Darwin is often invoked to legitimize new (and old) ideas and people who want to respond to criticisms that music is an invention rather than a product of evolution are happy to take support where they can get it, but that is not much of a defense. Darwin's second book had to address evolution directly, a move he carefully avoided in "Origin of Species", because language was the mechanism critics such as Friederich Max Müller, professor of linguistics at Oxford University, used in attacking the whole theory: "language is the Rubicon which divides man from beast, and no animal will ever cross it … the science of language will yet enable us to withstand the extreme theories of the Darwinians, and to draw a hard and fast line between man and brute."
If language was the great divide between man and other animals a 'multicomponent' view of language had to be in effect for evolution to meet critics. While mammals have music as a rarity, it is more common in birds. This went well with Darwin's insistence that the issue should not be human evolution but rather a broad theory of evolution in which general principles like sexual selection provided explanations of unique human traits.
The old boy had something going for him. Basically in a few pages he laid out that evolution brought a general increase in mental abilities but then sexual selection brought about the capacity for more complex vocal control, what we know as singing. Attributing meaning to the 'songs' by early man spurred further increases in intelligence.
Today we are discovering a lot more about music and the brain but some of it feels like old ground. We get that electricity is the 'currency' of the nervous system and that mapping changes in that flow can tell us something but, like with fMRI studies, it is difficult to make it anything more than correlation.
Correlation is not always a bad thing, of course. Nina Kraus, Professor, Neurobiology&Physiology, Otolaryngology at Northwestern, analyzes learning-associated brain plasticity and says that music is a multisensory auditory experience, which we all instinctively know, but that stimulus and response are too closely correlated to be ignored. As an example she used 'da' as a music recording and showed it as brain perception. She did this with three intonations of 'da' and it had its value but suddenly the three "da"s became Deep Purple's "Smoke on the Water" and a much different thing, to the delight of a room full of cynical journalists.
Gottfriend Schlaug of Harvard Medical School does something a little more direct that may be circumstantial but is a powerful exclamation point for a 'music came first' argument. His work with patients who have suffered severe lesions on the left side of their brain showed that while they could not speak - no language skill as we might define it - they were able to sing phrases like "I am thirsty", sometimes within two minutes of having the phrase mapped to a melody.
This 'melodic intonation therapy' might help patients who lack spoken language ability to communicate in ways they now lack - the cost is enormous, as you can imagine, but most new research is. Of course, the big question is, how much does music relate to other complex cognitive abilities?
Hard to say. Nouns and verbs have no musical counterpart and music lacks linguistic syntax; as Aniruddh Patel, Senior Fellow at the Neurosciences Institute puts it, "who did what to whom." So for every piece of evidence showing overlap there are likely as many examples of cases where dissociation is just as obvious. Patel seeks to unify those inconsistencies in a hypothesis he called "Shared Syntactic Integration Resource Hypothesis" (SSIRH), saying basically that language and musical representations may be stored in separate areas, so can be damaged independently, but that there is overlap in the networks themselves.
Is that evidence for evolution of musical ability first? Not so much. Schlaug's results, for example, are preliminary and part of an ongoing clinical trial so he does not overreach their significance. But it is sure interesting to see people who cannot communicate with words use words in a melody.
Musical syntactic processing in agrammatic Broca's aphasia, Patel et al, http://dx.doi.org/10.1080/02687030701803804
"The Singing Neanderthals: The Origins of Music, Language, Mind and Body" by Steve Mithen, Weidenfeld & Nicolson: 2005.
Musical protolanguage: Darwin's theory of language evolution revisited by W. Tecumseh Fitch