You wrote: IMO, a more thorough argument is to point out that the only way species-wide, complex morphological features like eyebrows can possibly exist is if t

You wrote: IMO, a more thorough argument is to point out that the only way species-wide, complex morphological features like eyebrows can possibly exist is if they serve an important purpose, even if that purpose has a miniscule impact on the outcomes of any given organism. This argument is resistant to any given explanation for the purpose of eyebrows turning out to be wrong. Plus it gets to a deeper understanding of evolution. That's not an argument. It's a tautology. In any field other than evolution, such a statement would be ridiculed. There are infinite reasons that could explain eyebrows. Design. Alien intervention. Lamarckisn processes. The Will of the Flying Spayhetti Monster. Out of this myriad of possibilities, if you suggest they arose because and persisted due to conferring a survival/reproductive benefit, the burden is on you to demonstrate such. This can be done in two ways. The first is by storytelling... creating a Kipling-esque 'just so' story involving, perhaps, sweat or sexual attraction or whatever. The second method is by using science. Namely, measuring the degree (if any) that eyebrows effect survival/reproduction. From that number, one can backtrack and determine mathemstically how many generations it would have likely taken for the phenotype to become ubiquitous, and see if it lines up with other assumptions. To summarize, before conjuring up 'just-so' stories, find 1,000 people with no eyebrows, a 1,000 with bushy eyebrows, and show me a measurable difference in their fecundity rates. Or, use chimps. Or, dogs. Doesn't matter. As mentioned, I personally don't think eyebrows have squat to do with survival/reproduction... you can't counter my null-hypothesis by assuming a priori that every phenotype must confer some sort of advantage.