HumanVariationBodyInfectiousDisease.pptxHuman Variation
Body Size & Infectious Disease Resistance
So far we have considered skin color as an example of human variation. This lecture focuses on other aspects of human variation, including weight, height, and susceptibility to infectious diseases. Why are some of us bigger? Taller? More likely to die at a young age? These are things we will be considering here.
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Why do we vary so much in one species?
Environmental and cultural pressures require the body to adapt to stress in order to maintain a homeostasis.
Sometimes these changes are the result of genetic mutations.
Others are the result of acclimatization. These are ways your body responds in your life time. If they happen during embryonic development, then they can become permanent.
Think back to our discussion about variation. Why does variation exist? Remember that if we’re all identical, then one disease or one change in climate could be lethal to us all. Variation allows us to have different traits. Much of our variation is genetic, but some of the ways that we survive has to do with out individual abilities to acclimate to a new environment. When it comes to temperature for instance, humans have many ways of maintaining body temperature without need any genetic evolution.
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Response to Heat
In order to maintain body heat as a warm blooded creature, we have several biological forces working.
Sweat glands allow for evaporative cooling
This can cause dehydration. 1 liter of water is 1.5% of body weight, 10% body water lost is deadly.
Vasodilation is the widening of capillaries on skin surface to increase blood flow to the skin and lose heat.
Ex. Red cheeks
Heat maintenance is important for normal functioning. We utilize sweat glands to release heat particles, but this can be dangerous if you have no water. We also have an acclimate technique called vasodilation, where our capillaries dilate in order to release heat particles.
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Response to Cold
Heat production: shivering and increased metabolism by breaking down more nutrients are acclimate ways to stay warm.
Heat retention: Vasoconstriction tightens the capillaries, preventing heat loss. This does not cost energy.
Subcutaneous fat layers.
Sometimes we want to keep heat in. We can produce heat by shivering and increasing our metabolism through exercise, but this costs a lot of energy. We can also use an acclimate technique called vasoconstriction. In this case capillaries constrict or close up to prevent the loss of heat particles.
Fat is also an important way for us to retain heat.
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Bergmann & Allen’s Rules
Bergmann and Allen’s rules work across species. One way we have evolved to deal with heat maintenance is in our size. Longer limbs allow us to release heat because there is more surface area on the skin. Shorter limbs and stockier bodies have less surface area, which allows them to retain heat. Our bodies have evolved to vary in size in order to help retain and/or release heat at appropriate times.
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“An Anthropological Perspective on Obesity”
Answer these questions using the reading above:
Why would increased fat have been naturally and sexually selected in early humans?
Why might living societies value being overweight more than they value being underweight?
What could be causing the dramatic incline in rates of obesity around the world?
Please read “An Anthropological perspective on Obesity” article for this discussion and answer these questions. These will be explored in the upcoming slides.
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“An Anthropological Perspective on Obesity”
What causes weight gain?
There are about 113 genes potentially associated with weight maintenance, 18 have been scientifically confirmed.
Sexual dimorphism in weight
“Adult men are larger than women in stature (+8%) and total body mass (+20%), whereas women have more subcutaneous fat as measured in skinfold thickness” (414). Why?
Why would increased fat retention have been favorable for us in early human societies?
Weight gain isn’t as simple as just being fat. How you build, maintain, and retain fat is genetically influenced. However, so are your eating habits, eat desires and likes, as well as the signaling that happens in your body to let you know that you’re hungry or full. We have about 113 potential genes involved in all the facets of how we eat, how we digest our food, and what we do with the nutrition from that food.
What we know is that men and women differ in size.
The following resource is a book that discusses genes associated with weight in great detail. http://www.ncbi.nlm.nih.gov/books/NBK19935/
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Evolution of Weight Retention
“In 28.7% of the societies, food shortages are rare, occurring every 10-15 years, whereas in 24.3% they happen every 2 to 3 years. Shortages occur annually or even more frequently in 47% of the societies” (417).
Fat retention is selected as favorable for survival in instances of food shortages, especially for women.
Would it have been sexually selected? Why?
“Malcom described the custom of fattening huts for the seclusion of elite Efik pubescent girls in traditional Nigeria. A girl spent up to two years in seclusion before marriage, and at the end of this rite of passage she possessed symbols of womanhood and marriage” (420).
“….the desirability of plumpness is found in 81% of societies in which there is data.”
It appears that the genes associated with retaining nutrition and thus storing it in fat cells, relates to the fact that our early ancestors had quite a few food shortages. Being able to retain weight during these times would have been crucial and adaptive and thus would have been naturally selection.
However, people likely began to sexually select for heaviness as well. A male or female who could retain a bit of weight would have been seen as a healthy mate.
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What makes this once adaptive trait, now maladaptive?
In 2014, 39% of adults 18 and over were overweight, 13% were obese.
Most of the world’s population lives in countries where overweight and obesity kills more people than underweight.
