All posts by skim549

The Possible Cause of Humans’ Dark Skin

I came across an interesting article called “Odd Cause of Humans’ Dark Skin Proposed”, which states that skin cancer could have directly driven evolution of dark skin in humans. According to the study, people who have albinism, which is an inherited disorder that prevents human from making black or brown pigments, almost die of skin cancer at young ages. The study suggests that the early death of albino people might be the reason why early humans evolved dark skin.

Individuals with white skin are more vulnerable to skin cancer than people with dark skin. For instance, the Skin Cancer Foundation reported that black or brown pigmented skin has a sun protection factor of 13.4, which is relatively higher than that of the white skin (3.4). When hunter-gatherers began to lose their body hair to keep their body temperature cool during their excessive movement for hunting and gathering, they probably had light skin similar to the chimpanzees that has white skin under their fur. However, the article stated that humans evolved to have dark skin about 1.8 million years ago, then evolved light skin again after they migrated to higher latitudes from Africa.

The paper mentioned that skin cancer itself could have driven humans to have dark skin. In addition to the fact that the probability of developing skin cancer is significantly higher for albino people than for people with darker skin, almost all albino people in Africa developed skin cancer at early ages due to high prevalence of outdoor labor. These early cancers in lighter skinned humans lead them to die before reproduction, thus leaving mostly darker skinned individuals to pass on their genes to next generations. Although this idea is still hypothetical, it was interesting to see that cancer can influence human evolution.

How is Darwinian medicine useful?

The article, “How is Darwinian medicine useful?”, written by Randolph M Nesse caught my attention, because it emphasized the usefulness of evolutionary principles in medicine. I think it is appropriate to post a blog about this article, since it is very relevant to our course in understanding evolution in sickness and in human health.

Evolutionary medicine uses an evolutionary perspective to learn why our body is not perfect and why diseases exist in human population. The article lists main principles of Darwinian medicine for explanations to these questions. One of the examples that the paper mentioned is that the obesity and its associated problems arise because our body is not designed to live in the modern environment. Natural selection shaped human appetite regulation mechanism in order to survive the periods of famine based on the environment in the ancient times. Another example the author mentioned is the usefulness of human defense systems. Pain, nausea, cough, fever, vomiting, diarrhea, fatigue, and anxiety are all common defense mechanisms. These are not problems of themselves but it is an attempt to cure a problem. In addition, the paper explores the reason why humans are vulnerable to certain diseases.  One main reason stated is that natural selection is a random process with no direction. Moreover, the purpose for the natural selection is not to make an organism either perfect or healthy, but to maximize reproduction.

The paper emphasizes that the evolutionary view offers a richer view of the human body as a product of natural selection. I agree with this statement, and I believe evolutionary medicine will advance our understanding of the pathogenesis of various diseases. In addition, health professionals will be able to find more effective treatments by understanding why people get the disease rather than by simply considering how they get the disease.

Human Behavior and the Evolution of Sexually Transmitted Diseases (Synopsis)

Human behavior is closely related to the evolution of sexually transmitted infections, and this paper talks about the impact of the determinants of the STI on disease prevalence and transmission. The paper lists numerous factors that influence infectious disease emergence, including ecological change, human demographics and behavior, international travel and commerce, technology and industry, and microbial adaptation and change. Usually, the combinations of multiple factors that are happening simultaneously affect the transmission of the disease. In addition, the review states that the most direct influence on the evolution of sexually transmitted infections is sexuality and sexual behavior. In order to establish how infectious disease is spread, evaluation of individual sexual behaviors, types of exposure, and the impact of economic, social and structural factor is required. The large variation in age structure in a population has been recognized as crucial factor in the spread of STIs.

