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.
A neuroma (; plural: neuromata or neuromas) is a growth or tumor of nerve tissue. Neuromas tend to be benign (i.e. not cancerous); many nerve tumors, including those that are commonly malignant, are nowadays referred to by other terms. Neuromas can arise from different types of nervous tissue, including the nerve fibers and their myelin sheath, as in the case of genuine neoplasms (growths) like ganglioneuromas and neurinomas. The term is also used to refer to any swelling of a nerve, even in the absence of abnormal cell growth. In particular, traumatic neuroma results from trauma to a nerve, often during a surgical procedure. Mortons neuroma affects the foot. Neuromas can be painful, or sometimes, as in the case of acoustic neuromas, can give rise to other symptoms. Neoplasms
Acoustic neuroma -  a slow-growing,  benign tumor of the  acoustic nerve. Symptoms, which most often start after the age of 30, can include dizziness, headache, vertigo, loss of balance, ringing sensations, and numbness. Ganglioneuroma - a tumor of the sympathetic nerve fibers arising from neural crest cells. Pacinian neuroma - a very rare, painful, benign hyperplastic tumor of Pacinian corpuscles (mechanoreceptors responsible for sensitivity to vibration and pressure), sometimes linked to a history of local trauma. Other nerve swellings
Some of the benign varieties of neuroma, in the broadest sense of the term, are  not neoplasms. Traumatic neuroma follows different forms of nerve injury (often as a result of surgery).