Category Archives: Evolution

A Tale of Cookies and Milk: How We Adapted to Consuming Grains and Dairy

Humans are curious creatures. We like to poke and prod at new things to see what will happen. This curiosity is part of the reason we are successful. Though it can sometimes lead to disastrous outcomes, curiosity can be the reason not only for cultural inventions, but biological changes. This is especially true for our diet, which has changed radically in the past 10 – 20 thousand years. Two of the biggest changes have been our ability to efficiently digest grains and dairy. The agricultural revolution led to a lot of changes in human diet, including grain and dairy. Humans were experimenting with many new types of food. I’m sure the first individual to started eating grain was met with a warmer  reception than the one who suggested we start drinking cow and goat milk. At any rate, both ventures wound up changing our biology and culture. Just think: without amylase and lactase, Santa would be having something other than cookies and milk.

The Short Story of Amylase

In order to digest grains or any other starchy food, an organism needs an enzyme called amylase. Amylase hydrolyzes starch, eventually getting to the glucose molecules contained within the food. Though amylase is not unique to humans, there are some unique aspects about human amylase. In humans there is a positive correlation between the number of copies of the gene responsible for production of amylase, AMY1, that exist in a genome and the expression of amylase in the saliva. Interestingly, the average human contains about 7 times as many copies of AMY1 as chimpanzees, suggesting evidence for amylase selection after our split from the common ancestor with chimpanzees. The small differences between DNA in human AMY1 genes suggest a fairly recent selection event. Moreover, populations with high-starch diets had more AMY1 copies than populations with low-starch diets, further supporting a more recent selection as well as fairly rapid evolution. When it comes to diet, it seems natural selection can act fairly quickly.

The process of carbohydrate digestion begins in the mouth with an enzyme called ptyalin, also known as α-amylase. Ptyalin hydrolyzes the glycosidic bonds within starch molecules, breaking them down into the disaccharide sugar known as maltose. In the walls of the stomach, specialized cells called parietal cells secret hydrogen and chloride ions, creating hydrochloric acid. Amylase, which works at an optimum pH of about 7, cannot function in the highly acidic environment of the stomach.

The second part of starch digestion is initiated in the small intestine by an enzyme called pancreatic amylase. Though pancreatic amylase and salivary amylase are coded by two different DNA segments, they are side by side in the genome. It has been suggested that an endogenous retrovirus inserted DNA in-between the two copies of amylase that existed in our ancestors’ genome; this interruption in the open reading frame of the gene caused a mutation that promoted amylase production in the saliva from one of the gene copies that originally coded for pancreatic amylase. This mutation would have had a clear advantage, allowing for greater breakdown of starchy foods. Further evidence for the positive selection of salivary amylase production can be seen in its independent convergent evolution in mice and humans.

So the story for amylase is fairly short. Our ancestors began with two pancreatic amylase genes, which split to create one pancreatic and one salivary amylase gene. Over time, copy-number variations in genes occurred and were either selected for or against. Random gene duplication in conjunction with varying diets among human populations has resulted in the amylase locus being one of the most variable copy-number loci in the entire human genome.

The Somewhat Longer Story of Lactase

            The Neolithic (agricultural) revolution brought about some of the biggest cultural changes that our species has ever seen. Small groups of hunter-gatherers began to morph into large societies of agricultural-based farmers that existed in tandem with a group of people who lived a nomadic herding lifestyle. Nomadic herders could travel between these newly formed cities, trading meat, milk, or animals for agricultural products such as recently domesticated plants and grains. This substantial change in lifestyle caused a rapid overhaul in many aspects of human biology, including immunity, body size, and prevalence of certain digestive enzymes.

Lactase is the enzyme that breaks down the disaccharide sugar lactose, found in dairy products, into the monosaccharides glucose and galactose. Lactase is an essential enzyme because it allows infants to break down the lactose in the mother’s milk. However, there is a down-regulation of the lactase gene during childhood for a significant portion of world’s population. Curiously, the portion of the world’s population that does not experience down-regulation of the lactase gene are mostly of European descent. There is an interesting correlation between geographic location and percent of the population with lactase persistence. The further North you go in Europe, the more lactase persistence you find. This probably has to do with the fact that the colder climate of Europe, especially northern Europe, left fewer options for food consumption. The ability to digest and reap the benefits of lactose into adulthood could have acted as a major factor in surviving to reproductive age, thus increasing the prevalence of lactase persistence in those cultures.

