [Richard W. Byrne, “Social and Technical Forms of Primate Intelligence,” in deWaal, ed., Tree of Origin, 148–49.]
“What protoform can we possibly envision that could have given birth to constraints on the extraction of noun phrases from an embedded clause? What could it conceivably mean for an organism to possess half a symbol or three quarters of a rule? . . . monadic symbols, on a yes-or-no basis—a process that cries out for a Creationist explanation.” [Elizabeth Bates quoted in Steven Pinker, The Language Instinct: How the Mind Creates Language (New York: Harper Perennial, 1994), 350, 377.]
Other Unevolvable Biological Features: