https://www.msn.com/en-ph/news/other/primates-have-been-practicing-self-love-for-millions-of-years-study-finds/ss-AA1czCVl?ocid=msedgdhp&pc=U531&cvid=73ef9ae692384527a6c726ca5fd6b458&ei=117#image=18
Evolutionary Biology researcher Dr. Brindle & co-researchers (in Proceedings of the Royal Society B) found our "dirty little secret" must have evolved around 40 million years ago among primates. Research on 105 out of 272 primate species found that 74.5% of females and 87.4% of males do this "ancient, evolved trait." Zoo keepers and Primatologists were also interviewed.
Q: Why?
A: No conclusive answer. While there seems to be no fitness benefits, the theory pf high libido was challenged and the possibility of clearing pathogens was advanced. In females, it could possibly increase mate selection as it made the environment more hospitable to male sperm.
The self-pleasuring activity correlated with mating systems with multiple partners and incidences of STDs.