The Evolution of the Human Body

Wednesday, November 17, 2010

From Trees to Cities: Evolution Through Urbanization

Throughout history, there have been a myriad of situations or shocks, which have helped, pave the way for the evolution of modern Homo sapiens. These include environmental changes; such as drought, an ice age, or volcanic eruption, the development of tools, larger brain size, and various others. However there is one change that has developed fairly recently in terms of evolution that can be argued as having grand affects on human evolution: urbanization. One major way in which city life has influenced human evolution is through diseases.

Cities have long been a part of our culture. Ever since the days when Catalhoyuk, Suberde, and Tepe Yahya joined Jericho's mesh of intercity trade, and four thousand years before the rise of the Sumerian cities of Ur, Uruk, and Kish, Stone Age metropolises from Anatolia to the edges of India were already rich in challenges and opportunities. The emergence of these ancient cities has played an integral role in our evolution. They brought our ancestors out of the caves and other isolated rural establishments they lived in, into densely populated cities. Bringing all of these people together has influenced our evolution. With people living close together in these metropolises, it allowed for diseases to be easily spread and much more common than before. Researchers believe that this cesspool of diseases has allowed us as modern day humans, to evolve and adapt to become resistant to several infections.

Evolutionary biologist, Ian Barnes, has set out to prove this thinking. In order to do this, he utilized the genetic variant, SLC11A1 1729+55del4 which is associated with the resistance of germs within cells (Charles Q. Choi). To test this hypothesis, he and his team took DNA samples from 17 urban centers ranging from Catalhoyuk, 6000 B.C. or roughly 8,000 years old, to Juba which was erected in the 20^th century and is barely a hundred years old (Matt Kaplan). One thing that he made sure of was to choose cities that had all been inhabited for different lengths of time. In other words, a city settled for 200 years compared with one settled for 5,000. In their findings, the team found that longer populated cities were more likely to have this variant than cities inhabited for only a hundred or so years. The article specifically states the city of Susa in Iran. This city has been inhabited for more than 5,200 years and the people here are “almost certain” to have this specific variant, which allows them to resist a number of diseases. However, in comparison with Yakutsk in Siberia, which was only settled for a few hundred years, only around 70 percent of the people would have the variant (Charles Q. Choi).

According to the article by Charles Q. Choir, Barnes states that “The research shows evolution happening.” Evolution takes more than a hundred years to take place so it makes complete sense that a population in a city that had been established for more than 5,000 years would have adapted to the conglomeration of diseases through acquiring this genetic variant. Eventually, through natural selection, only those with this variant would survive.

Information is from: http://www.msnbc.msn.com/id/39345149/ns/technology_and_science-science

http://news.nationalgeographic.com/news/2010/11/101108-cities-immune-system-tuberculosis-tb-evolution-dna-genetics-science/

Thursday, November 11, 2010

Evolution of Dentition

All primates express dietary plasticity, which means that they eat a wide variety of different foods. In order to keep this varied diet, primates’ dentition, or teeth, have adapted in various ways. Primates have four distinctive tooth types: incisors, canines, premolars, and molars. Each has different functions; incisors and canines are for biting and tearing food while premolars and molars are for grinding and crushing food. Evolution has allowed primates to retain these different teeth because there is a purpose for all of them based on what primates eat. Primates also have a reduced number of teeth compared to their earlier mammalian ancestors. This reduction has occurred over the course of primates’ evolution, meaning that higher level primates have fewer teeth than lower level primates. Old World higher primates, including humans, have the least number of teeth: 2 incisors, 1 canine, 2 premolars, and 3 molars in each quadrant of their dentition.

While human teeth do match primate teeth in many ways, human dentition has also evolved on its own due to dietary changes. The most prominent difference between human teeth and primate teeth is the fact that humans have nonhoning chewing. Approximately 5.5 million years ago humans lost their large, projecting canines. This is a common feature among primates, which allows them to slice and shred their food. Early humans learned to make and use tools that allowed them to process their food before putting it in their mouths. These changes in lifestyle lead humans to lose their honing canine and develop a vertical, incisor shaped canine in its place. Going along with this, humans also lack the diastema, or gap, that primates have between their lower canines and first premolars. This gap allows room for the large honing canine, as well as a surface on which to sharpen the canine and slice food materials. Over time, human teeth have also become smaller than primate teeth.