Milk has a decent amount of calories and fat to keep energy reserves up, allowing people to survive harsh winters in Northern Europe. In addition, it provides nutrients such as calcium, protein, and vitamins B12 and D. Today in the Western world we see the high caloric and fat content of milk as a threat of weight gain. However, people living in 7000 B.C. would have seen this as a gold mine for survival. As essential as the calories and fats were to Northern European Neolithic people, the vitamin D content of milk may have been equally as important. In order for the body to synthesize vitamin D, it needs UVB rays from sunlight. This is an issue at northern latitudes, where it’s colder and there’s less sunlight than many other areas on Earth. Moreover, the amount of UVB light that can be absorbed is dependent upon angle at which the Sun’s rays strike the Earth. So even during a clear sunny day in the winter, people living in northern latitudes may not be absorbing UVB rays.

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One way to combat the low levels of UVB rays is to have fair skin. UVB rays that strike the skin will cause the synthesis of cholecalciferol (Vitamin D3) from 7-dehydrocholesterol that is already present in the skin, eventually leading to the production of a usable form of vitamin D. Specifically, 7-dehydrocholesterol is found predominately in the two innermost layers of the epidermis. This can be an issue for UVB absorbance since, melanin, which is the pigment responsible for darker skin, absorbs UVB at the same wavelength as 7-dehydrocholesterol. Indeed, it turns out that fair-skinned people (who tend to live in colder and more northern climates) are more efficient at producing vitamin D than darker skinned people.

Vitamin D is really an underappreciated nutrient. It is essential for absorption of calcium, which is nearly ubiquitous in its usage throughout the body, from brain functioning to muscle contraction. Recent research has illuminated other uses for vitamin D, including regulation of genes associated with autoimmune diseases, cancers, and infection. One study in Germany found that participants (average age of 62) with low vitamin D levels are twice as likely to die, particularly of cardiovascular problems, in the following 8 years than those with the highest vitamin D levels.

Though it isn’t too important to us today, lactase persistence might have saved the populations of Neolithic people in Northern Europe. Its dose of fat and calories helped bump up energy stores while the calcium and vitamin D found in whole milk reaped significant nutritional benefits. Though there are still many questions surrounding the evolution of lactase persistence in sub-populations of humans, the selection of this phenotype is quite clear. Those with lactase persistence would have had supplemental nutrition that might have helped them survive the Northern European winters.

 

 

An Evolutionary Explanation For Why You Wear Glasses

Empirically testing health-related hypotheses formulated through an evolutionary lens can prove to be difficult. The environment and the human experience are radically different from the first 6 million years of human evolution. Living on the edge of human existence and the top end of the techno-scientific scale, we are far removed from the environment to which many of our genes are hypothesized to be properly suited. Fortunately, the human race is a diverse group of individuals who have dispersed across the globe and have acclimated to a variety of circumstances. Accordingly, a few hunter-gatherer societies remain in parts of Africa. Though neither their genes nor their cultures are identical to original hunter-gatherers, they do retain the closest genetic and sociocultural similarity to human ancestors in the modern world. This is not to say that they are “less evolved” than other human societies. This notion is elementary and indicative of evolutionary ignorance. They are very well suited for their habitat, both genetically and culturally. Fortunately, those of us who are less suited for our environments, both genetically and culturally (i.e., everyone else, particularly in the US), can glean incredible insights about the functioning our own bodies and to what dietary and daily circumstances our physiology is best suited.

I recently wrote a primer on evolutionary medicine (which can be found here), which might be beneficial to read before getting into the specifics. This post will focus on myopia, or “near-sightedness,” the visual condition where objects at a distance are out of focus. Myopia affects about 15% of Africans, a third of Americans and Europeans, and over 75% of Asians – a curious bias that I’ll address later in the article. Fortunately (sort-of), myopia is easy to treat with glasses or contacts, and can even be cured to some extent with Laser-Assisted in situ Keratomileusis, commonly known as LASIK. Myopia occurs when the eye is too long, causing the focal point of light to occur prematurely, resulting in a blurry image. As a result, corrective lenses refract the light before it hits the cornea, essentially “overshooting” the refraction. For example, myopic corrective lenses will be thicker on the sides and thinner in the middle, causing the light to spread out slightly more before it hits the cornea, ultimately moving the focal point further back in the eyeball. With LASIK, a high frequency laser is used to vaporize (note: no heat is used. The vaporization is due to the light wavelength) tissue on the center of the cornea, thus reshaping the cornea so that light will be correctly refracted.

