Nice to finally see this comment after much text implying that eyebrows are some crazy useless feature of humans. The ability of humans to sweat for cooling is one critical feature that has helped the species survive. The full body sweating that allowed humans higher endurance than any other animal on the African plains would have been much less useful if they were continuously blinded by the salty sweat pouring down off their foreheads.

FWIW, I was specifically trying to leave the actual purpose of eyebrows out of the argument.

We will probably never have a definitive answer to the question of why we have eyebrows. The best we'll ever do is some decent hypotheses. So if the argument is that eyebrows serve function X, but we find out in the future that eyebrows do not, in fact, serve function X, then a naive observer will think that eyebrows did not evolve to serve any purpose and are, in fact, just a random feature.

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.

P.S. I'm skeptical about the sweat in the eyes explanation for eyebrows. Anecdotally, my eyebrows have never kept sweat from getting in my eyes. And less anecdotally, if you spend a few minutes looking at somebody's face and imaging sweat rolling down their forehead, you'll see immediately that eyebrows aren't very effective for sweat diversion.

My money is on protecting our eyes, which are our most important sense organs. If you google image search "mma black eye", you'll see what I think is kind of remarkable: despite serious trauma to the surrounding areas, most people's eyes are intact.

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.

One might get some idea if the eyebrow was evolving due to some usefulness by looking at various isolated tribes living with different cultures and within ecosystems/climates (would be much easier to do if one had a time machine).

I tend to agree with you that a lot of things that are called "science" are mainly story telling. There is "physics envy" for a reason. Physics focuses on a very small subset of reality that have observations that are true with uncertainties of 5, 6, 9 sigmas. This is the level of truth one needs to build up a models of complex things that match reality almost all the time. Physicists at LIGO or CERN don't publish in journals some observation with P=0.05 and expect others to use it. Hopefully the reproducibility crisis in psychology will get many truth seeking fields to spend more time on understanding what is true at the lowest level. Feynman had a good essay about this issue many years ago about rat studies[1,2].

[1]Feynman's Caltech speech:http://calteches.library.caltech.edu/51/2/CargoCult.htm [2]More recent short essay on the rat study: https://www.realclearscience.com/blog/2014/02/the_rat_experi...

Your mistake is in thinking the null hypothesis is that a species-wide, complex morphological feature is non-adaptive.

Sure thing. Like most "things" in complex systems, a certain feature will have many functions and can evolve subtly to have yet another one. My eyebrows definitely help keep sweat out of my eyes, especially when the air is not very humid. The sweat hits the hair, wicks, and evaporates. Since human ancestors used to be covered in hair, the eyebrow is more of a patch of hair that was useful enough to be selected for when most of the rest of out hair was being selected out.

I guess I consider the eye brow just the hair on the surface and maybe the skin/muscles to move it, not the whole bone structure underlying it.

Agree. It’s probably partly about sweat, partly about protecting the eyes, and partly about communicating facial expressions.