Even more recent changes in human dentition have come about as the result of farming and agriculture becoming humans’ primary source of food. Within only the last 10,000 years humans have changed from foraging for wild plants and animals to producing domesticated plants and animals. In short, foods became more processed and softer over time, which lead to less stress on the chewing muscles. The reduction of stress on the muscles led to a reduction of the jaw bone underneath the muscle. However, tooth size is based more on genetics than bone size, and environmental changes had little effect on the size of the teeth in the increasingly smaller human jaw. All of this has lead to two extremely common problems among humans today: tooth crowding and malocclusion (overbites and underbites). Crowded and misaligned teeth are the direct results of having smaller jaws than our ancestors but still retaining similar sized teeth. The most common way to correct problems that are brought about by tooth crowding is braces. Orthodontics has become a multibillion-dollar industry in the United States because of how common teeth straightening has become recently.

Tooth decay is another recent problem facing humans as a result of dietary changes. Domesticated plants, which are carbohydrates, produce more tooth decay because bacteria in the mouth digest carbohydrates and then produce lactic acid, which dissolves the enamel of the teeth. Humans surprisingly have rather thick enamel on their teeth. Thick enamel is usually associated with eating harder foods, but since humans eat softer foods it is odd that the thick enamel has remained. Some types of domesticated plants, such as rice, cause very little tooth decay, while others, such as corn, cause considerable amounts of tooth decay.

Source:

Larsen, Clark Spencer. Our Origins: Discovering Physical Anthropology. New York: W.W. Norton &, 2008. Print.

Wednesday, October 27, 2010

Height, Weight, and Shape of the Human Body

The human body varies in many different ways between and within populations. Among all the variations, the size and shape of the body are of the most interesting. According to the article “Variation in Human Body Size and Shape” written by Christopher Ruff, “Mean body mass (weight) varies by 50% or more, within sex, in a worldwide sampling of populations…Variation in height is smaller (about 10%)…Variation in breadth is larger (about 25%),” (211).

One of the main factors that contributes to body mass and shape is geographic location. As latitudes increase, so does body mass. As people migrate further from the equator, temperatures decrease and body mass increases in order to retain more heat. As for body shape, it is also greatly influenced by climate. In colder climates, humans generally have shorter limbs and wider bodies whereas humans in warmer climates tend to have long limbs and a more lean body shape. This is due to surface area of the body and the ability to retain heat. In colder climates, shorter limbs and wider bodies mean less surface area to lose heat whereas in warmer climates, more surface area is better to more efficiently cool the body (220). Other factors, such as nutrition, contribute to body shape as well. Malnourishment leads to smaller body size than those that receive adequate nutrition, but the most important factor in body size and shape is geographic location.

Height also varies in living humans, although it does not generally depend on geographic location. As mentioned above, height varies by about 10% in living humans. However, it is also important to look at height changes over time as the human body has evolved. Since about 50,000 years ago, there has been a decrease in body size, caused by multiple factors. “These include technological improvements that decreased the selective advantage of a larger body (which is also metabolically expensive to maintain), a decline in nutritional quality, climatic factors (adaptation to a warming environment), and reduced gene flow (inbreeding),” (216). These factors have led to smaller bodies than those of our Neanderthal relatives. However, over the past few hundred years, there has been a slight upward trend in body size in some areas of the world (generally in more developed areas), due to improved nutrition and health overall (217).

Body size and shape vary greatly in living humans and over evolutionary relatives. While there are multiple explanations for these trends, geographic location and nutrition seem to be the most commonly agreed upon reasons for the variation that exists today.

Article Used:

Ruff, Christopher. "Variation in Human Body Size and Shape." Annual Review of Anthropology 31 (2002): 211-32. JSTOR. ITHAKA, 21 May 2002. Web. 24 Oct. 2010. <http://www.jstor.org/stable/4132878>.

Link: http://www.jstor.org/stable/4132878

Wednesday, September 29, 2010

Evolution for Distance Running

Runners always seem to be prone to injury. Shin splints, strained muscles and tendons, and other small maladies seem to plague the marathon runners of the modern world. Some coaches and trainers will even go so far as to claim that running itself causes these injuries. According to an article in the New York Times, however, the human body evolved for distance running.

