Do the damaging effects of the Epstein Barr virus, the cause of the Kissing Disease, weaken the idea that kissing is an adaptation favored by sexual selection?
– by Paul Ewald
In modern societies kissing is a widespread and important aspect of social interactions. It appears to be a sexually selected adaptation that facilitates mate evaluation, pair bonding, and/or sexual activity. This evolutionary explanation is paradoxical because kissing seems unnecessary for mate selection and is associated with transmission of diseases, which might outweigh the evolutionary advantages of kissing. The most widely recognized of these diseases is infectious mononucleosis, commonly known as the Kissing Disease. Although people with mononucleosis experience fever, body aches, and extreme fatigue, the illness itself is almost never life-threatening. People generally recover within a few weeks without any specific treatment. If that were the whole story, suffering infectious mononucleosis as a consequence of kissing would not be compelling evidence against the hypothesis that kissing is a sexually selected adaptation. However, the mounting knowledge about EBV-associated diseases indicates that kissing has more serious consequences.
The Epstein Barr virus was first accepted as the primary cause of infectious mononucleosis in 1968. About a decade later its major route of transmission in affluent populations was generally recognized to be salivary exchange during intimate kissing. Over the past half-century, EBV has been associated with severe diseases such as cancers, multiple sclerosis and systemic lupus erythematosus. Together with infectious mononucleosis, these diseases would seem to generate fitness costs that could offset or even completely negate the mating advantages of kissing, and make an adaptive explanation of kissing questionable.
If these diseases were not present during the evolutionary history of humans, this concern would disappear.
Comparisons across populations help clarify whether intimate kissing has been associated with these diseases during our evolutionary history and therefore with a high fitness cost. Specifically, if these diseases tend to occur only in modern populations with particular attributes, then their presence would not weaken the hypothesis that kissing evolved as a sexually selected adaptation.
The existing evidence indicates that infectious mononucleosis occurs primarily in modern affluent populations in which environmental disinfection is emphasized. In areas with poor hygiene, EBV infections tends to occur prior to adolescence and rarely causes infectious mononucleosis. When infectious mononucleosis does occur, it tends to be more mild than when it occurs in affluent populations. The general explanation for this difference is that kids tend to be exposed to low doses of EBV in areas with poor hygiene and therefore develop some immunity that protects them from the high doses that are associated with intimate kissing. Comparative evidence therefore indicates that infectious mononucleosis has been largely a recent consequence of EBV infection resulting from hygienic activities that increase the probability of severe infections from kissing. This conclusion has ramifications for understanding the fitness costs of kissing that arise from the more serious EBV-associated diseases.
EBV has been strongly associated with breast cancer. Comparisons of EBV positivity in breast cancers relative to control breast tissue together with effects of EBV on cellular proliferation indicate that EBV plays a causal role in about 20% of breast cancer. EBV is also now generally accepted as a cause of multiple sclerosis. The associations of EBV with breast cancer and multiple sclerosis are statistically significant only among patients who had experienced infectious mononucleosis. Because mononucleosis is a modern disease associated with delayed EBV infection, the associations of these diseases with mononucleosis indicates that they too are modern diseases associated with delayed EBV infection.
EBV is now a generally accepted cause of about half of Hodgkin’s lymphoma. Breast cancer is associated with young adult Hodgkin’s lymphoma, which tends to occur within two to three years after infectious mononucleosis in EBV positive tumors. EBV-negative tumors show no association with time since infectious mononucleosis. Like breast cancer, Hodgkin’s lymphoma therefore is associated with the more severe EBV infections in modern affluent populations.
EBV is now generally accepted as a cause of systemic lupus erythematosus. Although studies have not assessed whether lupus is restricted to individuals who have had infectious mononucleosis, lupus, like multiple sclerosis, tends to occur in affluent modern populations. The main genetic risk factor for EBV-associated multiples sclerosis is also a risk factor for lupus. It codes for a protein that EBV uses as a receptor to enter cells and became prevalent about 5000 years ago. This information suggests that lupus, like multiple sclerosis, would not have been a factor disfavoring kissing during the broader span of human evolution.
These findings indicate that the relationship between kissing transmitted EBV and these severe diseases is relatively recent, and that these diseases therefore did not generate fitness costs that would have selected against intimate kissing during the evolutionary history of Homo sapiens. The flip side of this conclusion is that intimate kissing is now more unsafe than it used to be during our evolutionary history, and humans have not evolved to be sufficiently wary of the present-day dangers of intimate kissing.
