For information and evidences of humans evolving out of foreskin visit this blog post: https://circumcisionandsex.wordpress.com/2017/03/23/humans-are-evolving-out-of-foreskin/
In 2007 the World Health Organization made recommendations for countries with a prevalence of heterosexually transmitted HIV to institute circumcision programs as part of a comprehensive package to reduce HIV prevalence . The World Health Organization felt that the benefits of male circumcision were significant enough to personally provide free circumcisions through Africa and set a goal of 90% circumcision rate among non-Muslims in Africa . The World Health Organization has provided the following three biologically plausible argument for the benefits of circumcision:
1). The inner part of the foreskin lacks keratinization that the outer foreskin and skin of the penile shaft has, which could make the inner foreskin more susceptible to HIV and other STD infection.
2). Foreskin is more susceptible to micro-tears during sex then other parts of the penis which allows for easier entry of HIV and other STD’s.
3) The environment under the foreskin could allow for pathogens to persist better then other areas of the penis.
Given that the World Health Origination now recommends circumcision in places that constitute most of the worlds population, and is even personally overseeing the mass circumcision of countries that already didn’t practice circumcision and are hit worst by the HIV epidemic, it is natural to ask why would men have it if they are better off without it? The first thing that should be states is that there are some ethnicities where shortened foreskin ‘naturally circumcised’ is very common and this is discussed in the last paragraph of this post as well as this blog post on evidence that humans are evolving out of foreskin: https://circumcisionandsex.wordpress.com/2017/03/23/humans-are-evolving-out-of-foreskin/ . The second thing that should be stated is that foreskin is apart if humans phylogenic heritage, meaning that our ancient ancestor had it so now humans and our relatives have it. This is the reason why humans have many things, even ones that serve no apparent use. For example, we have hair on our arms and legs because our ancient evolutionary ancestor did. Even though it does not serve a purpose for humans today, evolution is not able to ‘get rid of it’ in humans. In this case there is little selective pressure for a human with or without arm hair, so arm hair persists. Wisdom teeth and the appendix are other examples, ancient humans probably needed both of these things because our past diet required them. They serve no use today though, and can even be somewhat detrimental to our health. Because the way humans live has undergone such a radical change in such a short amount of time, there has not been a long enough time for humans to adapt to our new environment and evolve out of something like wisdom teeth. For a last example, look at the structure of the human throat. This is an example given in many books on evolution. Both our esophagus (food passage) and trachea (air passage) share an initial common path (the pharynx), which leaves humans prone to choking. The answer to why the human throat has this poor design is because it originated back when our ancestors still lived in water, and did not have the problem of choking. Even though humans ancestors left water over 300 million years ago, this trait has persisted in mammals (including humans), and many other land animals, probably because it is a difficult task for natural selection to restructure the throat.
The following is an evolutionary argument for why men have foreskin even if it is beneficial to be circumcised. (a more detailed discussion of this is done in another blog post on this site here: https://circumcisionandsex.wordpress.com/2017/03/23/humans-are-evolving-out-of-foreskin/ ) Now, a few functions of foreskin have been put forth, mainly in relation to sexual pleasure. Since 2007, when the World Health Organization began to recommend circumcision, many studies on adult circumcision have taken place. They have revealed that adults who undergo circumcision rarely report a decrease in sexual pleasure, in fact more men report an increase in sexual pleasure and almost all men report no change or an increase in pleasure. This is backed up by a meta-analysis by Danish researched which found grade A/B evidence that circumcision increases penile sensitivity , a Chinese meta-analysis which found circumcision does not decrease sexual pleasure or function and go on to purpose that many men report an increase in sexual pleasure because the glans of the penis make better contact with the vagina after circumcision , a meta-analysis by Australian researchers that found studies of high quality unanimously find circumcision does not decrease sexual pleasure , the American Academy of Pediatrics 2012 report on circumcision which stated that under no definition of sexual function or pleasure does circumcision decrease sexual function or pleasure , the Center of Disease Control 2014 guidelines on circumcision which states that few men report a decrease in sexual pleasure, more men report an increase, and almost all report either an increase or no change in sexual pleasure after undergoing circumcision , and many individual studies [9-15].
