Fill in the blank, but remember that when you do, a whole view of language comes with it. So don't try to argue that the word you chose is neutral or can serve as the basis of a consensus.
One of the big differences between science and philosophy is the way scientists agree about many points while philosophers can dispute everything. That fact alone suggests that language studies in general, and the study of language origins in particular is still much more philosophy than science.
Tecumseh Fitch says of biolinguistics, “I know of no other field where scholars seem so ready to champion their own pet hypothesis uncritically, while rejecting those of others as ludicrous” [p. 286 of “Prolegomena to a Future Science of Biolinguistics”].
Historically, pretty much every scholarly field was like that. But as astrology became astronomy and alchemy was transformed into chemistry something new appeared. Disputes persisted on the edge of research, but a consensus had been achieved on fundamentals. Fitch’s essay in the current issue of the online journal Biolinguistics is an attempt to propose a consensus. It’s a noble ambition. Does he succeed?
Typically, there are two ways to bring consensus out of disputation. The classic way is the Newtonian, theoretical triumph that explains and links the data along systematic lines. Everybody would like to follow suit, but those grand moves are rare. Another, less exciting but equally useful way, follows Ben Franklin. He wrote a book setting out all that was known about electricity, organizing it into a whole. For the first time, electricity made sense to people and they could look at electric phenomena with some feeling of understanding.
Philosophers, like mathematicians, tend to point toward definitions, but Newton, Franklin and the other heroic consensus builders pointed to observable facts and presented links between them. If we can agree on the truth of observation A and observation B, then we might be willing to join a consensus if somebody finds a natural way of linking the two. If Fitch, or anybody else for that matter, wants to move discussions of language origins onto the level of scientific consensus, he’s going to have to link an A and a B.
Fitch, however, has only one “Core Explanandum,” i.e. the central fact to be explained: Human children start talking quickly and easily. As he puts it:
Any normal child raised in a human household will quickly, and apparently effortlessly, acquire the language(s) of the family and community while no non-human animal will do the same. 
It is an important fact, but any single fact can be interpreted and explained in a large number of ways. We need to link the “core” with at least one more fact before we can hope to create a consensus. The really great theories like gravity, evolution, and plate tectonics link many facts from many sources.
Facts are ambiguous. Fitch’s core explanandum rules out any theory that does not posit a difference between humans and animals, but there are many possibly relevant differences: humans are bipedal, communal, bigger brained, and unusually hairless for a primate. Are any of those distinctions relevant? It takes a theory to tell us.
Einstein used to say that all facts secretly embed theories. For example, Aristotle knew that the sun rose in the east. It seemed obvious, but biased all attempts at explanation to assume that it’s the sun that moves. I’m afraid Fitch has stated his fact in just such a biasing way. His use of the word acquire assumes a whole underlying psychology and theory of language. It could be the correct psychology and linguistic theory, but he’ll never win over skeptics this way.
Acquire implies the existence of something to be acquired, perhaps the rules and processes required for generating the sentences of a particular language. Another description of the core explanandum might be, Human children start using words and sentences quickly and easily. That version gets rid of the assumption that something is acquired and Fitch knows the difference. At one point in his paper he refers to the “human capacity to acquire and use language” [284 and again on 287].
A biased fact is never neutral, but it doesn’t always impede progress. Ptolemy’s astronomy shows just how much great science can be done, even when you’ve got one of the basic facts wrong. But in Fitch’s case, the man declares himself stuck.
…the problem of language that …[is] most difficult to solve: The problem of meaning…the problems of meaning are very broad, and extend far beyond the confines of language or linguistics. [306-307]
For most people, the origin of language is synonymous with the origin of speaking meaningfully. The rest has its place, but when a leader of the inquiry says he cannot even “offer sketches of a solution” to the meaning question, he is saying he is stuck.
Sadly, instead of looking for ways to get moving, Fitch seems resigned to being stuck. He certainly isn’t talking about rethinking his axioms. He is quite confident that he already knows what language is and seems to expect nothing to change as a result of thinking about language in a biological context. That’s why he can be stuck without rethinking; he already knows about language, it’s the biology that is puzzling.
Those of us who are not so sure we already know the story of language can be a little more inventive. Why not take a look at that word acquire and see if it can be done away with? That might get a wheel or two turning.
Here’s another fact, one that Fitch does not mention. Apes can learn to use some hand signs. I don’t say they acquire sign language and Fitch surely would not either. So it turns out that he and I agree it is possible to use words without acquiring language.
Our second fact leads to a new question. If apes can be taught to use words, why don’t they do so in the wild? Whatever the answer is, it will not be because they aren’t smart enough. We know they are smart enough because they can be taught to do so. So there is something else afoot. What? That’s to be found out.
But whatever the reason it won’t be a linguistic one. It will have something to do with the difference between apes and humans, and when an answer is found we will have two observations linked by a theory:
If you can find a link between those two facts, you will begin to have an actual theory and some hope of building a consensus. Until then, or at least until Fitch’s observation A is linked to some observation B, we will have plenty of philosophizing about language origins but not much science.
A — Human children start talking quickly and easily, while other animals don’t.
B — Captive apes can be taught to use some signs, even though apes in the wild never do.