It is possible Neanderthals might have used "hmm". Nicholas Christenfeld, a psychologist at the University of California, San Diego, and an expert on filled pauses, attests "hmm" is popular largely since its such a neutral sound and that "its easier to say than anything else". The earliest attestations of "hmm" are from Shakespeare, "I cried hum... But markt him not a word" (1598 Shakespeare Henry IV, Pt. 1 iii. i. 154). It may be a vocable that grew out of lexicalized throat-clearing. Use as a filler word
"Hmm" is a "filler" word, such as "um" and "er". Use of "hmm" for "filled pauses" has been considered by many as stupidity and showing a lack of skill or competence, but many linguists  attest this judgement is unjustified. Typically, "hmm" is uttered when the person is being especially conscious about whom they are talking with, and as a result are thinking deeply about what to say. Moreover, the use of "hmm" is often interactional and cognitive. The interactional function is to do with politeness: if someone is invited to a party and responded "no" without a filled pause, they might appear rude, but a reply of "Hmm, sorry, no" might appear much more polite, as it seems the speaker is giving the offer some thought, rather than abruptly declining. Thoughtful absorption
The use of "hmm" is typically used during "thoughtful absorption", which is when one is engrossed in their flow of ideas and associations, that lead to a reality-oriented conclusion.
Erard, Michael (January 3, 2004). "Just Like, Er, Words, Not, Um, Throwaways". The New York Times. Retrieved November 10, 2013. Ing, John (September 26, 2019). "Pause Fillers for Speaking". Retrieved May 3, 2020. Schuessler, Jennifer (November 9, 2013). "The Syllable That Everyone Understands". The New York Times. Retrieved November 9, 2013.