There is a problem with how natural selection is influenced by sexual transmitted diseases. At the heart of natural selection is that the most fit humans will have the most children and thus the better traits become more prevalent. So say one human is more resistant to a disease like tuberculosis than another human, then the human that is more resistant to tuberculosis should be more likely to mate and have children, thus if the resistance to tuberculosis is contained in his DNA, it will get past on to his many children and the gene will become more prevalent. The problem with STD’s is that a man already needs to be having sex, and therefore passing on his genes, for resistance to STD’s to matter. In addition, most STD’s typically lay dormant in a persons system for many years before any symptoms are shown (HIV takes many years to turn into AIDS, HPV usually takes over 20 years to turn into cancer, ect…). That would mean that even if man A has a natural resistance to HIV that man B does not have, man A really doesn’t have much of an advantage to having children over man B, and so natural selection is unable to select for the man with a natural resistance to STD’s like man A. So both man A and man B are able to pass on their genes equally as well. This would mean that natural selection would have little power to push human evolution in the direction towards men becoming natural circumcised, In spite of the weak strength though, there is strong evidence that natural selection favors men with shortened foreskin which will be explained in the next few paragraphs.
Studies from some ethnicities indicate that humans are in fact evolving out of foreskin. One study  measured the foreskin length of a large cohort of uncircumcised Chinese men using the following classification.
The study found that the general Chinese male population (‘the control group’) fell into the following classifications.
As is shown, 22.9% of men in the general population, (‘control group’) have level 1 foreskin length, which is naturally circumcised. Another 18.2% of the population has their foreskin cover less than half of the glands of their penis, which is quit close to being naturally circumcised. All together over 40% of Chinese men are more or less naturally circumcised. This study as well as others [17,18] also find that men that test positive for STD’s have longer foreskin on average, and the difference grows greater when accounting for frequency of sex and for age. All together these studies show that natural selection favors men with shortened foreskin and humans are heading towards being natural circumcised.
 World Health Organization and UNIADS: New Data on Male Circumcision and HIV Prevention: Policy and Programme Implications http://apps.who.int/iris/bitstream/10665/43751/1/9789241595988_eng.pdf http://www.who.int/hiv/pub/malecircumcision/research_implications/en/
 A Framework for Sub-Saharan Voluntary Medical Male Circumcision. http://www.who.int/hiv/pub/malecircumcision/vmmc-policy-2016/en/
 2007 World Health Organization and UNIADS: Male Circumcision, Global Trends and Determinates of Prevalence, Safety, and Acceptability http://www.unaids.org/sites/default/files/media_asset/jc1360_male_circumcision_en_0.pdf
 Male Circumcision Does Not Result in Perceived Male Sexual Function – A Systematic Review. Shabanzadeh et al http://www.danmedj.dk/portal/page/portal/danmedj.dk/dmj_forside/PAST_ISSUE/2016/DMJ_2016_07/A5245
 Effects of Male Circumcision on Male Sexual Functions: A Systematic Review and Meta-Analysis. Tian et al https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3881635/?report=reader
 Does Male Circumcision Affect Sexual Function, Sensitivity, or Satisfaction? A Systematic Review http://www.jsm.jsexmed.org/article/S1743-6095(15)30172-7/abstract
 The American Academy of Pediatrics 2012 evaluation of circumcision:
 2014 Center of Disease Control Guidelines on Male Circumcision https://www.cdc.gov/hiv/pdf/prevention_research_malecircumcision.pdf
 Adult Male Circumcision Performed with Plastic Clamp Technique in Turkey. Results and Long-Term Effects on Sexual Function. Senel et al. http://www.urologyjournal.org/index.php/uj/article/view/1802/693
 Longer-Term Follow-Up of Kenyan Men Circumcised Using the ShangRing Device. Feldblum et al. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4569077/?report=reader
 Does Circumcision Affect Male’s Perception of Sexual Satisfaction? Cortes-Gonzales et al. https://www.ncbi.nlm.nih.gov/pubmed/19955598
 Effects of Adult Male Circumcision on Premature Ejaculation: Results from a Prospective Study in China. Gao et al. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4324807/
 Adult Male Circumcision: Effects on Sexual Function and Sexual Satisfaction in Kisumu, Kenya. Krieger et al. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3042320/?log$=activity
 Effects of Circumcision on Male Sexual Function: Debunking a Myth? Collins et al. https://www.ncbi.nlm.nih.gov/pubmed/11956452
 Long-Term Follow-Up for Shang Ring Male Circumcision. Cheng at al. https://www.ncbi.nlm.nih.gov/pubmed/24824249
Histological Correlates of Penile Sexual Sensation: Does Circumcision Make a Difference? Cox et al. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4498824/
 Redundant Prepuce Increases the Odds of Chronic Prostate/Chronic Pelvic Pain Syndrome. Zhao et al. (the table with foreskin lengths is the 4th image and 3rd table given in the paper. it is labeled ‘OR for CP/CPPS with respect to foreskin length’. The general Chinese population is the control group.) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4215657/
 Acceptability of Voluntary Medical Male Circumcision Among Male Sexual Transmitted Disease Patients in China. Wang et al. https://www.ncbi.nlm.nih.gov/pubmed/26905739