The article, entitled “The Human Body Is Built for Distance” and published in October of 2009, states that general wisdom seems to agree that “distance running leads to debilitating wear and tear, especially on the joints.” However, several new articles, from journals such as the Sports Medicine and Current Anthropology reveal that the human body appears to have been made for endurance running. Christopher McDougall, a runner who had been plagued with injuries, wrote a book (“Born to Run”) discussing the debate on distance running.

One of the main arguments for the evolution of the human body for endurance running is the history of the genus Homo. Around two million years ago, hominids were hunter-gatherers, and are thought to have caught their prey by persistence hunting. A group of hunters would follow their selected prey, chasing it across their habitat, until the animal could not elude them any longer. In order to be successful in this attempt, the hominids needed to be able to outrun their prey. Traits such as cooling by sweating rather than panting, little amounts of body hair, and short toes, as well as a narrow waist that is able to rotate, all suggest that the human body was, indeed, adapted to distance running.

“So if we’re born to run, why are runners so often injured?” The New York Times makes a valid point in asking this final question. The best answer, according to the article, is a combination of factors. Most people do not begin to run until later in life, when muscles and tendons which are used for running are already developed and are not accustomed to running distances. Artificial surfaces and the high-tech shoes which are so popular also increase the risk of injury. All these problems, however, are easily corrected by running early in life on natural surfaces, and staying away from complex running shoes.

Here’s the article:

http://www.nytimes.com/2009/10/27/health/27well.html?_r=1

And here are a few papers/studies which are referenced in the article:

http://www.fas.harvard.edu/~skeleton/pdfs/2007c.pdf

http://www.mattmetzgar.com/matt_metzgar/files/persistence_hunting.pdf

http://jeb.biologists.org/cgi/content/abstract/212/5/713

Tuesday, September 21, 2010

Introduction to the Evolution of the Human Body

This blog will be covering how the human body has evolved over time. Over the past 7 million years, the human body has evolved to adapt to climates and various geographies. The human race has its roots in Africa but as groups of people traveled and spread out further around the globe, each group developed its own unique characteristics for survival. The gradual changes led to the differences in humans around the globe today. Although we may all look, talk, and act differently, each human is still 99.9% similar in our DNA makeup. We are not only similar to each other, but we are also extremely similar to chimpanzees. In fact, “Studies indicate that humans and chimps are between 95 and 98.5 percent genetically identical,” (National Geographic).

However, the human body has evolved in several ways that make it completely different from all other species. One of these distinctive characteristics is bipedalism, which is the ability to walk on two legs as opposed to four. Bipedalism was the first physical characteristic that marked the difference between humans and other animals. There are several theories about why humans switched to bipedalism, but all point to selective pressures that caused the evolutionary change. These possible selective pressures include access to food, increase in offspring, and energy and water conservation.

The next major development that marked the difference between the human body and that of other animals was the loss of a honing canine tooth. Apes have a honing canine to shred their food during chewing. Humans developed nonhoning chewing because they gained the ability to make and use tools that could preprocess their food. The nonhoning canine is smaller than the honing canine, does not project out as far, and is also not sharpened against the lower premolars.

A third, and much more recent, development that distinguishes humans from other animals is speech. The development of the hyoid bone allowed for this characteristic. The hyoid bone is a part of the human vocal structure. The shape of the hyoid bone is unique to humans and reflects their ability to speak, whereas other animals can only make noises to communicate.

Although the human body has evolved over time, humans still have many functions and parts that were once important to survival, but are now essentially useless. We still get goose bumps when we are afraid or cold, but why? When early hominids had thick hair covering their bodies, goose bumps could keep them warm by trapping air between the hair and skin, which created insulation. Goose bumps also made the hair stand on end to frighten any threatening animal. In present day, however, we do not have enough hair for insulation, or to make us seem larger.

Extra ear muscles, the plantaris muscle in the foot, wisdom teeth, the third eyelid, and the tailbone are other examples of superfluous body parts left over from ancestors who lived millions of years ago. We no longer need the muscles to wiggle our ears, though some people still have them. The plantaris muscle was used for gripping and manipulating objects with our feet, something most modern-day humans no longer do. Wisdom teeth became unnecessary when our diets changed, and the third eyelid is now only common in birds, reptiles, and fish. Our tailbone (coccyx) is still believed to be used to support muscles, but is no longer an aid for swinging through trees and can be surgically removed without any effect to our health.