Many species on the planet employ a unique form of communication. Birds sing, and dolphins whistle and click. Yet, despite decades of searching, scientists have not found any form of animal communication that exhibits the structure of human language, which involves weaving together phonological patterns.

The reason for this unique trait is still up for debate. Some say humans’ capacity for language comes from a special genetic adaptation. Others say there’s nothing linguistically special about the human brain, it’s just bigger and smarter. From our general increased knowledge, combined with our unmatched articulatory capabilities, we create structured systems of language.

In her new book, The Phonological Mind, psychology professor Iris Berent revisits the dogma that has long supported the first of these explanations and presents a compelling argument for why things might be a little more nuanced than previously thought.

The arguments for the notion that humans’ capacity for language comes from a special genetic adaptation have been based almost entirely on syntax, according to Berent. But this is just one slice of the double layer cake of language. Syntax is how we put meaningful words together to create sentences. But even before that, we must form words by combining meaningless fragments like sound. The capacity to do so is called phonology.

Every known language uses these two layers to assemble its own method of communication. Even isolated communities of deaf people who were never previously exposed to a conventional language eventually drift toward one that consists of both a syntactic and a phonological layer, Berent explained.

At the phonological level, each human language follows a specific set of rules. For instance, in English we can say “blog” but we won’t say “lbog.” How do we know this? Some linguistic rules are learned at an early age. Others, however, might be innately available to all humans. Indeed, many of these rules are similar across languages. Just as the English “pencil” and Spanish “lápiz” each has two syllables, so do words in sign languages form from patterns of syllables. A person who knows nothing of sign language is able to track those syllables in signs, as Berent showed in a paper released yesterday.

To make things even more interesting, birds, dolphins, and most other animals that have been studied also follow specific rules in their own natural modes of communication. For instance, birds reared in isolation will, after a few generations, come to sing songs that are structurally similar to their species-mates outside the lab. Vervet monkeys have specific calls for different elements of their environment, like snake and eagle, which elicit different responses.

“The genius of using phonology is that we don’t need so many signs,” Berent said. “We don’t need distinct auditory elements for every word we speak; instead, we combine and reuse a select group of elements. Moreover, distinct languages form those combinations in similar ways.”

This is what makes human language special, Berent argues. It isn’t that we use rules, or that our rules are specifically adapted for our particular mechanical challenges. Rather it’s the capacity to combine these two ingredients–to form rules that are themselves designed to adapt to mechanical challenges–that is potentially unique to human vocal patterns.

Berent believes that phonology is a system of core knowledge—like our innate ability to quantify things, or our innate sense of distinct objects. And just like mathematics or physics, cultures use this core knowledge to create sophisticated theories and technologies. Human language is one.