Merge is a controversial belief by some that human language faculty arose in humans through a single gene mutation, rather the evolving gradually.

This human burst of genetic exceptionalism is embraced by some linguists, such as Noam Chomsky, but the science community has doubts. The basis for Merge is that humans are genetically equipped with a unique cognitive capacity that specifically allows us to implement computations over hierarchically structured symbolic representations.

"Put it simple, Merge takes two linguistic units (say, words) and combines them into a set that can then be combined further with other linguistic units, effectively creating unbounded linguistic expressions," says  Cedric Boeckx, linguist at University of Barcelona. "These, in turn, are claimed to form the basis for our cognitive creativity and flexibility, setting us aside from other species."

Image: University of Barcelona

The basis for Merge needs to be a single genetic mutation, a macromutation, that emerged instantaneously in a single hominin individual who is an ancestor of all modern humans, and spread through the population.

In a recent paper, the authors simulated the evolutionary dynamics of such a scenario, using different parameters such as how long ago this mutation would have happened and the population size at the time. These kinds of paramater-based models with boundary conditions were first used in physics and then spread to biology simulations; done properly, they create way to quantify the probability of a complex trait like language evolving in a single step, in many small steps, or in a limited number of intermediate steps, within a specific time window and population size. Of course, evolution did not always obey the parameters programmers think to build in but models are a reasonable guess.

The simulation concluded that, instead of a single mutation with an extremely large fitness advantage, the most likely scenario is one where higher number of mutations, each with moderate fitness advantages, accumulate. A gradual accumulation of smaller biological changes meant syntax evolving from phonological form, from rapid manual actions or from much simple pragmatic sequencing of words.

In a second paper, a team addressed the logic of Merge. Defendants say that because it is such a simple operation it had to be the result of a single genetic mutation that endowed one individual with the necessary biological equipment for language. In addition, because Merge is either fully present or fully absent, in other words, there cannot be such a thing as half-Merge, the human language faculty had to emerge suddenly, as the result of this single mutation.

That creates a chasm in biological, in which anything can be placed. Though linguists might do it as speculation, there is no scientific way to derive such simplistic evolutionary scenarios for any complex trait. It always feels like a cop-out to invoke multifactorial but sometimes that is the answer. Neurobiology, genetics, cognitive science, comparative biology, archaeology, psychology, and linguistics will all need to agree for Merge to be valid, and they do not. 

A scenario of sudden emergence of human language by means of a single mutation still doesn't add up, and until it does language evolved gradually.