Although the determinants of STI prevalence such as urbanization, migration, and poverty were around for a long time, the relatively recent industrialization caused greater economic inequality and changing migration patterns that facilitated the spread of STIs into new populations. The norms may also influence the spread of STIs and size of the population susceptible to STIs. The paper also states that public health and medicine have both positive and negative effects on STI control. Behavioral interventions such as giving sex education, prevention of disease progression, and giving vaccines against HPV strains have been developed to give positive effects. In contrast, the introduction of oral contraceptives and Viagra has increased sexual risk behavior.

In conclusion, the writer emphasizes that human developments affect the spread of sexually transmitted disease agent and its pathogenesis. For example, economic and gender inequality, increased mobility, and global variability of sexual behavior affect the evolution of the agents by changing age structures, unequal gender rates, and globalization.


Citation: Nahmias, S. B. and Nahmias, D. (2011), Society, sex, and STIs: human behavior and the evolution of sexually transmitted diseases and their agents. Annals of the New York Academy of Sciences, 1230: 59–73. doi: 10.1111/j.1749-6632.2011.06079.x

The Role of Human Movement in the Transmission of Vector-Borne Disease

Human movement in terms of spatial and temporal scales is an important factor that affects many vector-borne disease by influencing exposure to vectors and transmission of pathogens. However, very little is known about individual human movement patterns and its effects on the dynamics of vector-borne pathogens. Studying and understanding the human movement and its influences on disease dynamics will enable scientists to come up with a better intervention and disease prevention. The study examines the role and the importance of human movement for pathogen transmission by mostly focusing on mosquito-borne virus. According to the paper, exposure that is due to human movement strongly influences the transmission dynamics of pathogens. For instance, the national or international individual movement drives pathogen introduction and reintroduction.

In order to study the dengue, the ‘activity space’ model was used to determine the few locations individuals visit and spend most of their time. By incorporating the knowledge of vector behavior and the outcome of the activity space model, scientists were able to identify places and individuals that contribute disproportionately to pathogen transmission dynamics. An interesting outcome that the activity space model presented was that the movements of pathogen transmission typically occur mostly at night when hosts are inactive. The paper emphasizes that a better understanding of the role of human movement on pathogen transmission is critical in predicting possible disease outbreak and coming up with better disease preventions schemes.

Citation: Stoddard ST, Morrison AC, Vazquez-Prokopec GM, PAZ Soldan V, KOchel TJ, et al. (2009) The Role of Human Movement in the Transmission of Vector-borne pathogens. PLoS Negl Trop Dis 3(7): e481. doi:10.1371/journal.pntd.0000481


Pathogen-Pathogen Interaction

In our recent discussion, we read about the principles of Hamiltonian medicine and discussed its usefulness in addressing human health problems and disease that are occurring worldwide. One of the domains that the paper mentioned and we talked about throughout the class was the microbe-to-microbe social interaction that affects the health of the human host. The “Pathogen-pathogen Interaction” paper was very interesting since I was able to learn about the impacts of the pathogen-pathogen interaction in human disease patterns and review specific examples of disease interactions.

The paper emphasized that infectious agents often do not act independently but cooperate with other pathogens to modify microbial phenotype to persist in the human host. The identified examples of pathogen-pathogen interaction are amoeba/bacteria interaction, HIV/HCV and HIV/fungal interactions, and tick-borne disease interaction. All of these identified case examples are known to impact the health of human populations in recent years.

Several pathogen-pathogen interactions show that pathogens can have significant impact on each other. Based on the pathways the pathogens interact upon one another, pathogens cause different impacts on the host. For example, one disease can promote the contagiousness of another disease by enabling them to penetrate to vulnerable area of the body. Another type of interactions between pathogens can accelerate the virulence of one another. For example, patients who are co-infected with HIV and HSV have a boosting of HIV load along with the higher providing of a portal for entry and exit of the virus.

Since the interactions between pathogens considerably impact the human health, a disease should not be considered as a distinct entity separate from other diseases. Instead, a health professional should pay attention to the syndemics model, which states that aggregation of diseases can have negative health affects, and recognize that interactions among diseases can influence the severity, transmission, and diffusion of the disease within human population.