Modernization:
“A classic natural experiment compared the diet and health of Polynesian populations at different stages of acculturation: the prevalence of obesity in the most traditional island was 15.4%, for a rapidly modernizing population, it was 29.3%, and for urban Maoris it was 35.4%” (415).
http://www.who.int/mediacentre/factsheets/fs311/en/
The issue with adaptation is that something can be adaptive once, but then become maladaptive once the environment changes. Think back to our Santanic leaf eating gecko. Perfectly adapted to that tree, but if the tree, goes, the species has nothing it’s adapted to and would likely die off very quickly. In the case of fat, our ability to retain fat was once adaptive de to food shortages, but is now maladaptive due to access to constant food, and changes in our diets.
Your reading suggests that the more a society modernizes to be like the U.S. the more quickly their obese populations rise.
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Obesity in the U.S.
Consider this map of obesity in the U.S. What do you think has caused this?
http://www.cdc.gov/chronicdisease/resources/publications/aag/obesity.htm
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Obesity & Diabetes
Obesity has been connected with diseases like type 2 diabetes, as well as complications like high cholesterol, heart disease, infertility, and certain cancers.
In 2010, diabetes was the 7th common cause of death in the U.S., which is likely underestimated.
In 2014, 9.3% of the American population has type 2 diabetes. Non-hispanic whites have a prevalence of 7.6%, while American Indians have 15.9%.
Obesity is something that has often been directly correlated with diabetes. The historical connection has been that people make bad exercise and diet choices, then they become overweight and eventually they develop diabetes. Diabetes has become more and more common our species and often times has been called a “disease of civilization” that becomes more common due to our modern ways of life. A medical anthropologist would be interested in understanding why this disease evolved and why some are more prone than others.
http://www.tedmed.com/talks/show?id=18029
http://www.cdc.gov/chronicdisease/resources/publications/aag/obesity.htm
http://www.cdc.gov/diabetes/pubs/statsreport14/national-diabetes-report-web.pdf
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Why would diabetes have survived evolution?
Consider the argument that foragers needed to retain fat in order to survive food shortages.
Consider also that a forager diet was high in protein, low in sugar.
Diabetes is caused by the bodies inability to utilize insulin properly in the body, leading to high blood sugar.
Insulin resistance
https://sciencelife.uchospitals.edu/2013/03/07/how-type-2-diabetes-survived-evolution/
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Diabetes and…Neanderthals?
Gene SLC16A11 that codes for protein that controls fats associated with diabetes.
Interestingly, some research suggests that diabetes is something we may have inherited from neanderthals. They have a gene we know codes for the development of this disease. We’re not certain how it impacted them or if it ever had any benefits.
http://evolution.berkeley.edu/evolibrary/news/140204_diabetes
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This TED talk is a fascinating look at the current research on diabetes. This belief that poor life choices obesity diabetes is destroyed in this film. Watch this and identify, what is the new order of events and how does this change what we know about the disease itself as it relates to obesity?
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Poor Applied Strategies
Since we’re spreading obesity around the world, how do we stop it?
“In an obesity prevention campaign in a Zulu community outside of Durban, one of the health education posters depicted an obese woman and an overloaded truck with a flat tire, with a caption “both carry too much weight.” Another poster showed a slender woman easily sweeping under a table next to an obese woman who is using the table for support; it has the caption “who do you prefer to look like?” The intended message of these posters was misinterpreted by the community because of the cultural connection between obesity and social status. The woman in the first power was perceived to be rich and happy, since she was not only fat, but has a truck overflowing with possessions. The second poster was perceived as a scene of an affluent mistress directing her underfed servant” (419).
It’s vital that we know how a disease is caused and why it evolved in our species, because it helps us treat it. Medical anthropologists are attempting to solve this problem, and we can start by looking at poor applied strategies and how they have failed. Why did this strategy fail?
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Height
Height is determined by about 50 genes, but also has a huge environmental influence.
In 1850, Americans were the tallest people in the world, averaging 5’6’’. Today we are 3rd at 5’8’’, with the Dutch moving from an average height of 5’4’’ to 5’10’’.
How can we explain this?
Height is also something that varies quite a bit in our species. It is especially influenced by environmental factors like nutrition. Scientists show that height is something you’re genetically predisposed to, but that heights have changed over time around the world.
http://www.ncbi.nlm.nih.gov/pubmed/19818695
http://courses.washington.edu/bioa101/articles/article38.pdf
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“The Tall and the Short of it”
In both the U.S. and the Netherlands, life got better, height increases as a result.
Research shows Mayan refugees saw a 2.2 inch increase in ONE generation after moving to the U.S.
Height is the product of plasticity, if a girl is undernourished and suffers poor health, the growth of her body and reproductive capabilities will be reduced. She is then likely to produce low birth weight children who will also be stunted in growth.
Why did the Dutch beat us out?
“In 1990 only 4% of Dutch babies were born at a low birth weight, compared to 7% in the U.S. For white Americans the rate was 5.7%, and for black Americans the rate was a whopping 13.3%” (Bogin, 1998, p. 184).
Infectious Disease Resistance
Infectious diseases are pathologies caused by bacteria and viruses. Bacteria are living cells that can survive/replicate on their own, viruses are non-living and can survive only by taking over host cell’s machinery.
