The Mystery of Our Sexual Evolution

Our two closest living primate relatives, chimpanzees and bonobos, leave scientists puzzled over the origins of human sexual behavior.
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Question: You teach a class on sex evolution and human nature. Do you consider science sexy?
Laurie Santos: Oh yeah. I think science is very sexy.  I mean, there are a lot of big scientific puzzles when it comes to sexual reproduction.  You know, why do we choose to have sex?  It would be much easier just to kind of bud a little clone, boop! It saves lots of time and energy, waste, and disease and all of these things.  But you now, many, many animals in the animal kingdom actually put in the time and energy and risk the disease and you know, all the pains and heartache to actually sexually reproduce.  So, it’s a big puzzle in the evolutional biology.  And then when you start looking at the different ways animals go about sexual reproduction, all the different way they go about finding mates and you know, strategizing to get mates and convincing individuals to mate with them, and so on, you know, it’s kind of just fascinating to see all the strategies that are out there.  And in many ways, humans are real outliers in this.  So, it’s fun to look at the techniques that we see across the animal kingdom.
In what ways are humans outliers when it comes to sex?

Laurie Santos: Well, humans are a very funny species when it comes to sexual reproduction.  We’re very weird relative to our closest living primate relatives, the other apes.  So, we, for the most part, seem to be a pair-bonded species.  In other words, a male and a female get together and form this pair bond and kind of cooperatively rear their young.  There’s nothing really like that in the rest of the great ape species. In chimpanzees, our closest living relative, you see lots of multi-male, multi-female reproductive behavior.  Meaning, you know females are mating with multiple males; males are mating with multiple females.  But also in chimpanzees, you also see that males and females just aren’t interested in sex during the period where females aren’t very reproductive. 
So, female chimpanzees advertise their receptivity, that period when they’re fertile by having this huge sexual swelling that kind of blows up and males are only interested during this period.  Somehow humans, unlike our closest living relatives, have kind of gotten rid of this.  You know, human females of course, don’t go around advertising with you know, really robust, blown-up, red, sexual skins when they’re receptive, which is a bit of a puzzle.  You know, why did we lose this trait just in the last 7 million years?  You know, these are the kinds of things that hang on for long periods of time in primate evolution, so the fact that we got rid of this very quickly is a bit of a puzzle.  So weird kind of weird relative to other primates regarding our sexual behavior.
Our other closest... so we have two closest living primate relatives, it’s a tie between chimpanzees, so common chimpanzees, and their sister species, bonobos.  And chimpanzees and bonobos greatly differ in their own sexual behavior.  Bonobos are sort of the "make love and not war" kind of primate.  They tend to be way less violent and aggressive than chimpanzees and one of the behaviors that allows them to overcome that is the fact that they are very sexually oriented.  So, in a lot of the situations that would cause aggression in chimpanzees, bonobos actually resort to having sex to kind of diffuse any of the anxiety or stress about the situation. 
So, it’s another puzzle for humans is that we have one of our closest living relatives is very aggressive, very warlike, they’re kind of a lot of the stuff that explicitly say we don’t like about the human species. Our other closest living relative is very peaceful, you know, very kind of social sexual-oriented has lots and lots of female bonds.  And there’s a bit of a puzzle in  the field of what parts of these two guys did the humans get, you know, are we more like chimpanzees, are we more like bonobos?  Still big puzzles in the field of primate cognition.

Recorded May 21, 2010
Interviewed by Andrew Dermont