6 million years is a long time.
You're also forgetting how phenotypic variation gets passed through the breeding process.
We turned only 6 naturally occurring breeds of dogs into hundreds in roughly 30 000 years, through selective breeding.
So yeah, in 6 million years, with environmental factors like temperature playing off of epidemics and infestations, plus mate preferences rounding out the process of our hair migration, I can imagine the changes to humans all happening naturally. No divine intervention required.
"it's not like not having eyebrows would kill you."
It could if the wrong parasites were to get into your eyeballs and make a nest.
You're not bullshitting about being from a cold country, eh?
I've seen things in places where there are no winters... 0_o
Also, as I was saying, science can't always explain the things that we and other creatures find attractive in a mate.
Like who wants to look up at a sexual partner with a furry forehead? Or no eyebrows at all...
Not on my list of hotness, thx.