(Photo: Jenn Peters/CC BY 2.0)

There came a time in your average ancient man’s life when he fell in love with a woman, and then he fell in love with another woman, and then he fell in love with another woman, and the maybe a fourth, fifth, and sixth, and then had sex with all of them for days, months, or years. 

In other words, polygyny was pretty common thousands of years ago. But, scientists said recently, around 10,000 years ago, something changed. Ancient humans became more monogamous. The theorized reason will be familiar to anyone who has sat through Sex Ed: sexually transmitted infections, or STIs. Chlamydia and gonorrhea, to be exact. 

All of that hot ancient human sex was killing the population, since many STIs can cause infertility. Which means, the Guardian reports, monogamists came to be regarded as the superior beings, fining or ostracizing their sex-happy brothers and sisters. Interestingly, monogamy also became dominant around the same time that humans adopted another more-sustainable pursuit: agriculture as the major means of food production.