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In order to focus, the eye depends on ciliary muscles that are attached to the lens. When focusing on something far away, as would often be the case outdoors, the ciliary muscles contract, stretching the lens to a flattened shape. When focusing on something up close, such as a book, television, computer, or phone, the muscles relax, allowing the lens to become more concave. Think of a camera lens: to focus on something far away, you use a longer lens or zoom in. Doing this moves the focal point of distant objects further back, allowing them to be in focus. To take up-close shots you use a macro lens, which is a very short, rounded lens that moves the focal point for near objects closer to the lens. This is how the eye works. Myopia is what happens when your zoom function is broken. Evolution and an analysis of our current sociocultural context might be able to tell us why this happens.

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I’m a student, and spend a lot of my time looking at a book, a laptop, or a phone. I love to get outside when I can, but, ultimately, most of my time is spent looking at things up-close. That means that the ciliary muscles in my eye – the zoom muscles – spend most of their time relaxed. Just like any other muscle that goes unused, the ciliary muscle will likely begin to atrophy and become weaker (as far as I’m aware, no quantitative studies have been performed on ciliary muscle size or mitochondrial count, probably because this would be difficult or impossible to do on a living person. Perhaps future studies can examine the ciliary muscles of recently deceased individuals and compare individuals who suffered from myopia with individuals who had normal vision). Over time, particularly if it occurs throughout critical stages of development during childhood, the muscles may become to weak to contract and properly pull the lens flat, thus preventing you from being able to focus on distant objects. Of course, this begs the question of whether or not the muscle be strengthened. I don’t know, and I’m not sure that I am willing to find out by using myself as a guinea pig. Unfortunately, that makes me part of the problem of “dysevolution,” as coined by Harvard paleoanthropologist and human evolutionary biologist, Daniel Lieberman. Dysevolution refers to the circle of treating diseases without trying to change or fix the cause. Our technology and scientific understanding has advanced so rapidly in the past 100 years that we can fix things such as myopia with ease. Often this cycle is perpetuated by comfort. Why change what the way I do things when I can just buy contacts or glasses? My previous post mentions several other possible mismatch diseases, and Lieberman’s book, “The Story of the Human Body,” goes into detail about many of them. For many of them – if not most – we simply ignore the possible cures and instead opt for a more “comfortable” and easy treatment. However, this cycle is sure to grow and intensify as time goes on.

Evolutionary medicine is sometimes difficult to empirically test. However, as mentioned above, modern day hunter-gatherer societies can offer incredible insight and points of comparison for how sociocultural differences may affect our “mismatch diseases.” Studies of this kind are unfortunately few and far between (possibly because research funding also focuses on treatments). However, studies with hunter-gatherer societies have shown that very few members suffer from myopia (as well as many other non-infectious ailments, such as type-2 diabetes, heart disease, osteoporosis, and even cavities). The thought is that they are exposed to a variety of visual stimuli and their visual environment is constantly changing. This “exercises” their ciliary muscles and keeps them strong. Experiments have also shown that animals that are deprived of visual stimuli will grow elongated eyeballs. Similarly, people who spend more time indoors, particularly with studying, as is common in many Asian cultures, exhibit much higher instances of myopia whereas those who spend some time outdoors, as is more common in many African cultures, tend to have a lower rate of myopia. Our eyes did not evolve to see things 2 feet from our face all day long. They evolved to keep up alive from the plethora of visual stimuli in nature and to help us search for food: 2 things that many people, particularly children in developed countries, no longer need to do.

The solution isn’t to give up studying and electronics. It’s much more simple than that. Nearly everyone uses books and electronics, so why doesn’t everyone have myopia? One possibility is genetics, though that doesn’t seem like a plausible explanation. Rates of myopia have only skyrocketed in the last century, and any latent mutation for poor vision would have most certainly been selected against in our ancestors. The likely “cure” for myopia is balance. Spend time outside, especially as a child. The data from lab experiments as well as social statistics seem to point in this direction. If we continue to ignore the cause and only treat the symptoms, we are trapping ourselves in an ever growing cycle in which we become more and more dependent upon technology.