The question is: What is the "original" language of mankind?
The experiment is: Raise children, from birth, without talking to them. Theoretically, whatever they start speaking--uninfluenced by their culture, as opposed to how everyone else learns to speak--should be "it".
Unfortunately, if you actually try to do this, the child will probably die (of hospitalism, I expect). Hence the experiment is "forbidden". Don't try this at home, kids.
Herodotus reports that an Egyptian king named Psammetikos raised two children this way, and their first word was "bekos", which turned out to be the word for "food" in Phrygian. Which, I suppose, is as good a guess as any.
Note: Even if there were an "original language" of mankind, a baby would not know it. Do teach your kids to talk.