Another way in which humans vary is in our susceptibility to disease and how our bodies respond to diseases. For the sake of our discussion, we’re going to consider viruses and bacterial infections. Viruses and bacteria differ in many ways. Viruses are non-living. We consider them to be non-living because they cannot self-replicate, they cannot break down food, and they cannot survive without a host. Bacteria is considered to be alive because it replicates, it breaks down its own food, and they can survive without a host.
In fact, some of the first life on earth was bacterial. Also, a large percentage of your own body is bacterial. You have a microbiome that includes all the bacteria in your body that helps you digest your food, fight off bad infections, etc.
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Evolution of Pathologies : “The Arrow of Disease”
Microbes have developed amazing ways to spread themselves. Some symptoms, like legions on the skin from smallpox, are symptoms of disease that also spread the disease through contact.
Salmonella bacteria waits to be eaten by hosts.
Some get free rides through the saliva of other animals like mosquitos.
Influenza gets you to cough, sending the virus to other agents.
Please read “The Arrow of Disease” article for this discussion.
This article starts by discussing all the ways in which our bodies have evolved to to fight infectious diseases. But it also discuses the issue of diseases evolving. Both viruses and bacteria evolve constantly, and they have themselves developed unique ways to spread themselves and be more virulent.
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Biological Responses to Pathologies
Raising body temperature through fever to kill microbes.
Our body mobilizes the immune system through white blood cells, which kill microbes.
Antibodies are created that recognize the antigens of a particular disease, with a memory that allows it to fight more effectively if you were to be infected again. For something like measles, this means we will unlikely be infected again.
In response to diseases having the ability to spread and infect, our bodies have evolved abilities to fight them off. All the methods above are ways in which we have gotten the ability to fight infections. However, as diseases evolve to become more virulent, our biological methods have become less effective. For instance, HIV spreads itself more quickly than our body can mobilize to fight it. Your body has no ability to fight this infection.
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Cultural innovations as a response to pathologies
Bacteria can be controlled with antibiotics, which kill the replication abilities of the bacterial cells.
But be CAREFUL! Antibiotic resistance…
Viruses cannot be controlled. You can get vaccinated in order to become resistant to the virus before being infected.
Since we have become less effective biologically at responding to disease pathogens, humans have developed cultural methods. For bacteria, we use antibiotics. For viruses, we use vaccinations. While these methods have proven effective, they have also altered the biology of the infections.
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Why vaccinate?
Vaccinations help to get the body ready for fighting a virus in the future
For people who cannot get a vaccine, it’s imperative that the rest of the population gain immunity through vaccines
This is called herd immunity
Vaccinated populations will see lower rates of death, helping to control population growth
In the last decade, people have been concerned about vaccinating for many reasons. One is a myth that vaccines are causes autism. Andrew Wakefield published a fraudulent essay about autism, which was removed from the scientific community. Please note that this information is FALSE and no evidence has ever been presented to support this. Vaccinations are crucial to human survival because while the body’s immune system might not be able to mobilize quick enough, a vaccine gives the body the ability to learn how to fight a disease before it actually gets infected. When most humans get vaccinated, it decreases the chances that a disease can enter into a population and thus protects those who cannot get vaccinated.
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Biocultural Influences on Infectious Diseases
“…the evolution of crowd diseases could only have occurred with the buildup of large, dense human populations, first made possible by the rise of agriculture about 10,000 years ago, then the rise of cities several thousands of years ago” (p. 6).
Agriculture sustains higher populations, and sedentation means early farmers “live amid their own sewage, providing microbes a quick path from one person’s body into another person’s drinking water.”
“Another bonanza was the development of world trade routes, which by late Roman times effectively joined the populations of Europe, Asia, and North Africa into one giant breeding ground for microbes” (p. 7).
“Given our proximity to the animals we love, we must constantly be getting bombarded by animal microbes. Those invaders get winnowed by natural selection, and only a few success in establishing themselves as human diseases” (p. 8).
Diseases evolve the ability to transfer from animals to people over time.
With a combination of biology and culture, our bodies have evolved the ability to deal with diseases. But why are they becoming more common? Your article discusses several examples of how humans are causes diseases to become more common and more virulent to our species. Consider the examples above.
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Old Epidemics Coming Back For More…
Measles virus: 145,000 deaths globally in 2013
Vaccinations dropped measles deaths by 75% between 2000-2013.
92% of all newly reported measles deaths are unvaccinated individuals in the U.S.
Tuberculosis bacteria: 1.5 million deaths in 2013.
In 2013, 480,000 of these cases were drug resistant.
Malaria parasite: 600,000 deaths in 2013, 200 million people infected.
Drug resistance, climate change, and pesticides influence malarial deaths.
In modern society, we’re seeing that some diseases that were once controlled by human innovation are coming back for more. This shows just how evolution can work, but also makes people who work in public health like medical anthropologists have to look at why these things are evolving and how we can develop ways in order to stop them in the future without helping them to become more virulent.
http://www.who.int/mediacentre/factsheets/fs286/en/
http://www.who.int/mediacentre/factsheets/fs104/en/
http://needtoknow.nas.edu/id/threats/global-killers/
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