They dont need to be literal pseudo-vaginas in a fruedian sense. Evolution found an opportune place to show off some red, which is the color of sex. This also ties in with blushing. Its all fairly sexual, I dont see why you cant accept it. All these areas become redder when aroused. It should be obvious.
But it’s not obvious (to me). I’m thinking along the lines of dracoi and Quercus, though I’m not saying that the sexual signal component couldn’t be part of the story. Our lips are extremely important to us in terms of communicating and it would seem to me that there would be a strong evolutionary pressure to make the lips sensitive, mobile, and prominent on the face. Thinner skin and more tissue (which needs more blood flow, I would assume) would be quite helpful to all three of these “goals”. Thin skin with lots of blood flow would produce reddish lips in paler-skinned humans which would help make the lips more prominent. Darker-skinned humans, who evolved darker skin to protect against UV rays, wouldn’t have red lips, but the benefit of UV protection would probably mitigate that. (Dark-skinned people’s lips aren’t necessarily the same color as the rest of their face anyway and other things help make human mouths more prominent, like fleshy lips).
This is not to say that sexual signals didn’t play a role in the evolution of noticeably red lips in humans (where it was possible due to a relative lack of pigment in the skin of certain genetic lines of humans), but I don’t see why this should be the primary or only thing driving the process. I rather suspect that the sexual selection part of things became more important (if it ever factored in the equation) later on after the process was started, once lips were already gaining prominence for other reasons.
As for women having redder lips, I have a feeling that many people’s idea of the color of women’s lips is skewed by the use of lipstick. I’ve seen plenty of women who have naturally pale lips and plenty of men who have naturally red lips.
As for lips becoming redder when a person is aroused, when people become aroused blood flow increases to the entire body, including the lips; you don’t need any other explanation than that. (Although given that redder lips are a sign of arousal, it seems like humans would pick up on that fact and that could cause some evolutionary pressure towards redder lips or lips that become more noticeably red when aroused).
I always get that one wrong.
Regards,
Banana Mouth
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Ah, those were the days…